CN111164518A - Determining pulse timing function for linear print head - Google Patents

Abstract

A method for controlling a print head comprising an array of light sources is described. A target exposure function is provided that gives the target exposure as a function of the integer pulse count. An initial pulse timing function is also provided that defines an exposure time as a function of the pulse count. A light output function for the light source is determined in response to the pulse timing function, wherein the light output function gives the light output of the light source as a function of the exposure time. The pulse timing function is updated in response to the light output function and the target exposure function. The process is repeated until a predetermined iteration termination criterion is met. The determined pulse timing function is used to control the print head, where each light source is activated for a pulse count corresponding to the pixel code value of the associated image pixel.

Description

Determining pulse timing function for linear print head

Technical Field

The present invention relates to the field of electrographic printing, and more particularly, to a method of determining a pulse timing function for controlling a linear print head.

Background

Electrophotography is a useful process for printing an image on a receiver (or "imaging substrate"), such as a sheet or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other object), as will be described below. In this process, an electrostatic latent image is formed on the photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to produce an electrostatic charge pattern (i.e., a "latent image") corresponding to the desired image.

After the latent image is formed, the charged toner particles are brought near the photoconductor and attracted to the latent image to develop the latent image into a toner image. Note that depending on the composition of the toner particles (e.g., transparent toner), the toner image may not be visible to the naked eye.

After developing the latent image into a toner image on the photoreceptor, a suitable receptor is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to the receiver to form the desired printed image on the receiver. The imaging process is typically repeated multiple times with a reusable photoreceptor.

The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., "fuse") the printed image to the receiver. Multiple printed images (e.g., separate images of different colors) may be overlaid on the receiver before fusing to form a multi-color printed image on the receiver.

Typically, a linear print head comprising an array of LED light sources is used to form the electrostatic latent image. The difference between the power provided by the individual light sources can lead to streak artifacts formed in the printed image. Even if the print head is carefully calibrated in the factory to equalize the power provided by each light source, it has been found that there may be residual streak artifacts when the print head is installed into a printing system, and these artifacts may change over time. Therefore, there remains a need for a method that can be implemented in practical applications to calibrate a print head to equalize the exposure provided by each light source.

Typically, a linear print head comprising an array of LED light sources is used to form the electrostatic latent image. The print head typically has an 8-bit interface that enables 256 different exposure levels to be provided by each light source. The exposure level provided by the light source is typically controlled by adjusting the time that the light source is activated, with each pixel code value being mapped to an exposure time that provides the target exposure level.

To control exposure time, some printheads utilize a stream of exposure clock pulses having non-uniform pulse widths, where the pulse widths are selected to provide a target exposure level. The exposure time for a particular pixel is controlled by counting the number of exposure clock pulses corresponding to the pixel code value. For example, for a pixel code value of 100, the light source will be activated for 100 exposure clock pulses. However, it has been found that the optical power provided by the light source is not constant over time and that the shape of the light output function is a function of the pulse width of the exposure clock pulse. Therefore, determining the pulse width required to provide the desired target exposure can be a complex process, as changing the pulse width to modify the exposure time changes the power, which in turn can affect the exposure time required to provide the target exposure. In some printing systems, the target exposure level (which is a function of pixel code value) may be updated in actual use as part of the printer calibration process. Therefore, it is necessary to update the pulse width of the exposure clock pulse accordingly. There remains a need for an efficient method for determining the pulse timing function that can be implemented in practical applications and for controlling a printing press with a pulse timing function that is appropriate for a particular printing mode.

Disclosure of Invention

The present invention represents a method for controlling a print head in a digital printing system, the print head comprising an array of light sources for exposing a photosensitive medium, the method comprising:

a) providing a target exposure function that gives a target exposure to be provided by the light source as a function of an integer pulse count;

b) providing an initial pulse timing function defining an exposure time as a function of a pulse count;

c) determining a light output function for the light source in response to the pulse timing function, wherein the light output function gives the light output of the light source as a function of exposure time;

d) updating the pulse timing function in response to the light output function and the target exposure function;

e) repeating steps c) -d) until a predetermined iteration termination criterion is met; and

f) controlling the print head using the pulse timing function, wherein each light source is activated for a pulse count corresponding to a pixel code value of an associated image pixel.

The invention has the following advantages: a pulse timing function may be determined that will provide a specified target exposure function for the print head, wherein the light output function varies as the pulse timing function changes.

Drawings

FIG. 1 is a front cross-sectional view of an electrophotographic printer suitable for use with the various embodiments;

FIG. 2 is a front cross-sectional view of one of the print modules of the electrophotographic printer of FIG. 1;

FIG. 3 illustrates a processing path for producing a printed image using a pre-processing system coupled to a print engine;

FIG. 4 is a flowchart illustrating processing operations for applying various calibration and artifact correction procedures in accordance with an illustrative embodiment;

FIG. 5 illustrates an exemplary quantization look-up table;

FIG. 6 illustrates an exemplary target exposure function;

FIG. 7 is a diagram illustrating how the master clock signal and the exposure clock signal are used to control the activation of the light sources;

FIG. 8 is a flowchart of an iterative process for determining a pulse timing function in accordance with an illustrative embodiment;

FIG. 9A compares an initial pulse timing function and an updated pulse timing function;

FIG. 9B compares the updated light timing function and the initial light output function corresponding to the pulse timing function of FIG. 9A;

FIG. 10 is a flowchart of a process for determining a current control parameter according to an exemplary embodiment;

FIG. 11 illustrates an exemplary test target for use with the process of FIG. 10;

FIG. 12 is a flow chart showing additional details of the step of analyzing the captured image of FIG. 10;

FIG. 13 illustrates an exemplary set of measured test patch data;

FIG. 14 illustrates an exemplary calibration function relating scanner code values to estimated exposure values;

FIG. 15 is a graph showing estimated exposure error as a function of light source for a particular test patch;

FIG. 16 is a graph showing estimated exposure error for a particular light source;

FIG. 17 is a diagram illustrating an exemplary set of gain corrections;

FIG. 18 illustrates an exemplary user interface that enables a user to select an option for specifying a print mode;

FIG. 19 shows a processing path including a print engine adapted to generate a printed image from image data using a plurality of print modes; and

FIG. 20 illustrates an exemplary set of pulse timing functions suitable for use with different printing modes.

It is to be understood that the drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.

Detailed Description

The invention includes combinations of the embodiments described herein. References to "a particular embodiment" or the like refer to features that are present in at least one embodiment of the invention. Separate references to "an embodiment" or "particular embodiments" or the like do not necessarily refer to the same embodiment or embodiments; however, unless so indicated, or otherwise apparent to those of ordinary skill in the art, such embodiments are not mutually exclusive. The use of the singular or plural in referring to "method" or "methods" and the like is not limiting. It should be noted that the word "or" is used in this disclosure in a non-exclusive sense unless the context clearly dictates otherwise.

As used herein, a "sheet" is a discrete sheet of media, such as receiver media for an electrophotographic printer (described below). The sheet has a length and a width. The sheet is folded along a folding axis (e.g., centered in the length dimension and extending the entire width of the sheet). The folded sheet contains two "lobes," each of which is that portion of the sheet on one side of the folding axis. The two sides of each blade are called "pages". "face" refers to one side of the sheet, whether before or after folding.

As used herein, a "toner particle" is a particle of one or more materials that is transferred to a receiver by an Electrophotographic (EP) printer to produce a desired effect or structure (e.g., a printed image, texture, pattern, or coating) on the receiver. As is known in the art, toner particles may be milled from larger solids, or chemically prepared (e.g., precipitated from a solution of pigment and dispersant using an organic solvent). The toner particles may have a range of diameters (e.g., less than 8 μm, about 10-15 μm, up to about 30 μm or more), where "diameter" preferably refers to a volume-weighted median diameter, as determined by a device such as a Coulter Multisizer. When practicing the present invention, it is preferred to use larger toner particles (i.e., those having a diameter of at least 20 μm) in order to obtain the desired toner stack height, which will enable the formation of macroscopic toner release structures.

"toner" refers to a material or mixture that contains toner particles and is useful in forming an image, pattern, or coating when deposited on an imaging member comprising a photoreceptor, photoconductor, or electrostatically or magnetically charged surface. The toner may be transferred from the imaging member to a receiver. Toners are also referred to in the art as marking particles, dry inks, or developers, but it is noted that "developers" are used differently herein, as described below. The toner may be a dry mixture of particles or a suspension of particles in a liquid toner matrix.

