GB2579653A

GB2579653A - Improvements in or relating to laser marking

Info

Publication number: GB2579653A
Application number: GB1820090.7A
Authority: GB
Inventors: L Phillips Tristan; Cridland John
Original assignee: DataLase Ltd
Current assignee: DataLase Ltd
Priority date: 2018-12-10
Filing date: 2018-12-10
Publication date: 2020-07-01
Also published as: GB201820090D0; WO2020120934A2; WO2020120934A3

Abstract

There is provided a method of calibrating a laser marking system 1 of the type operable to mark a substrate moving relative to the marking system at a substrate speed. The marking system includes a plurality of emitters 10, each emitter operable to emit light on to a substrate for marking. The emitted light is controlled in response to a variable marking setting. The method includes the steps of activating the emitters to mark a series of block images at a range of different marking settings and a range of different substrate speeds, determining the average optical density of each marked block, identifying the marking settings that correspond most closely to a selected optical density value for each substrate speed, and thereby determining a calibration relationship between marking setting and substrate speed. An apparatus and system operable to this method are also provided. There is also provided a method of determining fixed pulse duration and a laser marking system characterised by the emitter pulse duration being selected from a predetermined range of values defined by 1.2(d0/V) ≤ tp ≤ 1.5(d0/V); where tp is the emitter pulse duration, V is the substrate speed and d0 is the emitter spot diameter.

Description

IMPROVEMENTS IN OR RELATING TO LABEL MARKING Technical Field of the Invention The present invention relates to improvements in or relating to laser marking. In particular, the present invention relates to calibration of a laser marking system comprising a multi-fibre array.

Background to the Invention

Traditionally, labels for products and/or packaging have been pre-printed in bulk and applied to the products/packaging as required. The drawback to such actions is that the information on pre-printed labels is fixed. This does not provide flexibility to customise labels or to adapt labels readily at short notice.

One solution to this issue is to digitally print labels on demand using inkjet printers. This requires the supply of liquid inks, solvents and the like at the point of printing. This can be undesirable in many sectors, such as food and beverages, particularly if the inks/solvents provide a contamination risk.

As an alternative, labels may be produced using laser marking. Various forms of laser marking apparatus are known and are used in conjunction with label substrates that comprise colour change material. Upon controlled exposure to laser light from the marking apparatus, portions of the substrate change colour forming a desired image. The image may be monotone or coloured depending on the material and/or the nature of the exposure. The image may comprise text, numbers, codes or the like as well as pictographic elements.

In some implementations, the substrate is provided in the form of an extended tape on a supply reel. The tape is unwound from the supply reel via one or more rollers and passes a laser marking head. This enables successive like or custom labels to be marked on the substrate. Subsequently, the tape can be wound on to a storage reel via one or more further rollers for later application to products/packaging or individual labels may be cut from the tape and applied directly to products/packaging In other implementations a suitable substrate is provided directly on the product/packaging and this is marked by laser exposure as the product/packaging passes the laser making head.

The laser making head may comprise a single laser or a multi source laser array. The benefit of the multi-emitter array is that the imaging speed is independent of image content. In a particular configuration the array comprises a ID or 2D array of fibres coupled to a plurality of laser diodes illuminates the target as it moves in front of the imaging head. The individual laser diodes are modulated based on the image requirements. This results in colour change where the spot emitted by each fibre is incident upon the substrate, forming an image from an array of fine dots or pixels generated by each spot.

In order to control image formation, it is necessary to synchronise emitter activation with substrate motion. This can involve the use of a sensor to detect substrate motion and thus determine the substrate speed relative to the marking head. In many cases, the sensor may produce an encoder type output which is then used to control the activation of the emitters. This encoder output may comprise an encoder count parameter being a number of encoder counts which substantially correspond to the spot diameter, do. In alternative implementations, count parameters corresponding to other distances larger or smaller than the spot diameter may be used. Typically, the encoder counts would correspond to a resolution of a fraction of the spot diameter do and for good alignment between image produced by displaced heads, each encoder count should be <do/10 and more typically <do/20.

It is not uncommon for the substrate speed to vary during the course of marking operation. In one example, the substrate speed will evidently vary during start up (from zero to a selected operating substrate speed) and shutdown (from the selected operating substrate speed back to zero. During such variations in substrate speed, the density of the marked image will vary. Intuitively one might expect the density to decrease as the substrate speed increases.

The average optical density of the image depends on individual dot density, dot or line narrowing and dot 'smearing' due to elongation as product motion is increased. The spaces between dots, in the direction of motion, may get smaller, the region between adjacent transverse dots may become less dense and get wider, the individual dot or line optical density may reduce and therefore average optical density of the image may increase or decrease with substrate speed. The result of these interactions is that the appearance of an image may get lighter or darker as substrate speed changes, the particular effects depend upon the specific marking setting (as defined by parameters such as pulse duration, longitudinal pitch and resolution or spot size etc). If the variation in image optical density with respect to substrate speed is too great, marked substrates may be unusable.

It is therefore an object of the present invention to provide a method and apparatus for calibrating a laser marking system that at least partially alleviates or overcomes the above problems.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of calibrating a laser marking system of the type operable to mark a substrate moving relative to the marking system at a substrate speed, the marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the emitted light controlled in response to a variable marking setting, the method comprising the steps of activating the emitters to mark a series of block images at a range of different marking settings and a range of different substrate speeds; determining the average optical density of each marked block; identifying the marking settings that correspond most closely to a selected optical density value for each substrate speed; and thereby determining a calibration relationship between marking setting and substrate speed.

This method provides for ready calibration of a laser marking system.

The selected optical density value may be a value equivalent to the optical density achieved at the highest substrate speed. In another example, the selected optical density value may be a value toward the centre of the range determined average optical densities the marked blocks.