As already mentioned, the toner includes toner particles; it may also comprise other types of particles. The particles in the toner may be of various types and have various properties. Such properties may include absorption of incident electromagnetic radiation (e.g., particles containing colorants such as dyes or pigments), absorption of moisture or gases (e.g., desiccants or getters), inhibition of bacterial growth (e.g., bactericides, particularly useful in liquid toner systems), adhesion to a receiver (e.g., adhesives), electrical conductivity or low magnetic resistance (e.g., metal particles), electrical resistivity, texture, gloss, remanence, fluorescence, etch resistance, and other properties of additives known in the art

In a single component or one-component development system, "developer" refers only to toner. In these systems, all, some, or none of the particles in the toner are themselves magnetic. However, the developer in the one-component system does not include magnetic carrier particles. In a two-component, or multi-component development system, "developer" refers to a mixture comprising toner particles and magnetic carrier particles, which may be conductive or non-conductive. The toner particles may be magnetic or non-magnetic. The carrier particles may be larger than the toner particles (e.g., 15-20 μm or 20-300 μm in diameter). The magnetic field is used to move the developer in these systems by exerting a force on the magnetic carrier particles. The developer is moved by a magnetic field to the vicinity of the image forming member or the transfer member, and the toner or toner particles in the developer are transferred from the developer to the member by an electric field, which will be described further below. The magnetic carrier particles are not intentionally deposited on the component by the action of an electric field; only toner is intentionally deposited. However, magnetic carrier particles, as well as other particles in the toner or developer, may be inadvertently transferred to the imaging member. The developer may include other additives known in the art, such as those listed above for toner. The toner and carrier particles may be substantially spherical or non-spherical.

The electrophotographic process may be implemented in devices including printers, copiers, scanners and facsimile machines, as well as analog or digital devices, all of which are referred to herein as "printers". Various embodiments described herein are useful with electrostatographic printers, such as electrophotographic printers that employ toner developed on an electrophotographic receiver, as well as ionographic printers and copiers that do not rely on an electrophotographic receiver. Electrophotography and ionography are types of electrostatography (printing using an electrostatic field) which are a subset of electrography (printing using an electric field). The present invention may be implemented using any type of electrographic printing system, including electrophotographic and ionographic printers.

Digital reproduction printing systems ("printers") typically include a digital front end processor (DFE), a print engine (also referred to in the art as a "marking engine") for applying toner to a receiver, and one or more post-press finishing systems (e.g., a UV coating system, a glosser system, or a laminator system). The printer can reproduce a desired black-and-white or color image onto the receiver. The printer may also produce selected toner patterns on the receiver that do not directly correspond to the visible image (e.g., surface texture).

In an embodiment of an electrophotographic module printing machine useful with various embodiments (e.g., a NEXPRESS SX3900 printer manufactured by Istman Kodak, NY), a color toner print image is formed by a plurality of color imaging modules arranged in tandem, and the print image is successively electrostatically transferred to a receiver attached to a conveyor web that is moved by the modules. Color toners include colorants (e.g., dyes or pigments) that absorb certain wavelengths of visible light. Commercial machines of this type typically employ an intermediate transfer member in each module for transferring the visible image from the photoreceptor and the printed image to a receiver. In other electrophotographic printers, each visible image is transferred directly to a receiver to form a corresponding printed image.

Electrophotographic printers having the ability to also use an additional imaging module to deposit clear toner are also known. Providing a clear toner overcoat for a color print is desirable for providing features such as protecting the print from fingerprints, reducing certain visual artifacts, or providing desirable texture or surface finish characteristics. Transparent toners use particles similar to toner particles of a color development station, but no colored material (e.g., dye or pigment) is incorporated into the toner particles. However, transparent toner overcoats can increase cost and reduce the color gamut of the print; it is therefore desirable to provide the operator/user with the option of determining whether or not a clear toner overcoat will be applied to the entire print. A uniform transparent toner layer can be provided. Layers that vary inversely with the height of the toner stack may also be used to establish a horizontal toner stack height. The respective color toners are deposited one after the other at respective locations on the receiver, and the height of the respective color toner stack is the sum of the toner heights of each respective color. The uniform stack height provides smoother or uniform gloss to the printed product.

Fig. 1 and 2 are front cross-sectional views illustrating portions of a typical electrophotographic printer 100 useful with various embodiments. The printer 100 is adapted to produce an image on a receiver, such as a single color image (i.e., a monochrome image), or a multi-color image, such as a CMYK, or a five color (five color) image. The multi-color image is also referred to as a "multi-component" image. One embodiment involves printing using an electrophotographic print engine having five sets of single color image producing or image printing stations or modules arranged in tandem, but more or less than five colors may be combined on a single receiver. Other electrophotographic recorder or printer devices may also be included. The various components of the printing press 100 are shown as rollers (rollers); other configurations are possible, including belts (belt).

Referring to fig. 1, a printer 100 is an electrophotographic printing apparatus having a plurality of electrophotographic image-forming printing subsystems 31, 32, 33, 34, 35 (also referred to as electrophotographic imaging subsystems) arranged in series. Each printing subsystem 31, 32, 33, 34, 35 produces a single color toner image for transfer to a receiver 42 that moves successively through the modules using respective transfer subsystems 50 (only one labeled for clarity). In some embodiments, one or more of the printing subsystems 31, 32, 33, 34, 35 may print a clear toner image, which may be used to provide a protective overcoat or tactile image feature. The receiver 42 is transported from the supply unit 40 into the printing press 100 using a transport web 81, the supply unit 40 may comprise an active feed subsystem as known in the art. In various embodiments, the visual image may be transferred directly from the imaging roll to the receiver, or sequentially from the imaging roll to one or more transfer rolls or belts and then to the receiver 42 in a transfer subsystem 50. The receiver 42 is, for example, a selected portion of a web or slice of a planar receiver medium, such as paper or transparent film.

In the illustrated embodiment, each receiver 42 may transfer in registration (in registration) thereon up to five individual color toner images during a single pass through the five printing subsystems 31, 32, 33, 34, 35 to form a five-color image. As used herein, the term "five colors" means that in a printed image, combinations of various ones of the five colors are combined to form other colors on the receiver at different locations on the receiver, and all five colors participate in forming process colors in at least some of the subsets. That is, each of the five colors of toner may be combined with one or more of the other colors of toner at a particular location on the receiver to form a color that is different from the color of the combined toner at that location. In an exemplary embodiment, the printing subsystem 31 forms a black (K) print image, the printing subsystem 32 forms a yellow (Y) print image, the printing subsystem 33 forms a magenta (M) print image, and the printing subsystem 34 forms a cyan (C) print image.

Printing subsystem 35 may form a red, blue, green, or other fifth printed image that includes an image formed from clear toner (e.g., a toner lacking pigments). The four subtractive primary colors cyan, magenta, yellow and black may be combined in various combinations of subsets thereof to form a representative color spectrum. The color gamut of the printer (i.e., the range of colors that can be produced by the printer) depends on the materials used and the processes used to form the colors. Therefore, a fifth color may be added to improve the color gamut. In addition to increasing the color gamut, the fifth color may also be a special color toner or a mottled color, such as a color used to make a proprietary logo or not producible with CMYK colors alone (e.g., metallic, fluorescent, or pearl colors), or a clear toner or a lightly tinted toner. The lightly dyed toners absorb less light than they transmit, but do contain pigments or dyes that shift the shade of light through them towards the shade of the light. For example, a bluish toner coated on white paper will make white paper appear bluish when viewed under white light, and make yellow printed under a bluish toner appear slightly greenish under white light.

Receiver 42a is shown after passing through printing subsystem 31. The printed image 38 on the receiver 42a includes unfused toner particles. After transferring the respective printed images (one from each of the respective printing subsystems 31, 32, 33, 34, 35, overlaid in registration), the receiver 42a advances to a fuser module 60 (i.e., a fusing or fixing component) to fuse the printed images 38 to the receiver 42 a. The transport web 81 transports the print image bearing receiver to the fuser module 60, which fuser module 60 typically fixes the toner particles to the respective receiver by application of heat and pressure. The receivers are continuously unloaded from the transport web 81 to allow them to be fed cleanly to the fuser module 60. The transport web 81 is then restored for reuse at the cleaning station 86 by cleaning and neutralizing the charge on the opposite surface of the transport web 81. A mechanical cleaning station (not shown) for scraping or sucking toner off the transport web 81 may also be used independently or together with the cleaning station 86. Mechanical cleaning stations may be provided along the transport web 81 before or after the cleaning station 86 in the direction of rotation of the transport web 81.

In the illustrated embodiment, fuser module 60 includes a heated fusing roll 62 and an opposing pressure roll 64 that form a fusing nip 66 therebetween. In one embodiment, the fuser module 60 also includes a release fluid application station 68 that applies a release fluid (e.g., silicone oil) to the fusing roller 62. Alternatively, a wax-containing toner may be used without adding a release liquid to the fusing roller 62. Other embodiments of the cage may be employed, including contact and non-contact. For example, solvent fixing uses a solvent to soften the toner particles so they bond with the receptor. Flash fusion uses short bursts of high frequency electromagnetic radiation (e.g., ultraviolet light) to melt toner. Radiation fixation uses low frequency electromagnetic radiation (e.g., infrared light) to melt the toner more slowly. Microwave fixing uses electromagnetic radiation in the microwave range to heat the receiver (primarily) to melt the toner particles by thermal conduction so that the toner is fixed to the receiver.

The fused receiver (e.g., receiver 42b carrying fused image 39) is serially transferred from the fuser module 60 along a path to an output tray 69, or returned to the printing subsystems 31, 32, 33, 34, 35 to form an image on the back of the receiver (i.e., to form a duplex print). The receiver 42b may also be transmitted to any suitable output accessory. For example, a secondary fuser or glosser assembly may provide a transparent toner overcoat. The printing press 100 may also include a plurality of fuser modules 60 to support applications such as overprinting, as is known in the art.

In various embodiments, between the fuser module 60 and the output tray 69, the receiver 42b passes through the finisher 70. The finisher 70 performs various sheet handling operations such as folding, stapling, saddle-stitching, collating (binding), and binding.

The printing press 100 includes a main press unit Logic and Control Unit (LCU)99 that receives input signals from various sensors associated with the printing press 100 and sends control signals to various components of the printing press 100. LCU 99 may include a microprocessor that incorporates a suitable look-up table and control software executable by LCU 99. It may also include a Field Programmable Gate Array (FPGA), Programmable Logic Device (PLD), Programmable Logic Controller (PLC) (with, for example, a program in ladder logic), microcontroller, or other digital control system. LCU 99 may include memory for storing control software and data. In some embodiments, sensors associated with the fuser module 60 provide suitable signals to the LCU 99. In response to the sensor signals, the LCU 99 issues command and control signals that adjust the heat or pressure within the fusion nip 66 and other operating parameters of the fuser module 60. This allows the printer 100 to print on receivers of different thicknesses and surface finishes, such as glossy or matte.

Fig. 2 shows additional details of printing subsystem 31, which are representative of printing subsystems 32, 33, 34, and 35 (fig. 1). The photoreceptor 206 of the imaging member 111 includes a photoconductive layer formed on a conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that charge remains on its surface. Upon exposure to light, the charge is dissipated. In various embodiments, photoreceptor 206 is part of the surface of imaging member 111 or is disposed on imaging member 111, which imaging member 111 may be a plate, drum, or belt. The photoreceptor may comprise a uniform layer of a single material, such as glassy selenium, or a composite layer comprising a photoconductor and another material. Photoreceptor 206 may also comprise multiple layers.

The charging subsystem 210 applies a uniform electrostatic charge to the photosensitive body 206 of the imaging member 111. In an exemplary embodiment, the charging subsystem 210 includes a wire grid 213 having a select voltage. The additional necessary components provided for control may be assembled around the various process elements of the respective printing subsystem. Meter 211 measures the uniform electrostatic charge provided by charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating a uniform electrostatic charge on the photoreceptor 206 in an image-wise manner (image-wise) by exposing the photoreceptor 206 to electromagnetic radiation in order to form an electrostatic latent image. The uniformly charged photoreceptor 206 is typically exposed to actinic radiation provided by selectively activating specific light sources in an LED array or laser device that outputs light directed onto the photoreceptor 206. In embodiments using a laser device, a rotating polygon (not shown) is sometimes used to scan one or more laser beams across the photoreceptor in a fast scan direction. One pixel location is exposed at a time and the intensity or duty cycle of the laser beam is varied at each point location. In embodiments using an array of LEDs, the array may include a plurality of LEDs arranged adjacent to one another in a linear array extending in a direction intersecting the track, such that all of a row of dot locations on the photoreceptor may be selectively simultaneously exposed, and the intensity or duty cycle of each LED may be varied over a line exposure time to expose each pixel location in the row over the line exposure time.

As used herein, an "engine pixel" is the smallest addressable unit on the photosensitive body 206, and the exposure subsystem 220 (e.g., a laser or LED) may be exposed with a selected exposure that is different from the exposure of another engine pixel. The engine pixels may overlap (e.g., to increase addressability in the slow scan direction). Each engine pixel has a corresponding engine pixel location, and the exposure applied to the engine pixel location is described by the engine pixel level.

Exposure subsystem 220 may be a write-white (write-white) or a write-black (write-black) system. In a write-white or "charged area development" system, exposure consumes charge on areas of photoreceptor 206 to which toner should not adhere. The toner particles are charged to be attracted by the charge remaining on the photosensitive body 206. Thus, the exposed areas correspond to the white areas of the printed page. In a black writing or "discharged area development" system, the toner is charged to be attracted by a bias voltage applied to the photoreceptor 206 and repelled by the charge on the photoreceptor 206. Therefore, the toner adheres to the area of the photoreceptor 206 where the charge has been consumed by exposure. Thus, the exposed areas correspond to the black areas of the printed page.

In the illustrated embodiment, a meter 212 is provided to measure the post-exposure surface potential in the patch area of the latent image that is formed from time to time in the non-image areas on the photoreceptor 206. Other instruments and components (not shown) may also be included.

The development station 225 includes a toning shell 226 (the toning shell 226 can be rotating or stationary) for applying toner of a selected color to the latent image on the photosensitive body 206 to produce a developed image on the photosensitive body 206 corresponding to the color of toner deposited on the printing subsystem 31. The development stations 225 are electrically biased by suitable respective voltages, which may be supplied by a power supply (not shown), to develop respective latent images. Developer is provided to the toning shell 226 by a supply system (not shown), such as a supply roller, auger, or belt. The toner is transferred from the developing station 225 to the photosensitive body 206 by electrostatic force. These forces may include coulombic forces between the charged toner particles and the charged electrostatic latent image, and lorentz forces on the charged toner particles due to the electric field generated by the bias voltage.

In some embodiments, the development station 225 employs a two-component developer that includes toner particles and magnetic carrier particles. The exemplary development station 225 includes a magnetic core 227 to form a "magnetic brush" with magnetic carrier particles near the toning shell 226, as is known in the art of electrophotography. The core 227 may be stationary or rotating and may rotate at the same or a different speed and direction as the toning shell 226. The core 227 may be cylindrical or non-cylindrical, and may include a single magnet or multiple magnets or poles disposed around the circumference of the core 227. Alternatively, the magnetic core 227 may comprise an array of solenoids that are driven to provide a magnetic field of alternating direction. The magnetic core 227 preferably provides a magnetic field of varying magnitude and direction around the outer circumference of the toning shell 226. The development station 225 may also employ a one-component developer containing a magnetic or non-magnetic toner without the need for separate magnetic carrier particles.

Transfer subsystem 50 includes a transfer support member 113 and an intermediate transfer member 112, the intermediate transfer member 112 for transferring the respective printed images from the photosensitive body 206 of imaging member 111 through the first transfer nip 201 to a surface 216 of intermediate transfer member 112 and then to a receiver 42, the receiver 42 receiving the respective toned printed images 38 from each printing subsystem in a superimposed manner to form a composite image thereon. The print image 38 is, for example, a separation of one color, such as cyan. The receiver 42 is transported by the transport network 81. The transfer to the receiver is effected by an electric field provided by power supply 240 to transfer support member 113, which is controlled by LCU 99. The receptor 42 may be any object or surface to which toner may be transferred from the imaging member 111 by application of an electric field. In this example, receiver 42 is shown prior to entering second transfer nip 202, and receiver 42a is shown after printed image 38 is transferred onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from photoreceptor 206 to intermediate transfer member 112 and from there to receiver 42. Registration of the individual toner images is achieved by registering the individual toner images on receiver 42, as is done with NexPress 2100. In some embodiments, a single transfer member is used to sequentially transfer the toner images from each color channel to receiver 42. In other embodiments, individual toner images may be transferred in registration directly from the photosensitive bodies 206 in the respective printing subsystems 31, 32, 33, 34, 25 to the receiver 42 without the use of a transfer member. Any transfer process is suitable when practicing the present invention. An alternative method of transferring toner images involves transferring individual toner images in registration onto a transfer member and then transferring the registered images onto a receiver.

LCU 99 sends control signals to, among other components, charging subsystem 210, exposure subsystem 220, and corresponding development station 225 of each printing subsystem 31, 32, 33, 34, 35 (fig. 1). Each printing subsystem may also have its own respective controller (not shown) coupled to LCU 99.