The block images may be of any suitable shape or size. Typically, the shape is a square or rectangle. In a preferred embodiment the block images are each of the same shape and of substantially uniform target optical density.

The marking setting may be varied in response to a variation in the target optical density. The marking setting may be defined by one or more different parameters. In particular, the making setting may be defined by any one or more of emitter power; emitter spot diameter, emitter pulse duration, emitter energy, emitter power density, emitter energy density. The emitter power parameter may be expressed as a greyscale value, say 0-255 for an 8 bit system. In other systems, the pulse duration may be expressed as a greyscale value. In such cases, the greyscale value may be used as a scale factor to scale within an operating range of pulse durations.

In some implementations, whilst the marking setting may be defined by multiple parameters, variation of the marking setting in response to a variation in the target optical density is achieved by varying a subset of marking setting parameters and maintaining other marking setting parameters constant. In some implementations, variation of the marking setting in response to a variation in the target optical density may be achieved by varying two marking setting parameters. For example, this may involve varying emitter power and emitter pulse duration. In such implementations, the method may involve varying a first marking setting parameter whilst a second marking setting parameter is fixed and subsequently repeating the variation of the first marking setting parameter with the second marking setting parameter at a series of other fixed values.

In other implementations variation of the marking setting in response to a variation in the target optical density may be achieved by varying a single marking setting parameter. In such implementations, the method may involve the step of determining an optimum value of one or more of the unvaried marking setting parameters. In one such implementation, the sole varied marking setting parameter is pulse duration. In another such implementation the sole varied marking setting parameter is emitter power.

The range of different marking settings selected may cover substantially a MI range of expected marking settings. Sampled values may be evenly spaced through the range. Alternatively sampled values may be more closely separated in more commonly expected operational ranges. This may include covering a range of values of the or each varied marking setting parameter from an expected minimum operational value to an expected maximum operational value. Additionally or alternatively, the range of different marking settings may be selected so as to correspond to a full range of achievable target optical densities.

The marking settings and/or substrate speeds identified are those one or more marking settings and/or substrate speeds for which the determined optical density value is closest to matching said selected optical density value.

Determining a calibration relationship may involve the steps of performing an interpolation between the closest pair of marking settings for marked block images with determined optical densities that correspond most closely to the selected optical density value for each substrate speed. In a preferred embodiment, the closest pair of marking settings include one marking setting for an image block with an optical density determined to be greater than the selected optical density value and one marking setting for an image block with an optical density determined to be less than the selected optical density value. The interpolation may be a linear interpolation.

Determining a calibration relationship may involve carrying out regression analysis to calculate a best fit function relating variation in marking setting to substrate speed. The regression analysis may be carried out in respect of the identified marking settings. The regression analysis may be carried out in respect of a single selected optical density value or may be carried out in respect of multiple selected optical density values.

Where variation of the marking setting involves varying more than one marking setting parameter, the method may involve performing independent regression analysis in respect of variation in each marking setting parameter. Alternatively, the method may involve performing independent regression analysis in respect of one marking setting parameter at a series of fixed values of the remaining marking setting parameters. In such instances, the method may include the step of carrying out regression analysis in respect of the values of the calculated regression coefficients for the first marking setting parameter with respect to variation of a second marking setting parameter. Additionally or alternatively, the method may include the further step of averaging regression coefficients or interpolating between regression coefficients calculated under each regression analysis.

Where variation of the marking setting involves varying only a single marking setting parameter, the method may involve performing regression analysis in respect of only the varied marking setting parameter. In such implementations, the method may include the step of selecting a fixed value of one or more of the remaining marking setting parameters. The selection may be made from a predetermined range of marking setting parameter values. In one implementation, where the fixed value is the emitter pulse duration, the predetermined range of values may be defined by do do 1.2 -V < t < 1.5.

P V

where tp is the emitter pulse duration, V is the substrate speed and do is the emitter spot diameter.

In instances where the other marking setting parameters may be varied, the upper and/or lower limits may be varied as appropriate. In one example, the upper limit may be varied to 1.8.-v. For instance, this would allow longer pulse durations at lower average power at lower substrate speeds.

Alternatively, the method may include the step of determining a fixed value of one or more of the remaining marking setting parameters. The fixed value may be determined by selecting two or more candidate values from within the predetermined range. In such implementations, the fixed value may be determined by marking block images at different substrate speeds using said candidate values of the marking setting parameter; and identifying the candidate value that corresponds most closely to a selected optical density value for all substrate speeds. In such embodiments, the method may involve the additional step of selecting the identified candidate value. Alternatively, the method may include the step of interpolating between two or more candidate values in order to identify an intermediate value for selection.

The regression analysis may be linear. In alternative embodiments, the regression analysis may be non-linear. In such cases, the regression analysis may involve a polynomial fit. The polynomial can be of any suitable order. In some implementations, the polynomial may be a second, third or fourth order polynomial.

The range of different substrate speeds selected may cover substantially a full range of expected operating speeds. Preferably, this includes operating substrate speeds expected for normal steady operation and operating substrate speeds expected during ramping up or shutting down of marking operation. In one example, where a maximum operating substrate speed is, say, 2.0 m/s, the range of substrate speeds used in the method may be from, say, 0.1 m/s to 2.0 m/s.

In one embodiment, determination of optical density of the marked blocks may be achieved by use of a spectrophotometer or an optical densitometer. In an alternative embodiment, determination of optical density of the marked blocks may be achieved by: capturing an image of each marked block; and processing the captured images to determine the average optical density of each marked block.

The method may include the additional steps of storing details of the determined calibration relationship in a calibration data store. The method may include the step of using the determined calibration relationship to generate a look up table relating marking settings and substrate speeds. The look up table may be stored in the calibration data store.

In addition to the above, the method may include carrying out calibration in respect of variation between individual emitters. Preferably, such calibration of individual emitters should be carried out before applying the present calibration method. The calibration details for individual emitters may be stored in the calibration data store. Alternatively, calibration details for individual emitters may be stored in a dedicated emitter calibration data store.

The emitters may comprise lasers or laser diodes. In some embodiments, the emitters may comprise individually addressable laser diode arrays (IALDA) or individually addressable laser arrays (IALA). In some embodiments, the emitters may comprise an emitting end of a marking fibre wherein an input end of the marking fibre is coupled to a laser diode. The emitters may be configured as a one dimensional or two dimensional array. Where the array is a one dimensional array, it may be a simple linear array or may be a staggered array. In particular, the emitters may collectively form a marking head.

The emitters may be operable to emit light with any suitable wavelength, including but not limited to visible or near infrared (NIR) wavelengths. Generally, for marking applications, wavelengths in the range 200nm to 20000nm might be suitable. In some embodiments, the emitters are operable to emit light with wavelengths in the ranges: 390 to 460nm, 500 to 550nm, 620 to 660nm, 900nm to 1100nm and 1400 to 1600nm.

The substrate may comprise a label. The label may be mounted on a product or packaging before marking. Alternatively, the label may be provided on a reel of label substrate for marking before subsequent application to a product or packaging. The substrate may comprise an extended tape.

The substrate may comprise a colour change material operable to change colour in response to illumination by the emitters. The colour change material may comprise substances including but not limited to any of a metal oxyanion, a leuco dye, a diacetylene, a charge transfer agent or the like. The metal oxyanion may be a molybdate. In particular, the molybdate may be ammonium octamolybdate (AOM). The colour change material may further comprise an acid generating agent. The acid generating agent may comprise thermal acid generators (TAG) or photo-acid generators (PAG). In one embodiment, the acid generating agent may be an amine salt of an organoboron or an organosilicon complex. In particular, the amine salt of an organoboron or an organosilicon complex may be tributyl ammonium borodisalicylate.

The substrate may comprise an NIR (near infrared) absorber material. The NIR absorber material may be operable to facilitate the transfer of energy from an NIR laser illumination means to the colour change material. The NW absorber material may comprise substances including but not limited to any of Indium Tin Oxide (ITO), non-stoichiometric reduced ITO, Copper Hydroxy Phosphate (CT-IP), Tungsten Oxides (WOx), doped WOx, non-stochiometric doped WOx and organic NIR absorbing molecules such as copper pthalocyanines or the like.

According to a second aspect of the present invention, there is provided a calibration apparatus for a laser marking system of the type operable to mark a substrate moving relative to the marking system at a substrate speed, the marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the emitted light controlled in response to a variable marking setting wherein the laser marking system is operable to mark a series of block images at a range of different marking settings and a range of different substrate speeds; and wherein the calibration apparatus comprises: a sensor operable to determine the average optical density of each marked block; and a calibration processer operable to identify the marking settings that correspond most closely to a selected optical density value for each substrate speed and thereby determine a relationship between marking setting and substrate speed.

The calibration apparatus may additionally comprise a calibration data store operable to store data corresponding to the determined relationship between marking setting and substrate speed for at least the selected optical density.

According to a third aspect of the present invention, there is provided a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a calibration data store and wherein the calibration data store is populated using the method of the first aspect of the present invention or the apparatus of the second aspect of the present invention.

The apparatus of the second and the system of the third aspects of the present invention may incorporate any or all features of the method of the first aspect of the present invention as required or as desired.

According to a fourth aspect of the present invention, there is provided a method of determining fixed pulse duration for use in a laser marking system of the type operable to mark a substrate moving relative to the marking system at a substrate speed, the marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the emitted light controlled in response to a variable marking setting, the method comprising the steps of: selecting a fixed value of the emitter pulse duration from a predetermined range of values defined by: do 1.2.-< t < 1.5.-V P V where tp is the emitter pulse duration, V is the substrate speed and do is the emitter spot diameter.

In instances where the other marking setting parameters may be varied, the upper and/or lower limits may be varied as appropriate. In one example, the upper limit may be varied to 1.8. If; The method may include the step of determining a fixed value of the emitter pulse duration by selecting two or more candidate values from within the predetermined range. In such implementations, the fixed value may be determined by marking block images at different substrate speeds using said candidate values of the emitter pulse duration; and identifying the candidate value that corresponds most closely to a selected optical density value for all substrate speeds. In such embodiments, the method may involve the additional step of selecting the identified candidate value. Alternatively, the method may include the step of interpolating between two or more candidate values in order to identify an intermediate value for selection According to a fifth aspect of the present invention, there is provided a laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a calibration data store and wherein the calibration data store, wherein the emitters are operable to emit pulses of a fixed duration, the emitter pulse duration being selected from a predetermined range of values defined by: do 1.2.-< t < 1.5.-

V -P V

In instances where the other marking setting parameters may be varied, the upper and/or lower limits may be varied as appropriate. In one example, the upper limit do may be varied to 1.8. -. I1