Various finishing systems may be used to apply features such as protection, glossing, or binding to the printed image. The finishing system scan is implemented as an integral component of the printing press 100, or may comprise one or more separate machines through which the printed images are fed after they are printed.

FIG. 3 illustrates a processing path that may be used to generate a printed image 450 using the print engine 370, according to an embodiment of the invention. The preprocessing system 305 is used to process the page-describing file 300 to provide image data 350 in a form ready for printing by the print engine 370. In an exemplary configuration, the pre-processing system 305 includes a Digital Front End (DFE)310 and an image processing module 330. The pre-processing system 305 may be part of the printing press 100 (fig. 1) or may be a separate system from the printing press 100. DFE 310 and image processing module 330 may each include one or more suitably programmed computers or logic devices adapted to perform operations suitable for providing image data 350.

DFE 310 receives page description file 300 that defines the pages to be printed. The page description file 300 may be of any suitable format (e.g., the well-known Postscript command file format or PDF file format) that specifies the content of a page in terms of text, graphics, and image objects. The image objects are typically provided by an input device, such as a scanner, digital camera, or computer-generated graphics system. The page-describing file 300 may also specify invisible content such as texture, gloss, or specifications for protective coating patterns.

DFE 310 rasterizes page description file 300 into an image bitmap for printing by the print engine. DFE 310 may include various processors, such as a Raster Image Processor (RIP)315, a color transform processor 320, and a compression processor 325. It may also include other processors not shown in fig. 3, such as an image location processor or an image storage processor. In some embodiments, DFE 310 enables an operator to set parameters such as layout, font, color, media type, or finishing options.

RIP 315 rasterizes objects in page description file 300 into an image bitmap comprising an array of image pixels at an image resolution suitable for print engine 370. For text or graphics objects, RIP 315 will create an image bitmap based on the object definition. For image objects, RIP 315 resamples the image data to the desired image resolution.

The color transformation processor 320 transforms the image data to the color space required by the print engine 370 to provide color separation for each color channel (e.g., CMYK). For the case where the print engine 370 includes one or more additional colors (e.g., red, blue, green, gray, or transparent), the color transform processor 320 will also provide color separation for each additional color channel. The objects defined in the page-describing file 300 may be in any suitable input color space (e.g., RGB, CIELAB, PCS LAB, or CMYK). In some cases, different color spaces may be used to define different objects. The color transformation processor 320 applies the appropriate color transformation to convert the object to the device dependent color space of the print engine 370. Methods for creating such color transforms are well known in the color management arts, and any such method may be used in accordance with the present invention. Typically, the color transformation is defined using a color management profile that includes a multi-dimensional look-up table. The input color profile is used to define the relationship between the input color space and a Profile Connection Space (PCS) defined for the color management system (e.g., the well-known ICCPCS associated with the ICC color management system). The output color profile defines a relationship between the device-dependent output color space and the PCS for the printing press 100. The color conversion processor 320 converts the image data using the color management profile. Typically, the output of the color transform processor 320 will be a color-separated set comprising an array of pixels stored in a memory buffer for each color channel of the print engine 370.

The processing applied to digital front end 310 may also include other operations not shown in fig. 3. For example, in some configurations, DFE 310 may employ halo correction processes (halo correction processes) described in commonly assigned U.S. patent 9,147,232(Kuo), entitled Reducing halo features in electronic printing systems, which is incorporated herein by reference.

The image data provided by the digital front end 310 is sent to the image processing module 330 for further processing. To reduce the time required to transmit the image data, the compressor processor 325 is typically used to compress the image data using a suitable compression algorithm. In some cases, different compression algorithms may be applied to different portions of the image data. For example, a lossy compression algorithm (e.g., the well-known JPEG algorithm) may be applied to portions of image data that include image objects, and a lossless compression algorithm may be applied to portions of image data that include binary text and graphics objects. The compressed image values are then transmitted over a data link to the image processing module 330 where they are decompressed using a decompression processor 335, the decompression processor 335 applying a corresponding decompression algorithm to the compressed image data.

The halftone processor 340 is used to apply halftone processing to the image data. Halftone processor 340 may apply any suitable halftone processing known in the art. In the context of the present disclosure, halftone processing is applied to a continuous tone image to provide an image having a halftone dot structure suitable for printing using the printer module 435. The output of the halftone may be a binary image or a multi-level image. In an exemplary configuration, the halftone processor 340 applies the halftone processing described in commonly assigned U.S. patent 7,830,569(Tai et al), entitled "multilevel ketone screen and sets thermoof," which is incorporated herein by reference. For this halftone processing, a three-dimensional halftone screen is provided that includes a plurality of planes, where each plane corresponds to one or more intensity levels of the input image data. Each plane defines a pattern of output exposure intensity values corresponding to a desired halftone pattern. The halftone pixel values are multilevel values at a bit depth suitable for the print engine 370.

The image enhancement processor 345 may apply a variety of image processing operations. For example, the image enhancement processor 345 may be used to apply various image enhancement operations. In some configurations, the image enhancement processor 345 may apply an algorithm that modifies the halftoning in the border regions of the image (see U.S. patent 7,079,281 entitled "Edge enhancement processor and method adjustable threshold setting" and U.S. patent 7,079,287 entitled "Edge enhancement of level images" (both Ng et al), both of which are incorporated herein by reference).

The pre-processing system 305 provides the image data 350 to a print engine 370, where the image data 350 is printed to provide a printed image 450. The preprocessing system 305 can also provide various signals to the print engine 370 to control the timing at which the image data 350 is printed by the print engine 370. For example, when a sufficient number of lines of image data 350 have been processed and buffered, the preprocessing system 305 can signal the print engine 370 to begin printing to ensure that the preprocessing system 305 will be able to keep up with the rate at which the print engine 370 prints the image data 350.

A data interface 405 in the print engine 370 receives data from the pre-processing system 305. The data interface 405 may use any type of communication protocol known in the art, such as a standard ethernet network connection. The printer module controller 430 controls the printer module 435 based on the received image data 350. In an exemplary configuration, the printer module 435 may be the printer 100 of fig. 1, including a plurality of individual electrophotographic printing subsystems 31, 32, 33, 34, 35 for each color channel. For example, the printer module controller 430 may provide suitable control signals to activate the light source in the exposure subsystem 220 (FIG. 2) to expose the photosensitive body 206 with an exposure pattern. In some configurations, the printer module controller 430 may apply various image enhancement operations to the image data. For example, algorithms may be applied to compensate for various sources of non-uniformity in the printing press 100 (e.g., streaks formed in the charging subsystem 210, exposure subsystem 220, development station 225, or fuser module 60). One such compensation algorithm is described in commonly assigned U.S. patent 8,824,907(Kuo et al) entitled "electrophoretic printing with column-dependent tone adaptation," which is incorporated herein by reference.

In some cases, the printing system may also include an image capture system 440. The image capture system may be used for purposes such as system calibration. The image capture system 440 may use any suitable image capture technology, such as a digital scanner system or a digital camera system. The image capture system 440 may be integrated into the printing system or may be a separate system in communication with the printing system.

In the configuration of fig. 3, the pre-processing system 305 is tightly coupled to the print engine 370 because it supplies the image data 350 in a state that matches the halftone state and printer resolution required by the printer module 435. In other configurations, the print engine may be designed to adapt to the characteristics of different pre-processing systems 305, as described in commonly assigned, co-pending U.S. patent application serial No. 15/135,607 to Kuo et al, entitled "printing with adaptive processing," which is incorporated herein by reference.

Aspects of the invention will now be described with reference to fig. 4, which fig. 4 shows a flowchart of processing operations that may be used to apply various calibration and artifact correction procedures, according to an example embodiment. Some operations may be applied in the data processing electronics 570 prior to passing the image data to the printer module 435 (e.g., in the printer module controller 430 (fig. 3)), while other operations may be applied in the print head electronics 580 associated with the exposure subsystem 220 (fig. 2) of the printer module 435.

The input to the flowchart is the pixel code values 500 for the image pixels in the array of image data to be printed by one of the electrophotographic printing subsystems 31, 32, 33, 34, 35 in the printer 100. In an exemplary embodiment, the pixel code values 500 may be pixels of image data 350, which image data 350 is input to the print engine 370 (see FIG. 3). Typically, the pixel code value 500 will be an 8-bit number between 0-255.

The apply calibration LUT step 510 is used to apply a calibration look-up table (LUT)505 to the pixel code value 500. Typically, the output of the calibration LUT will be an exposure value EV that is linear with the exposure level provided by the print head. In an exemplary arrangement, the exposure value EV is represented by a 12-bit integer in the range of 0-4095. The exposure value EV corresponds to the exposure that should be provided to the photoreceptor 206 (FIG. 2) by the exposure subsystem 202 so that the printer 100 (FIG. 1) produces a target density value that is appropriate for the pixel code value 500.

The apply gain correction step 520 is used to apply the gain correction values 515 on a pixel-by-pixel basis to compensate for various sources of non-uniformity in the printing press 100 (e.g., streaks formed in the charging subsystem 210, exposure subsystem 220, development station 225, or fuser module 60). In an exemplary embodiment, the apply gain correction step 520 applies the compensation algorithm described in the aforementioned U.S. patent 8,824,907. The method involves determining two gain correction values 515 (i.e., G1 and G2) for each light source in the linear printhead. The output of the apply gain correction step 520 is the modified exposure value EV.

Although the exposure value EV is a 12-bit number in the exemplary configuration, only 256 different code values will be used because the pixel code value 500 is an 8-bit number. Applying the gain correction step 520 will modify the exposure value EV for each light source in a different way according to the associated gain correction value 515. Thus, the modified exposure value EV will generally utilize more of the available 12-bit code values. The exact set of code values used will depend on the gain correction values 515, which gain correction values 515 are necessary to correct for streak artifacts.

The interface to the print head is typically an 8-bit number. As a result, application quantization steps must be usedStep 530 is to determine a quantized exposure value 540 by applying a suitable quantization LUT 525. To minimize quantization error, a vector quantization process may be used to select a range of exposure values that are mapped to each quantized exposure value 540. Vector quantization processes are well known in the art and any suitable process may be used in accordance with the present invention. An example of a quantization LUT 525 is shown in fig. 5. Quantization LUT 525 defines bin (bin) B_jCorresponding to a range of modified exposure values that map to the ith quantized exposure value. A target exposure value E may also be defined for each bin_A，iWhich specifies a target exposure value representing the ith quantized exposure value. This set of target exposure values defines a target exposure function 605, which may be represented as a vector E_a：

E_a＝[E_a，0，E_a，1，...E_a，i，...E_a，255](1)

An exemplary target exposure function 605 is illustrated in fig. 6.

Over time, it has been found that the characteristics of the streak artifact can change. It is therefore desirable to perform a calibration procedure to determine the light source dependent gain correction values 515 on a periodic basis or as needed. For example, the calibration process may be performed at the beginning of each day, or if the operator observes the presence of streak artifacts, the calibration process may be initiated. Because the optimal quantization LUT 525 will be a function of the gain correction value 515, it is generally desirable to determine the updated quantization LUT 525 at the same time. In a preferred embodiment, as part of the calibration process, a determine gain correction process 590 is performed to determine the gain correction values 515, quantization LUTs 525, and corresponding target exposure functions 605 for each light source.

The quantified exposure values 540 are passed to the print heads where they are used to control the exposure provided by the corresponding light sources. In an exemplary embodiment, the control light source exposure time step 550 provides a target exposure value E corresponding to the associated quantized exposure value 540 by activating each light source in the print head_A，iRequired exposure timeTo control the exposure.

In some embodiments, the print head has an associated master clock that provides a master clock signal 660, as shown in FIG. 7. For example, the master clock may run at 80 MHz. An exposure clock signal 670 is then formed, the exposure clock signal 670 having a stream of pulses formed by clocking a corresponding number of pulses in the master clock signal 660. The exposure may then be controlled by activating the light source at time t-0 and then deactivating the light source after counting to the number of exposure clock signal pulses corresponding to the quantized exposure value 540. The time (t) of the ith pulse is defined by the pulse time S_iGiven, the set of pulse times for each quantized exposure value together define the pulse timing function 610 (S):

S＝[S₀，S₁，...S_i，...S₂₅₅](2)

in an exemplary configuration, the pulse time S_iExpressed in terms of the number of master clock pulses. Fig. 7 illustrates a light source activation function 680 corresponding to the quantized exposure value 540 of EQ ═ 5, where the light source is activated at time t ═ 0 and deactivated at time S5 when the falling edge of the fifth exposure clock signal pulse is detected.

In the simplest case, the power provided by the light source (i.e. the light output) is constant during the time that the light source is activated, so that the exposure will simply be proportional to the exposure time. However, it has been found that the power provided by the light source generally varies over time (see, e.g., exemplary light output function 630 in fig. 9A). More complicated is the fact that the time dependence varies as a function of the number of pulses that make up the exposure clock signal 670. For example, for some common driver chips used in LED print heads, it has been found that when the pulses in the exposure clock signal 670 are closer together, the light output is generally lower than when the pulses in the exposure clock signal 670 are farther apart.

The determine pulse timing function process 600 is used to determine a pulse timing function 610 that will deliver the specified target exposure function 605. In order to determine the pulse timing function 610, the shape of the light output function 630 must be known in order to be able to calculate the exposure that is provided for a particular exposure time. However, as already discussed, the shape of the light output function 630 depends on the pulse timing function 610. Therefore, it is not possible to use a simple procedure to determine the pulse timing function 610.

Fig. 8 illustrates an iterative process that has been developed for use by the determine pulse timing function process 600 in accordance with an exemplary embodiment. The determine light output function step 620 is used to determine an initial light output function 630 based on the initial pulse timing function 615(S ^). The initial pulse timing function 615 may be provided in a variety of ways. In some embodiments, it may be a previously determined pulse timing function that is determined for a similar target exposure function 605. In other embodiments, the initial pulse timing function 615 may be determined based on the assumption that the optical output function 630 is constant over time.

Determining the light output function step 620 may use any suitable means to determine the light output function 630. In one exemplary configuration, one or more light sources in the print head may be controlled using an initial pulse timing function 615, and a light output function 630 may be measured using a light detector that measures the light output of the one or more light sources as a function of exposure time. In a preferred configuration, the determine light output function step 620 determines the light output function 630 using a light output model 625, which light output model 625 predicts light output as a function of exposure time given a pulse timing function.

It has been found that the following functional form for the light output model 625 yields a good prediction for normalized light output as a function of exposure time for a common type of driver chip for LED print heads (e.g., a type LC 46611C dryer chip available from ONSemiconductor):

wherein S_iIs the ith pulse time, Δ t_i＝(S_i+1-S_i) Is at the timeThe time difference between two consecutive exposure clock signal pulses at time t, and α is a constant that can be experimentally determined for the driver chip and operating conditions.a typical value for α is about 0.01-0.02 msec.

Next, an update pulse timing function step 635 is performed to determine an updated pulse timing function 640, which updated pulse timing function 640 will provide the exposure value given by the target exposure function 605 given the determined light output function 630. The updated pulse time for the ith quantized exposure value 540 and the jth iteration may be determined by calculating an updated pulse time that satisfies the following equation

Wherein, P^(j-1) _(t)Is the light output function 630, E determined for the previous iteration_a，iIs the target exposure value for the ith quantized exposure value 540, and

is the corresponding normalized target exposure value. (Note that the method determines an updated pulse timing function 640 that will provide an exposure value having the same normalized shape as the target exposure function 605. Absolute exposure value can be matched by adjusting the total current provided to the light source.) an updated pulse time that satisfies this equation

This can be determined using well known numerical integration techniques. Updated pulse timing function 640 (S) for jth iteration^j) Vector corresponding to each pulse time:

completion test 645 is usedA determination is made whether a predetermined iteration termination criterion is met. In an exemplary embodiment, the updated pulse timing function 640 (S)^j) And the pulse timing function (S) for the previous iteration^(j-1)A comparison is made to determine if the results have converged. For example, the iteration termination criterion may be determined by determining the magnitude of the vector difference between two pulse timing functions and comparing it with a predetermined threshold ε_sA comparison was made to evaluate:

|s^j-s^(j-1)|＜ε_S(6)

in other variations, rather than determining the magnitude of the vector difference, the maximum difference may be determined for the elements of the vector difference. In this case, if there is a significant difference in pulse timing for one quantized exposure value, the iterative process will continue even if the total difference is small.

In another embodiment, the iteration termination criteria may include calculating a current iteration for the current iteration

And compares it with the normalized target exposure function 605

A comparison is made. For example, the iteration termination criterion may be determined by determining the magnitude of the vector difference between the exposure functions and comparing it to a predetermined threshold ε_eA comparison was made to evaluate:

wherein for the current iteration

Is determined by using the updated pulse timing function (S)^j) To integrate the light output function P for the current iteration^j _(t)And determining that:

the pulse timing function 610 that provides the specified target exposure function 605 may be a function of the printer configuration. For example, some printers may be configured to print at various in-track spatial resolutions (e.g., 600dpi or 1200 dpi). If the total print speed remains the same, this means that 1200dpi pixels must be printed in half the time as 600dpi pixels. As a result, the associated pulse time will also nominally be about half as long. This will typically have a significant effect on the shape of the optical output function 630 and will therefore require a corresponding re-optimization of the pulse timing function 610. Thus, in this case, the method of FIG. 8 may have to be applied to determine the appropriate pulse timing function 610 for each relevant printer configuration. Each resulting pulse timing function 610 may be stored and used when the printer is used in the corresponding configuration.