The method of the fourth and apparatus of the fifth aspects of the present invention may incorporate any or all features of the method of the first three aspects of the present invention as required or as desired.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure I is a schematic block diagram of a laser marking system according to the present invention; Figure 2 is a schematic block diagram of a laser marking system according to the present inventions where the emitters are an individually addressed laser diode array; Figure 3 is a schematic illustration of a series of calibration blocks marked in the method of the present invention; Figure 4 is a plot of determined optical density against substrate speed for image blocks marked at various different marking settings (variable emitter pulse duration (tp) and fixed emitter power parameter (GS) value), Figure 5 is a plot of pulse durations required to compensate for particular substrate speeds at different emitter power parameters as determined from figure 4 and a linear fit determined by regression analysis; Figure 6 is a plot of regression coefficients corresponding to a linear regression analysis of figure 5; Figure 7 is a plot of pulse durations required to compensate for particular substrate speeds as calculated from the regression of figure 6; Figure 8 is a plot of pulse durations required to compensate for particular substrate speeds at different emitter power parameters as determined from figure 4 and a 3"1 order polynomial fit determined by regression analysis; Figure 9 is a plot of regression coefficients corresponding to a polynomial regression analysis of figure 8; Figure 10 is a plot of pulse durations required to compensate for particular substrate speeds as calculated from the regression of figure 9; Figure 11 is a plot of pulse durations required to compensate for particular substrate speeds as calculated by determining the regression coefficients by interpolation between adjacent data pairs; Figure 12 is a plot of determined optical density against substrate speed for image blocks marked at various different marking settings (fixed emitter pulse duration (tp) and variable emitter power parameter (GS) value) and two polynomial fits determined by regression analysis; Figure 13 is a plot of power parameter values required to compensate for particular substrate speeds as calculated from the regressions of figure 12; and Figure 14 is a plot of determined optical density against substrate speed for image blocks marked at various different marking settings (variable emitter pulse duration (tp) and fixed emitter power parameter (GS) value) and two polynomial fits determined by regression analysis.

Turning now to figure 1, a multisource laser diode fibre array imaging system 1 comprises a marking head 10 in the form of a linear fibre array and a plurality of laser diodes 21-26. Each laser diode 21-26 is provided with a corresponding emission element 31-36. The emission elements 31-36 provide a coupling between the output of the laser diodes 21-26 and the input ends of corresponding fibres 11-16 making up the array. The emitting ends of the fibres 11-16 are maintained in position within the emitting head 10 by suitable formations. The skilled man will appreciate that the particular layout of figure 1 is a simple example for the purposes of explanation. In practice, the system 1 may have many more than six laser diodes and the emitting ends of the corresponding fibres may be arranged in formations other than a linear one dimensional array, including but not limited to two dimensional arrays and skewed or staggered arrays. In use, a substrate 2 is positioned in front of the marking head 10, the substrate 2 comprising a colour change material that changes colour in response to exposure to laser light. Suitable modulation of the output of the laser diodes 21-26 in combination with relative motion between the marking head 10 and the substrate 2 enables a pattern, image, text or the like to be marked on the substrate.

The laser diodes 21-26 are selected to have an output wavelength that is effective for initiating colour change in the substrate 2. Examples of suitable wavelengths range form 200-20000nm. More typically wavelengths in the range 390 to 460nm, 500 to 550nm, 620 to 660nm, 900-1100nm and 1400 to 1600nm may be utilised.

The emission elements 31-36 may be formed integrally with the laser diodes 21-26 or may be a separate component. Typically, the emission elements 31-36 comprise optical fibres, ferrules or optical fibre connectors.

A substrate motion sensor 29 is operable to detect motion of substrate 2 and thus determine the substrate speed relative to the making head 10. This can enable image formation, by synchronising activation of laser diodes 21-26 with substrate motion.

The substrate motion sensor 29 may produce an encoder type output, for instance an encoder count parameter being a number of encoder counts which substantially correspond to the spot diameter of emitted light incident on substrate 2. The encoder counts would typically correspond to a resolution of a fraction of the spot diameter do. For good alignment between image marked by displaced marking heads 10, each encoder count should be <d0/10 or, in preferred embodiments, <do/20.

In an alternative embodiment as illustrated in figure 2, the marking head 10 may comprise one or more individually addressable laser diode arrays (IALDA) 7. As in this example, the IALDA 7 may comprise an array of laser diodes 21-26. In addition, in such embodiments, the marking head 10 comprises a beam conditioning optic 8.

Typically, the beam conditioning optic may be a fast axis correction optical element.

The marking head 10 may additionally comprise an imaging lens assembly 9 to further correct and focus the emitters 21-26 onto the substrate 2.

The system further comprises a control unit 3. The control unit 3 is operable to control the output of each diode 21-26 in response to a marking setting. In this context the marking setting is defined by reference to one or more of emitter power; emitter pulse duration (tp); emitter spot diameter (do); emitter duty cycle or the like. The power parameter for each emitter 21-26 is related to the power input signal applied to the emitter and may be defined as target image greyscale value (GS).

The control unit 3 is operable to activate the emitters in response to the substrate motion sensor 29. This enables a desired image to be formed on the substrate 2 as it moves relative to the marking head 10. In some embodiments, the control unit 3 is operable to control the substrate speed V relative to the marking head 10. In other embodiments, substrate speed V is controlled by a production line controller or motion system controller.

For calibration, the control unit 3 is operable to control the diodes 21-26 so as to mark a series of calibration blocks 101-106 on the substrate 2, as shown on figure 3. The calibration blocks 101-106 are uniform block images marked using a different marking setting and/or a different substrate speed V. In the examples shown in figure 3, for simplicity, there are six separate block images 101-106, at different target optical densities. In this particular example, as will be discussed in more detail below, each block image 101-106 is marked at the same substrate speed V using a common emitter power parameter but each block 101-106 is marked at a different emitter pulse duration.. The skilled man will however appreciate that more block images may be used for calibration if desired and/or that alternative bock images shapes can be used if desired.

The optical density of block images 101-106 is measured by a sensor 4. As the block images 101-106 are intended to be uniform, the sensor 4 is generally operable to measure the average optical density across the block 101-106 or at least a average of two or more samples of different areas of the block 101-106. Typically, the sensor 4 may comprise a spectrophotometer or an optical densitometer. Example of suitable sensors 4 include but are not limited to the SpectroEyeTM or eXactTM available from XriteTm.

The determined optical density values for each block 101-106 are then output to a calibration processing unit 5. The optical density values can then be processed alongside marking setting and substrate speed values to determine a calibration relationship between marking setting and substrate speed. The calibration relationship I5 can then be stored in a calibration data store 6. The control unit can then apply the calibration relation in response to signals indicative of the substrate speed received from substrate motion sensor 29 when marking images.