If the completion test 645 determines that the iteration termination criteria have been met, a store final pulse timing function step 650 is used to store the results of the final iteration in processor-accessible memory as the pulse timing function 610(S) for use in controlling the print head to print image data. Otherwise, another iteration is performed by again applying the determine light output function step 620 and the update pulse timing function step 635. It has been found that the process typically converges in 10-200 iterations.

Fig. 9A shows an example of an initial pulse timing function 615. The corresponding initial light output function 630 is shown in fig. 9B. The determine pulse timing function process 600 of fig. 8 is applied using this initial pulse timing function 615 to determine an updated pulse timing function 610 shown in fig. 9A, which updated pulse timing function 610 will provide the target exposure function 605 of fig. 6. The corresponding optimized light output function 632 is shown in fig. 9B. It can be seen that the optimized light output function 632 is quite different from the initial light output function 630. This indicates the dependence of the light output function on the shape of the pulse timing function.

Returning to the discussion of fig. 4, the pulse timing function 610 determined by the determined pulse timing function process 600 is used by a control light source exposure time step 550, which control light source exposure time step 550 is applied in the print head electronics 580 to control how long each of the individual light sources in the print head is activated in response to the corresponding quantized exposure value 540.

In an exemplary embodiment, the same pulse timing function 610 is used for all light sources in the linear print head. However, when different light sources are operated at the same current, there will generally be a difference between their light outputs. This may result in various artifacts in the printed image, such as streaks. To compensate for these artifacts, the current supplied to each light source may be adjusted using the control light source current step 550 to equalize the light output of the light sources. A calibration operation comprising the determine current control parameter process 700 may be performed to determine a set of current control parameters 710 that are used by the control light source current step 560 to control the current of each light source.

In some embodiments, determining current control parameters process 700 may determine current control parameters 710 by placing the print head in a test fixture (test fixture) that includes light sensors and measuring the light output of each light source. In this way, the current supplied to each light source may be adjusted until the light output from each light source is equalized to within a predetermined tolerance.

In an exemplary embodiment, a plurality of driver chips are used to control the light sources in the print head, where each driver chip controls an associated set of light sources. For example, the print head in an exemplary printing system includes a linear array of 17,280 light sources controlled by 90 driver chips, where each driver chip controls 17,280/90 ═ 192 light sources. In this case the print head is divided into 45 segments along its length. In each segment, one driver chip controls odd-numbered light sources, and a second driver chip controls even-numbered light sources.

In an exemplary configuration, current control parameter 710 includes a global current control value (V)_REF) Chip related current control value (C)_REF) And the source-related current control value (D)_REF) A collection of (a). Global current control value(V_REF) Is to set the total current level I supplied to all light sources in the print head_GThe parameter (c) of (c).

Chip dependent current control value

Can be represented by an array of control values (one for each driver chip) that are used to independently adjust the current provided by each driver chip:

C_REF＝[C₁，C₂，...C_m，...C_M](9)

wherein M is the number of driver chips, and C_mIs the chip dependent current control value of the mth driver chip. In an exemplary configuration, each C_mThe values are 4-bit integers ranging from 0-15 that specify gain adjustments in 3% increments. In this case, the chip dependent gain adjustment may be denoted as G_c，m＝0.03×(C_m-7)。

The source dependent current control value (D)_REF) Can be represented by an array of control values (one for each light source) which are used to independently adjust the current provided by each light source:

D_REF＝[D₁，D₂，...D_n，...D_n](10)

wherein N is the number of light sources, and D_nIs the source dependent current control value for the nth light source. In an exemplary configuration, each D_nThe values are 6-bit integers ranging from 0-63 that specify gain adjustments in 1% increments. In this case, the source-related gain adjustment may be denoted as G_d，n＝0.04×(D_n-31)。

The current supplied to each light source will be a global current as modified by the chip-related gain adjustment and the source-related gain adjustment. In the equation form, the current supplied to the nth light source (which is controlled by the mth driver chip) is given by:

fig. 10 illustrates a flow diagram of an exemplary embodiment of a determine current control parameter process 700, the determine current control parameter process 700 determining a current control parameter 710 based on an analysis of printed test targets. In this process, the print head is configured to control the set of parameters 715 using the initial current. The initial current control parameter 715 may be obtained in a variety of ways. For example, they may be a set of current control parameters determined using a test device that includes a light sensor and measures the light output for each light source, as previously described. Alternatively, they may be a set of current control parameters determined using a previous calibration procedure.

The print test target step 725 is used to print test target image data 720 of a test target 760 that includes one or more uniform patches. FIG. 11 illustrates an exemplary test target 760 that may be used in exemplary embodiments. The test target 760 comprises a collection of uniform patches 800 that span the width of the print head in a cross-track direction 810. Each uniform patch 800 is located at a different in-track location in the in-track direction 812. Each uniform patch 800 has a different density level ranging from a light uniform patch 802 to a darkest uniform patch 804. The test target 760 also includes a set of alignment marks 806 having known positions relative to the print head, which can be used to determine the alignment of the printed test target with the print head.

Typically, prior to printing the continuous tone digital image data of the test target 760, it is processed through a halftoning process to provide halftone image data. In an exemplary embodiment, the halftone process is a random halftone process. The use of a random halftoning process is advantageous because its characteristics are more homogeneous and moire artifacts do not occur all the time during image capture. The halftone image data is then printed using the process of fig. 4. Preferably, in determining the current control parameters 710, the gain correction values 515 are all set to unit element values (unity value) so that no gain correction is applied by applying the gain correction step 520.

The printed test target 730 produced by the print test target step 725 is next digitized using a scan test target step 735. The scan test target image step 735 uses the digital image capture system 440 (fig. 3) to provide a captured image 749 of the printed test target 730. In a preferred embodiment, the digital image capture system 440 is a digital camera system or an optical scanner system integrated into a digital printing system. In some configurations, the digital image capture system 440 is used to automatically capture an image of a printed test object 730 as it passes through the digital printing system.

The captured image is then analyzed 740 using an analyze captured image step 745 to determine an estimated light source dependent exposure error 750. FIG. 12 shows a flowchart of an exemplary process that may be used to perform the analyze captured image step 745. First, an align image step 900 is used to detect the position of the alignment marks 806 (FIG. 11) and remove any skew (skew) from the captured image 740. The determine light source location step 905 determines the location of each light source within the image that intersects the track based on the detected location of the alignment marks 806.

Then, determine illuminant correlation code values step 910 is used to determine an average code value within each uniform patch 800 for each illuminant. This is done by averaging the code values in the vertical columns within the uniform patch at the determined track-crossing location for the light source. FIG. 13 shows a graph 920 illustrating a sample set of curves showing scanner code values as a function of illuminant for a set of six uniform patches. (Note that the collection of light sources at either end of the head is outside the effective printing area of the printing system, so that the number of light sources in diagram 920 is less than the total number of light sources in the print head.)

Returning to the discussion of FIG. 12, the determine light source related exposure error step 915 is then used to determine a corresponding estimated light source related exposure error 750. In an exemplary embodiment, the digitized scanner code values are mapped to exposure values by applying a calibration curve 930 (such as the calibration curve 930 shown in FIG. 14). The calibration curve 930 may be determined by printing a patch with a known exposure and measuring the resulting code value in the scanned image. Note that the "exposure" value in fig. 14 and subsequent figures is the exposure time (in microseconds) for which the light source is activated. These values will be proportional to the actual exposure, which can be determined by multiplying these values by the power of the light source (which is approximately 180 picowatts).

To evaluate exposure errors, the measured exposure values may be smoothed (e.g., by fitting a spline function) against the illuminant function to determine a set of smoothed exposure values. The difference between the smooth and non-smooth functions will be an estimate of the exposure error for each light source. FIG. 15 shows a graph 940 showing estimated exposure error as a function of light source for one of the uniform patches 800 (FIG. 11).

Returning to the discussion of fig. 10, next an updated current control parameter 710 is determined using determine updated current control parameter step 755. In an exemplary embodiment, the exposure gain error is determined for each illuminant by combining the estimated exposure errors for each uniform patch 800. FIG. 16 is a graph 950 illustrating the estimated exposure error determined by six uniform patches 800 (FIG. 11) for two light sources. A linear function may be fitted to the points for each light source to provide an estimated gain error. In a preferred embodiment, the linear function is constrained to pass through the origin, and thus the slope of the resulting linear function is an estimate of the exposure gain error. A positive slope indicates that the light source provides too much exposure, while a negative slope is an indication that the light source provides too little exposure.

Fig. 17 shows a graph 960, said graph 960 showing an exemplary set of gain corrections determined for each light source. (in this figure, the x-axis has been scaled by the number of control chips across the print head.) these gain corrections can then be combined with the gain values associated with the initial current control parameters 715 (FIG. 10) to determine an updated set of gain adjustment values. The updated gain adjustment values are then used to determine a corresponding set of current control parameters 710.

In an exemplary embodiment, the global current control value (V) is not adjusted during this process_REF) And therefore uses the same value as in the initial current control parameter. But, instead, allLocal current control value (V)_REF) Is set to produce the desired maximum exposure level at the quantized exposure value 540 of EQ ═ 255. To determine a chip dependent current control value (C) for the updated current control parameter 710_REF) Is averaged and quantized to a chip-dependent current control value (C) associated with each control chip_m) The associated bin. Calculating an associated chip dependent gain adjustment for each control chip (e.g., using equation G)_c，m＝0.03×(C_m-7))) and subtracting the associated chip dependent gain adjustment from the gain adjustment value to determine a remaining gain adjustment value. The remaining gain adjustment value for each light source is quantized to a source-related current control value (D) that is available_n) The associated bin. Chip dependent current control value (C)_m) Is used to form a chip dependent current control value (C)_REF) And the associated current control value (D)_n) The source-related current control value (D) used to form the updated current control parameter 710_REF). The resulting graph of chip-related current control values is shown in fig. 962, and the resulting graph of source-related current control values is shown in fig. 964.

Once the updated current control parameters are determined 710, they are stored in a processor-accessible memory for use in printing subsequent digital image data. In some embodiments, the determine current control parameter process 700 of fig. 10 may be iteratively performed to further refine the gain correction, with the updated current control parameter 710 used as the initial current control parameter for the next iteration. For example, the determine current control parameter process 700 may be repeated until the determined light source related exposure errors 750 are less than the predetermined threshold.

Returning to the discussion of fig. 4, in the exemplary embodiment, determine current control parameters process 700 is performed at the factory to determine a set of current control parameters 710, which current control parameters 710 are stored in a printing system when the printing system is shipped to a customer. In general, the gain correction process 590 will be determined to be used in actual applications to make corrections for any streak artifacts (e.g., due to degradation of the print head or other components such as the charging subsystem 210 or the development subsystem 225) present in the printed image. However, the determine current control parameter process 700 may also be performed in a practical application on an as-needed basis. For example, the determine current control parameters process 700 may be performed when a new printhead is installed or when a service technician observes that performance degradation has occurred. The gain correction value 515 and quantization LUT 525 are typically set to nominal values when the determine current control parameter process 700 is performed. After determining the updated current control parameters 710, a determine gain correction process 590 may be performed to correct for any remaining error that may remain.

As previously discussed with respect to fig. 8, it has been found that depending on the printer configuration, a different pulse timing function 610 may be required to provide the defined target exposure function 605. In particular, different pulse timing functions 610 will typically be required for different print modes having different line print times (i.e., the time it takes for the print head to print a line of image data). The line print time will define the maximum pulse time available for the pulse timing function 610, which in turn will have a significant impact on the light output function 630. Aspects of the print mode that would have a direct impact on line print time would be in-track printer resolution (i.e., the number of printed lines/inch printed in the in-track direction, and print speed (i.e., the number of pages/minute printed) — for example, doubling the in-track printer resolution or doubling the print speed would have the effect of reducing the line print time to 1/2 times.

In an exemplary embodiment, the printing system is adapted to print in a set of different printing modes having the following characteristics:

table 1: exemplary printing modes

Each of these five print modes has a different line print time and therefore requires a different pulse timing function 610 in order to provide the defined target exposure function 605.

In some embodiments, a user interface (e.g., in the pre-processing module 305) may be provided that enables a user to select different printing modes on a job-by-job basis. Thus, in a preferred embodiment, a mechanism is provided to select the appropriate pulse timing function to use with each print job. For example, FIG. 18 shows an exemplary user interface 970 having user-selectable options for specifying aspects of a print mode. In this example, the user selections for specifying the print mode include a resolution selection 972 for selecting a printer resolution within the track and a print speed selection 974 for selecting a print speed. While resolution selection 972 and print speed selection 974 are shown in numerical selection, in other embodiments, text labels may be used. For example, a 1200 line/inch printer resolution may be labeled "MaxHD" and a 600 line/inch printer resolution may be labeled "classic".

In an exemplary embodiment, only certain combinations of printer resolution and print speed may be allowable. For example, if a 1200 line/inch printer resolution is selected, the print speed selection may be limited to 82 pages/minute or 100 pages/minute, such that a 120 page/minute selection is obscured. The user interface 970 may also include other selections for controlling other attributes of the print job (e.g., number of copies to print, pages to print, type of halftone to apply, etc.).

Fig. 19 shows a processing path that includes a print engine 400, which print engine 400 is adapted to generate a printed image from image data 350 using a plurality of print modes. This processing path represents an extension to the processing path described in the aforementioned U.S. patent application serial No. 15/135,607 to Kuo et al. In this configuration, the pre-processing system 305 provides image data 350 and associated metadata 360. In a preferred embodiment, metadata 360 includes print mode metadata that provides an indication of the print mode to be used to print image data 350. In an exemplary configuration, the print mode metadata may be an integer that specifies a print mode from a predetermined set of print modes (such as those shown in table 1). In other configurations, the print mode metadata may include various parameters that specify various attributes of the print mode, such as printer resolution parameters and print speed parameters specified using the user interface 970 (fig. 18). Metadata 360 may also include other parameters, such as image resolution metadata and halftone status metadata.

Print engine 400 receives image data 350 and metadata 360 using a suitable data interface 405 (e.g., an ethernet interface). The print engine includes a metadata interpreter 410 that analyzes metadata 360 to provide appropriate control signals 415 for various aspects of print engine 400. In an exemplary configuration, the control signals include a resolution modification control signal for controlling the resolution modification processor 420 and a halftone algorithm control signal for controlling the halftone processor 425, as described in the aforementioned U.S. patent application serial No. 15/135,607 to Kuo et al. A resolution modification processor 420 and a halftone processor 425 are used to process the image data 350 to provide processed image data 428, which is in a suitable state to be printed by a printer module 435. The printer module controller 430 then controls the printer module 435 to print the processed image data 428 to produce a printed image 450.

In a preferred embodiment, control signal 415 includes a pulse timing function selection parameter for selecting pulse timing function 610 (FIG. 8). Metadata interpreter 410 determines pulse timing function selection parameters in response to metadata 360, which metadata 360 specifies a print mode to be used to print image data 350. In an exemplary configuration, the print mode metadata includes an in-track printer resolution parameter specifying an in-track printer resolution (e.g., 600 or 1200 lines/inch) and a print speed parameter specifying a print speed (e.g., 83, 100, or 120 pages/minute). As illustrated in table 1, a set of print modes may be defined corresponding to permissible combinations of these parameters, where each print mode has an associated row print time. In addition to selecting the pulse timing function 610, the control signals 415 determined from the print mode metadata may also include parameters for controlling other aspects of the printer module 435. For example, the control signals 415 may be used to select a set of current control parameters 710 (FIG. 4) appropriate for the selected print mode and to adjust the speed of various motors to control the print speed.

The pulse timing function 610 for each print mode is preferably predetermined using the method of fig. 8 for the line print time associated with each supported print mode and stored in a processor-accessible digital memory 460. Fig. 20 shows an exemplary set of pulse timing functions 610 corresponding to the print modes in table 1. The pulse timing function selection parameters included in the control signals 415 are used to select an appropriate pulse timing function 610 for the selected print mode, which appropriate pulse timing function 610 is then used by the printer module controller 430 to control the print heads in the printer module 435.