Turning to the calibration method in more detail, this involves marking as many block images 101-106 as required, each block image marked using a different combination of marking setting and substrate speed. Whilst the skilled man will appreciate that any possible marking setting parameters may be varied, for the sake of clarity, the following description will limit discussion to variations of the emitter power parameter, typically expressed as a greyscale (GS) value and the emitter pulse duration (tp).

In the example of figure 3, the block images 101-106 comprise a series of images marked at different emitter pulse durations at a fixed substrate speed, in this instance lm/s. Specifically: the block 101 corresponds to a pulse duration of 64tis; the block 102 corresponds to a pulse duration of 68ps; the block 103 corresponds to a pulse duration of 82)ts, the block 104 corresponds to a pulse duration of 96p.s, the block 105 corresponds to a pulse duration of 112ps; and the block 106 corresponds to a pulse duration of 126tis.

The emitter power setting for each block image 101-106 in figure 3 is fixed, in this example at 255, being the maximum emitter power setting in an 8 bit system. All other possible marking setting parameters are also fixed. Subsequent series of block images 101-106 used in the calibration method may be marked at different substrate speeds (say, 0.l m/s, 0.5m/s, 1.5m/s & 2m/s). in addition, series of block images 1 0 1 -106 can be produced at additional emitter power parameters, say, 128, 140, 170, 200 & 240 on an 8 bit system.

Turning now to figure 4, this shows a plot of the measured optical densities for blocks 101-106 produced at a fixed emitter parameter of 255 with emitter pulse durations of 64ps, 68)ts, 82ps, 96)ts, 112Rs and 126ps and substrate speeds of 0.1m/s, 0.5m/s, 1m/s, 1.5m/s & 2m/s. The plot in figure 4 clearly illustrates the non-intuitive variation of optical density as the pulse duration and substrate speed varies.

At this point, an optical density value can be selected. In the example of figure 4, the selected optical density value is the value obtained at maximum substrate speed by all pulse durations tp. The skilled man will of course appreciate that any other optical density value within the measured range could be selected.

The pulse duration tp required to maintain the optical density at the value obtained at maximum speed can be determined by drawing a line parallel to the speed axis through the optical density value obtained at maximum substrate speed. At substrate speeds below 2m/s (for example at 0.1 to 1.5m/s) the pulse duration tp that correspond most closely to the selected optical density value are identified. In the example of figure 4, these would be the plotted data points closest to the line parallel to the speed axis.

The identified optical density values and pulse durations thus comprise a series of data point pairs. These data point pairs are the subjected to regression analysis, in this example linear regression analysis, to determine the slope (m) and intercept (c) of a straight line connecting the data point pairs. The skilled man will appreciate that higher order regression analysis could be applied if desired or appropriate. The slope and intercept can then be used to calculate an estimated pulse duration tp to produce the selected optical density value at any substrate speed for the fixed emitter power parameter.

In order to determine calibration relationships applicable to varying emitter power parameters it is possible to repeat the steps for a series of different emitter power parameters (say additional GS values of 128, 140, 170, 200 & 240 on an 8 bit system). This can be used to plot a relationship between the required pulse durations for particular substrate speeds at each different emitter power parameter, as is shown in figure 5.

Regression analysis can then be used to fit a suitable function (linear or higher order polynomial (say 3' order) as required or desired) to the relation of pulse duration and substrate speed for each power parameter (GS) value. The coefficients of the polynomial fit to the data can be further plotted against GS value. in the case of a polynomial fit of order 2 or more the coefficients or the polynomial may be plotted against GS value. The coefficients can then in turn be subject to a regression analysis, say quadratic or higher polynomial fit. In this manner polynomial coefficients for a relation between any GS value and optical density may be determined.

As an example, figure 6 shows a plot of the coefficients 'm' and 'c' for a linear (1st order) fit against GS value. It is apparent from figure 6 that the variation in slope 'm' and intercept 'c' can be approximated with reasonable accuracy as straight lines in this case. In such a case, for power parameter (GS) value, the slope and intercept can be determined from the equations of the type m = ai. GS + bi (1) c= a2. GS + b2 (2) where ai, a2, b1 and b2 are the coefficients determined by regression analysis. For a particular power parameter (GS) value the slope 'm' and intercept 'c' may be calculated from equations 1 & 2. Subsequently, the required pulse duration for a particular substrate speed may be calculated from: I5 t = V C (3) Figure 7 contains an illustration of a plot of pulse durations calculated using equation 3 for the present example. The coefficients, slope and intercept values or a look up table of pulse durations for particular speed and power parameter values may be stored in the calibration data store 6 for subsequent use by the control unit 3.

As an alternative to the above method, it is possible to determine the slope 'm' and intercept 'c' for each power parameter (GS) value as discussed above. Subsequently, the slope 'm' and intercept 'c' for intermediate power parameter (GS) values may be determined by linear interpolation between adjacent pairs of data points (slope or intercept and GS value). This will provide a measure of compensation for errors induced by fitting to a straight line.

A further alternative is to determine the slope 'm' and intercept 'c' for each power parameter (GS) value and then calculate the average values of slope 'ma' and intercept 'ca'. A single equation can then be used for determining the change of pulse duration for all power parameter (GS) values.

tp = ma. V + ca (4) Whilst the specific implementation above have relied upon a linear regression analysis of the relationship between the required pulse durations for particular substrate speeds at each different emitter power parameter value (figure 5), this is only a first order approximation. Nevertheless, this is likely to be sufficiently accurate for most applications. If greater accuracy is required, a polynomial of higher order can be used for regression analysis. Turning now to figure 8, this presents the same data as figure 5 but fits a 3rd order polynomial applied to the relationship between the required pulse durations for particular substrate speeds at each different emitter power parameter value.

As with the process described in relation to figure 6, it is then possible to determine the coefficients (ml, m2, m3, c) at intermediate power parameter (GS) values. This can be achieved by one possibility is to fitting another polynomial to the coefficient data, any suitable form of regression analysis. A non-limiting example of such a technique would be the LINEXT function in EXCELTM. The resulting fitting coefficients (ml, m2, m3, c) are shown plotted against power parameter (GS) value in figure 9.

In this particular example, it is clear that a linear fit is not optimal and thus a higher order polynomial is required. Accordingly, figure 9 shows a second order polynomial fit to each set of coefficient data. The coefficients of the second order polynomial may then be used to calculate new 3rd order coefficients for intermediate power parameter (GS) values. Using the calculated coefficients the third order polynomial may be used to calculate the required pulse durations as function of speed for each power parameter (GS) value. The outcome of this calculation is shown in figure 10. As previously, the coefficients, slope and intercept values or a look up table of pulse durations for particular speed and power parameter values may be stored in the calibration data store 6 for subsequent use by the control unit 3.

Whilst the relation calculated above is good, there can still be errors. For instance, in the example of figure 10 which plots the so calculated relations for each power parameter (GS) value, it can be seen that there is an anomaly at substrate speeds of 2m/s. At these speeds, the pulse durations should all tend to -64us but in the figure the relations for power parameter (GS) values of 240 and 255 do not trend correctly. This may result from the quadratic fit for the coefficients not being optimal.

Such errors described above may be overcome or at least mitigated by determining the fitting coefficients (ml, m2, m3, c) at intermediate greyscale values by linear interpolation between adjacent data pairs. The resulting coefficients ml, m2, m3, & c for each greyscale setting are used to calculate the required pulse duration using the third order polynomial relating pulse duration to substrate speed. The resulting relations between pulse duration and substrate speed for each power parameter are plotted in figure 11. In this case the relations have the correct shape and also tend to the pulse duration of 64us when the speed tends to 2m/s.

In many instances, linear interpolation between adjacent data pairs in order to calculate fitting coefficients results in better predictions and is typically preferred over the polynomial or functional methods.

Another alternative to fitting all results in order to calculate relations between pulse duration and substrate speed for particular power parameter (GS) values is to apply a series of linear interpolations to successive data point pairs. This would involve calculating a straight line fit between the data point pair at one extreme of the tested substrate speeds and subsequently moving on to calculate another straight line fit between the data point pair (which includes one previous data point) until the other extreme of tested speed is reached. For each speed interval V, to Vi+i, it is then able to calculate the required pulse duration from equations of type: tp = m(GS). V + c(GS) for V, < V < VE+1 (5) Where Vi and Vi+i define the speed interval for which the linear fit was applied and m(GS) and c(GS) are the slope and intercept for the particular GS value for that speed interval. The slope and intercept for intermediate GS values may also be determined by linear interpolation between adjacent GS settings. As previously, the coefficients, slope and intercept values or a look up table of pulse durations for particular speed and power parameter values may be stored in the calibration data store 6 for subsequent use by the control unit 3.

Whilst the above discussions have centred on selecting an appropriate pulse duration to compensate for a variation in substrate speed, surprisingly it has been found that the variation of image optical density with changes in substrate speed can be significantly reduced by choosing an appropriate fixed pulse duration.

The maximum pulse duration before two adjacent dots merge is determined by the spot diameter (or selected pitch and spot diameter) and the substrate speed. For example, marking at a resolution of at 200dpi (dots per inch) with a spot diameter of 127pm the time to travel a distance equivalent to one spot diameter at a speed of 2m/s is 63.5µs. For two adjacent pixels in the direction of travel and a pulse setting of 64ps the emitter does not have time to activate, deactivate and activate again between adjacent pixels. Therefore the emitter will stay activated constantly for the adjacent pixels.

The energy received by any pixel at maximum speed is determined by the time to travel a spot diameter (or the pulse width setting) and the power parameter (GS) value. If the pulse duration is fixed at, say, 63.5ps the energy input per pixel is independent of speed up to a speed of 2m/s. Typically when the pulse duration is set to this value the optical density decreases with reduced speed. However, as the product speed slows down there is more time available between pulses and so at slower speeds longer pulse durations can be accommodated.

If the pulse duration is set to a value larger than the time to travel a distance equivalent to one spot diameter (or pitch) at maximum substrate speed, then as the substrate speed reduces the exposure time per pixel (and hence the effective pulse duration) is determined solely by the substrate speed and spot diameter (or pitch). When the substrate speed has slowed to the point where the time to travel a spot diameter (or pitch) is greater than the pulse duration then the exposure time will correspond to the pulse duration. In this way the system automatically increases the effective pulse duration, in a linear manner as substrate speed reduces until the selected pulse duration is reached.

In view of the above, it might be expected that setting the pulse duration to an arbitrarily large value and letting the spot diameter (or pitch) and substrate speeds determine the exposure per pixel as speed changes will automatically compensate the entire speed range. However due to the nonlinear nature of the relation between optical density and substrate speed this is not the case. Instead it has been found that there is a small range of fixed pulse durations where good compensation occurs. This range can be defined by an equation of the type: 1.2 -cia < t < 1.5.-cia (6) v In instances where the other marking setting parameters may be varied, the upper and/or lower limits may be varied as appropriate. In one example, the upper limit do may be varied to 1.8.-v. For instance, this would allow longer pulse durations at lower I0 average power at lower substrate speeds.

For example, using DataLase 'Diversity' colour change coating on a PET label, a fixed pulse duration of 82us has been found to correct for image density variations to good effect over the substrate speed range of 0.5 to 2m/s.