Partial list

31 printing module

32 printing module

33 printing module

34 printing module

35 printing module

38 printing images

39 fusing images

40 supply unit

42 receiver

42a receiver

42b receiver

50 transfer subsystem

60 fusion cage module

62 fusion roller

64 pressure roller

66 fusion nip

68 Release fluid application substation

69 output tray

70 finishing device

81 transport network

86 cleaning station

99 Logic and Control Unit (LCU)

100 printing machine

111 imaging member

112 intermediate transfer member

113 transfer support member

201 first transfer nip

202 second transfer nip

206 photosensitive body

210 charging subsystem

211 instrument

212 meter

213 grid

216 surface

220 exposure subsystem

225 developing station

226 toning shell

227 magnetic core

240 power supply

300 page description file

305 preprocessing system

310 Digital Front End (DFE)

315 Raster Image Processor (RIP)

320 color transform processor

325 compression processor

330 image processing module

335 decompression processor

340 halftone processor

345 image enhancement processor

350 image data

360 metadata

400 print engine

405 data interface

410 metadata interpreter

415 control signal

420 resolution modification processor

425 halftone processor

428 processed image data

430 printer module controller

435 printing press module

440 image capture system

450 printed image

460 digital memory

500 pixel code value

505 calibration LUT

510 apply the calibration LUT step

515 gain correction value

520 apply a gain correction step

525 quantization LUT

530 quantization step

540 quantized Exposure values

550 step of controlling light source exposure time

560 controlling the light source Current step

570 data processing electronic device

580 print head electronics

590 determine gain correction procedure

600 determining a pulse timing function procedure

605 target exposure function

610 pulse timing function

615 initial pulse timing function

620 determining a light output function

625 light output model

630 light output function

632 optimized light output function

635 update pulse timing function step

640 updated pulse timing function

645 finish testing

650 store the final pulse timing function

660 master clock signal

670 exposure clock signal

680 light source activation function

700 determining a current control parameter process

710 current control parameter

715 initial current control parameter

720 test target image data

725 printing test target step

730 print the test target

735 Scan test target step

740 captured image

745 step of analyzing the captured image

750 light source dependent exposure error

755 determining updated Current control parameters step

760 test target

800 even patch

802 brightest uniform patch

804 darkest uniform patch

806 alignment marks

810 cross direction of the track

812 in-track direction

900 alignment image step

905 determining the position of the light source

910 determining a value of a correlation code for a light source

915 determining light source dependent exposure error

920 figure

930 calibration curve

940 scheme

950 scheme

960 (diagram)

962 scheme

964 fig. 4

970 user interface

972 resolution selection

974 print speed selection

Claims (31)

1. A method for controlling a print head in a digital printing system, the print head comprising an array of light sources for exposing a photosensitive medium, the method comprising:

a) providing a target exposure function that gives a target exposure to be provided by the light source as a function of an integer pulse count;

b) providing an initial pulse timing function defining an exposure time as a function of a pulse count;

c) determining a light output function for the light source in response to the pulse timing function, wherein the light output function gives the light output of the light source as a function of exposure time;

d) updating the pulse timing function in response to the light output function and the target exposure function;

e) repeating steps c) -d) until a predetermined iteration termination criterion is met; and

f) controlling the print head using the pulse timing function, wherein each light source is activated for a pulse count corresponding to a pixel code value of an associated image pixel.

2. The method of claim 1, wherein the iteration termination criterion is that a difference between the updated pulse timing function and the pulse timing function for a previous iteration does not exceed a predetermined threshold.

3. The method of claim 2, wherein the difference between the updated pulse timing function and the pulse timing function for the previous iteration is a magnitude of a vector difference between a vector representing the updated pulse timing function and a vector representing the pulse timing function for the previous iteration.

4. The method of claim 1, wherein the iteration termination criterion is that a difference between the target exposure function and an actual exposure function does not exceed a predetermined threshold, wherein the actual exposure function is the actual exposure provided by the light source as a function of pulse count and is determined in response to the updated pulse timing function and the updated light output function.

5. The method of claim 4, wherein the difference between the target exposure function and an actual exposure function is a magnitude of a vector difference between a vector representing the target exposure function and a vector representing the actual exposure function.

6. The method of claim 1, wherein the light output function is determined using a predetermined parametric light output model.

7. The method of claim 6, wherein the parametric light output model has the form:

wherein

Is the time difference between two successive exposure clock signal pulses at time t, and a is a constant.

8. The method of claim 1, wherein the light output function is determined by controlling one or more light sources in the print head using the pulse timing function and measuring the light output of the one or more light sources as a function of exposure time using a light detector.

9. The method of claim 1, wherein different pulse timing functions are determined for a plurality of different printer configurations.

10. The method of claim 9, wherein the different printer configurations include a first printer configuration having a first in-orbit spatial resolution and a second printer configuration having a second in-orbit spatial resolution.

11. The method of claim 9, wherein the different printer configurations include a first printer configuration having a first print speed and a second printer configuration having a second print speed.

12. The method of claim 1, wherein the target exposure function is modified in response to a printer calibration process, and wherein a new pulse timing function is determined corresponding to the modified target exposure function.

13. The method of claim 1, wherein the pulse timing function specifies a number of master clock pulses by which the light source should be activated as a function of the pixel code value.

14. The method of claim 1, wherein an exposure clock signal is formed comprising exposure clock pulses corresponding to each pixel code value, wherein the pulse timing for each exposure clock pulse is specified by the pulse timing function.

15. A print engine adapted to print image data, comprising:

a printer module for printing image data in a plurality of different print modes, the printer module comprising a printhead for exposing a photosensitive medium, wherein each print mode has an associated line print time, the line print time being the amount of time it takes for the printhead to print a line of image data;

a data interface to receive image data and associated metadata for a print job from a pre-processing system, wherein the metadata includes print mode metadata that provides an indication of the print mode to be used to print the image data;

a digital memory storing a plurality of pulse timing functions, each pulse timing function defining an exposure time as a function of an integer pulse count, wherein each pulse timing function corresponds to one of the line print times associated with the plurality of print modes;

a metadata interpreter that interprets the metadata and determines the printing mode to be used for printing the image data; and

a printer module controller that controls the printer module to print the image data in response to the determined print mode, wherein the printer module controller controls the print head using the pulse timing function corresponding to the line print time associated with the print mode, each light source being activated for a pulse count corresponding to a pixel code value of an associated image pixel of the image data.

16. The method of claim 15, wherein the pulse timing function associated with each of the line printing times is determined to provide the same target exposure function that defines a target exposure to be provided by the light source as a function of the pulse count.

17. The method of claim 16, wherein the pulse timing function is determined by a process comprising:

a) providing an initial pulse timing function for a particular line print time, the initial pulse timing function defining an exposure time as a function of a pulse count;

b) determining a light output function for the light source in response to the pulse timing function, wherein the light output function gives the light output of the light source as a function of exposure time;

c) updating the pulse timing function in response to the light output function and the target exposure function;

d) repeating steps b) -c) until a predetermined iteration termination criterion is met; and

e) storing the pulse timing function in the digital memory for use by the particular row print time.

18. The method of claim 17, wherein the iteration termination criterion is that a difference between the updated pulse timing function and the pulse timing function for a previous iteration does not exceed a predetermined threshold.

19. The method of claim 18, wherein the difference between the updated pulse timing function and the pulse timing function for the previous iteration is a magnitude of a vector difference between a vector representing the updated pulse timing function and a vector representing the pulse timing function for the previous iteration.

20. The method of claim 17, wherein the iteration termination criterion is that a difference between the target exposure function and an actual exposure function does not exceed a predetermined threshold, wherein the actual exposure function is the actual exposure provided by the light source as a function of pulse count and is determined in response to the updated pulse timing function and the updated light output function.

21. The method of claim 20, wherein the difference between the target exposure function and an actual exposure function is a magnitude of a vector difference between a vector representing the target exposure function and a vector representing the actual exposure function.

22. The method of claim 17, wherein the light output function is determined using a predetermined parametric light output model.

23. The method of claim 22, wherein the parametric light output model has the form:

wherein

Is the time difference between two successive exposure clock signal pulses at time t, and a is a constant.

24. The method of claim 17, wherein the target exposure function is modified in response to a printer calibration process, and wherein a new pulse timing function is determined corresponding to the modified target exposure function.

25. The method of claim 15, wherein the pulse timing function specifies a number of master clock pulses by which the light source should be activated as a function of the pixel code value.

26. The method of claim 15, wherein an exposure clock signal is formed comprising exposure clock pulses corresponding to each pixel code value, wherein the pulse timing for each exposure clock pulse is specified by the pulse timing function.

27. The method of claim 15, wherein the print mode metadata associated with the image data is defined in response to one or more user-selectable options in a user interface.

28. The method of claim 15, wherein the print mode metadata comprises an in-track printer resolution parameter.

29. The method of claim 15, wherein the print mode metadata comprises a print speed parameter.

30. The method of claim 15, wherein the data interface receives image data and associated metadata for a plurality of print jobs, and wherein the print mode metadata associated with the successive print jobs indicates that the successive print jobs are to be printed in different print modes having different associated row print times, such that the printer module controller prints the image data for the successive print jobs using different pulse timing functions.

31. The method of claim 15, wherein the print head comprises a linear array of light sources.

Images