Where the suitable fixed pulse durations are derived from the equation 6 above, it is still possible to select an optimum fixed pulse duration. This may be achieved by selecting an initial pulse duration that satisfies equation 6, say, 82ps and alterative longer and shorter pulse durations, say, 70ps and 96ps. The method may then involve marking block images 101-106 at each of the selected pulse durations at a variety of different substrate speeds and power parameter (GS) values as above. If the optical density values obtained for any series of image blocks at any of the selected pulse durations differs from the target value, an optimised pulse duration may be determined by linear interpolation with the data produced by the other pulse durations.

In implementations using a fixed pulse duration, the power parameter (GS) value may be varied to compensate for substrate speed variation and thereby achieve a marked image of a target optical density. In order to determine a suitable calibration relation between power parameter and substrate speed, it is first necessary to select a suitable fixed pulse duration, either using equation 7 or by the optimisation method described above. In this instance the upper limit of equation 6 is varied so as to allow longer pulse durations at lower average power at lower substrate speeds 1.2 ='d < tP < 1.8 ='d v -V (7) Subsequently, a series of image blocks 101-106 are marked at various different substrate speeds (say, 0.1m/s, 0.5m/s, lm/s, 1.5 m/s & 2m/s) and various different power parameter (GS) values (say 128, 145, 170, 200, 240 & 255 for an 8 bit system).

At this point, an optical density value can be selected for each GS value In the example of figure 12, the selected optical density value is the value obtained at maximum substrate speed, shown here for the GS value of 200 by the line parallel to the speed axis and intercepting the optical density axis at -1.2. The skilled man will of course appreciate that any other optical density value within the measured range could be I 0 selected.

The GS value required to maintain the optical density at each substrate speed the selected value can be determined by identifying the pairs of GS values corresponding to optical density values closest to the selected optical density value. In the example of figure 12, these would be the plotted data points closest to the line parallel to the speed axis. Subsequently, linear interpolation between the respective pairs of GS value can determine the GS value (GSr) that would produce the selected optical density at each measured substrate speed.

The determined GSr values and corresponding substrate speeds can then be subject to regression analysis to determine a calibration relationship. This can be achieved by fitting a polynomial (say quadratic or the like) to the GSr values and substrate speeds. This can thus provide a calculate GSr values for intermediate speeds as is illustrated in figure 13, which also illustrates the relatively similar outcomes that are achieved if different fixed pulse durations (96ps or 112ps) are selected. As above, the calibration relationship or a look up table of GS values for particular substrate speeds may be stored in the calibration data store 6 for subsequent use by the control unit 3.

In the above implementation, it is also possible to calculate a scale factor Fos for each speed, where Fos is the ratio between the initial GS value and the calibrated GS value GSr. The scale factor FGs for each speed can thus be applied to all GS values to provide a ready first order compensation for substrate speed variation.

In a further implementation, it is possible to fit polynomials to the relationship between GS value and substrate speed for fixed pulse durations. This is illustrated for a GS value of 240 and a series of pulse durations in figure 14, with a third order polynomial fit to the relationship for 64tts pulse duration and a fourth order polynomial fit to the relationship for 112(ts pulse duration.

A generalised 4th order polynomial for optical density (OD) with respect to substrate speed V takes the form OD = aV4 + bV3 + cV2 + dV + e (8) The coefficients 'a' to e' can be determined by regression analysis with respect to GS value setting or by linear interpolation for GS values intermediate between those of the measurements. In the fitting example, the coefficients 'a' to e' may have a quadratic dependence on GS value as set out below: a = auGS2 + igaGS + ye, (9a) ab__ rb--b = GC + R GS + v (9b) c = acGS2 + ficGS + y, (9c) d = adGS2 + f3dGS + yd (9d) e = ce,GS2 + fleGS + ye (9e) The equations 9a to 9e apply for each fixed pulse duration. Substituting for 'a' to e' in equation 8 and letting OD be OD(GS) (the required optical density at the selected GS value and substrate speed) it is possible to solve for a new grey scale value GSN as set out below: -W± VW 2 -4. U.Z GSN = 2U (10) where U= abV4 + abV3 + ay2 + adV + a, (Ha) W = Sart + fib V3 + ficV2 + 13e (11b) Z= yali4 ybV3 + ycli2 + ydV + ye-OD, (He) The new greyscale values GSN may be calculated for the fixed pulse durations of the measurements using equation 10. GSN values for intermediate pulse durations may be determined by linear interpolation between adjacent data pairs. In this way the GSN value may be found for any pulse duration to achieve the required optical density as a function of speed. Typically, the pulse duration should be selected according to equation 7.

An alternative to fitting polynomials is to search the determined optical density values from all marked image blocks 101-106 to identify an image block matching the target optical density or the image blocks having an optical density value closest to a target optical density value. Once such image block or blocks are identified, the corresponding GS values and pulse duration tp can be identified. Where a matching image block is identified, the corresponding GS and tp can simply be stored in the calibration data store 6. Where the closest image blocks are identified linear interpolation can be carried out between the respective GS values and pulse durations to determine a GS value and pulse duration that will correspond to the target optical density. These values can be stored in the calibration data store 6. In some instances, there may be more than one GS value and pulse duration combination that correspond to a target optical density. In such instances, a preferred combination may be selected that uses the lowest GS value (and thus lowest power) that satisfies the inequality: tmin < tp < tmax (12) Where train = 1.254 /V (12a) trna, = 1.75d0 /V (12b) The one or more embodiments are described above by way of example only.

Many variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

CLAIMS1. A method of calibrating a laser marking system of the type operable to mark a substrate moving relative to the marking system at a substrate speed, the marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the emitted light controlled in response to a variable marking setting, the method comprising the steps of: activating the emitters to mark a series of block images at a range of different marking settings and a range of different substrate speeds; determining the average optical density of each marked block; identifying the marking settings that correspond most closely to a selected optical density value for each substrate speed; and thereby determining a calibration relationship between marking setting and substrate speed.
2. A method as claimed in claim 1 wherein the marking setting is defined by any one or more of emitter power; emitter spot diameter, emitter pulse duration, emitter power density, emitter energy density.
A method as claimed in claim 2 wherein variation of the marking setting is achieved by varying a subset of marking setting parameters and maintaining other marking setting parameters constant.
4. A method as claimed in claim 2 wherein variation of the marking setting in response to a variation in the target optical density is achieved by varying a single marking setting parameter.
5. A method as claimed in any one of claims 2 to 4 wherein determining a relationship between marking setting and substrate speed involves performing an interpolation between the closest pair of marking settings for marked block images with determined optical densities that correspond most closely to the selected optical density value for each substrate speed.
A method as claimed in any preceding claim wherein determining a calibration relationship involves carrying out regression analysis to calculate a best fit function relating variation in marking setting to substrate speed.
A method as claimed in claim 6 wherein if variation of the marking setting involves varying more than one marking setting parameter, the method involves performing independent regression analysis in respect of variation in each marking setting parameter.
A method as claimed in claim 6 wherein the method involves performing independent regression analysis in respect of one marking setting parameter at a series of fixed values of the remaining marking setting parameters.
9. A method as claimed in any one of claims 6 to 8 wherein the method includes carrying out regression analysis in respect of the values of the calculated regression coefficients for the first marking setting parameter with respect to variation of a second marking setting parameter.
10. A method as claimed in any one of claims 6 to 8 wherein the method includes averaging regression coefficients or interpolating between regression coefficients calculated under each regression analysis.
11. A method as claimed in any preceding claim wherein if variation of the marking setting involves varying only a single marking setting parameter, the method involves performing regression analysis in respect of only the varied marking setting parameter.
12. A method as claimed in claim 11 wherein the method includes the step of selecting a fixed value of one or more of the remaining marking setting parameters.
13. A method as claimed in claim 12 wherein where the fixed value is the emitter pulse duration, and the method involves selecting a pulse duration that satisfies 1.2.---V < tP < 1.8.---do
- Vwhere tp is the emitter pulse duration, V is the substrate speed and do is the emitter spot diameter 14 A method as claimed in claim 12 or claim 13 wherein the method includes determining a fixed value of one or more of the remaining marking setting parameters by selecting two or more candidate values; marking block images at different substrate speeds using said candidate values of the marking setting parameter; and identifying the candidate value that corresponds most closely to a selected optical density value for all substrate speeds or interpolating between two or more candidate values in order to identify an intermediate value for selection.
15. A method as claimed in any preceding claim wherein the method includes storing details of the determined calibration relationship in a calibration data store.
16. A method as claimed in claim 15 wherein the method includes using the determined calibration relationship to generate a look up table relating marking settings and substrate speeds and storing the look up table in the calibration data store.
17. A method as claimed in any preceding claim wherein the emitters comprise individually addressable laser diode arrays (IALDA) or individually addressable laser arrays (IALA).
18. A method as claimed in any preceding claim wherein the emitters comprise an emitting end of a marking fibre wherein an input end of the marking fibre is coupled to a laser diode.
19. A method as claimed in any preceding claim wherein the substrate comprises a label or an extended tape.
20. A method as claimed in any preceding claim wherein the substrate comprises a colour change material operable to change colour in response to illumination by the emitters, the colour change material comprising substances including but not limited to any of: a metal oxyanion, a leuco dye, a diacetylene, or a charge transfer agent.
21 A method as claimed in claim 20 wherein the colour change material further comprises an acid generating agent.
22. A method as claimed in claim 20 or claim 21 wherein the substrate comprises an NIR (near infrared) absorber material.
23. A calibration apparatus for a laser marking system of the type operable to mark a substrate moving relative to the marking system at a substrate speed, the marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the emitted light controlled in response to a variable marking setting wherein the laser marking system is operable to mark a series of block images at a range of different marking settings and a range of different substrate speeds; and wherein the calibration apparatus comprises: a sensor operable to determine the average optical density of each marked block; and a calibration processer operable to identify the marking settings that correspond most closely to a selected optical density value for each substrate speed and thereby determine a relationship between marking setting and substrate speed.
24. A calibration apparatus as claimed in claim 23 wherein a calibration data store operable to store data corresponding to the determined relationship between marking setting and substrate speed for at least the selected optical density is provided.
25. A laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a calibration data store and wherein the calibration data store is populated using the method of any one of claims 1 to 22 or the apparatus of claim 23 or claim 24.
26. A method of determining fixed pulse duration for use in a laser marking system of the type operable to mark a substrate moving relative to the marking system at a substrate speed, the marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, the emitted light controlled in response to a variable marking setting, the method comprising the steps of: selecting a fixed value of the emitter pulse duration from a predetermined range of values defined by: do 1.2.-oV < tP V < 1.5.-where tp is the emitter pulse duration, V is the substrate speed and do is the emitter spot diameter..
27. A method as claimed in claim 26 wherein the method includes determining a fixed value of one or more of the remaining marking setting parameters by selecting two or more candidate values; marking block images at different substrate speeds using said candidate values of the marking setting parameter; and identifying the candidate value that corresponds most closely to a selected optical density value for all substrate speeds or interpolating between two or more candidate values in order to identify an intermediate value for selection
28. A laser marking system comprising a plurality of emitters, each emitter operable to emit light on to a substrate for marking, a control unit operable to modulate the output of each emitter by varying a marking setting wherein the control unit is operable to look up a marking setting associated with a target optical density in a calibration data store and wherein the emitters are operable to emit pulses of a fixed duration, the emitter pulse duration being selected from a predetermined range of values defined by: do do 1.2 -< t < 1.5.-Vwhere tp is the emitter pulse duration, V is the substrate speed and do is the emitter spot diameter