EP2109094A1 - LCD-Umkehrungssteuerung - Google Patents

LCD-Umkehrungssteuerung Download PDF

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Publication number
EP2109094A1
EP2109094A1 EP08154287A EP08154287A EP2109094A1 EP 2109094 A1 EP2109094 A1 EP 2109094A1 EP 08154287 A EP08154287 A EP 08154287A EP 08154287 A EP08154287 A EP 08154287A EP 2109094 A1 EP2109094 A1 EP 2109094A1
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EP
European Patent Office
Prior art keywords
inversion
liquid crystal
signal
backlight
colour
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP08154287A
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English (en)
French (fr)
Inventor
Jeroen Debonnet
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Barco NV
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Barco NV
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Priority to EP08154287A priority Critical patent/EP2109094A1/de
Publication of EP2109094A1 publication Critical patent/EP2109094A1/de
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    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3614—Control of polarity reversal in general
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    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/3406—Control of illumination source
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    • G09G2360/00—Aspects of the architecture of display systems
    • G09G2360/14—Detecting light within display terminals, e.g. using a single or a plurality of photosensors
    • G09G2360/145—Detecting light within display terminals, e.g. using a single or a plurality of photosensors the light originating from the display screen
    • G—PHYSICS
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    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2360/00—Aspects of the architecture of display systems
    • G09G2360/18—Use of a frame buffer in a display terminal, inclusive of the display panel
    • G—PHYSICS
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    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/2007—Display of intermediate tones
    • G09G3/2018—Display of intermediate tones by time modulation using two or more time intervals
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/3406—Control of illumination source
    • G09G3/342—Control of illumination source using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines

Definitions

  • the present invention relates to liquid crystal devices and to corresponding systems and methods.
  • an LCD displays images by controlling light transmittance of a liquid crystal having dielectric anisotropy, using an electric field.
  • the LCD device has an LCD panel having pixel regions arranged in a matrix arrangement and a drive circuit for driving the LCD panel.
  • the LCD panel has pixel electrodes and common electrodes formed to apply an electric field to each of the pixel regions.
  • Periodic inversion of the field is needed to prevent polarization (and rapid permanent damage) of the liquid crystal material.
  • Various schemes for periodic inversion are known, such as frame inversion, line-column inversion, and dot inversion.
  • inversion relies on the fact that in liquid crystal pixel cells, it is only the magnitude of the applied voltage which determines the light transmission (the transmission vs. voltage function is symmetrical about 0V). Unfortunately it is very difficult to get exactly the same voltage on the cell in both polarities, so the pixel-cell brightness will tend to flicker to some extent at half the frame-rate. If the polarity of the whole screen were inverted at once then the flicker could be highly objectionable. Instead, it is usual to have the polarity of nearby pixels in anti-phase, thus cancelling out the flicker over areas of any significant size.
  • the polarity of the data signals supplied to the liquid crystal cells on the LCD display panel is inverted whenever a frame is changed.
  • the polarity of the data signals supplied to the liquid crystal cells is inverted according to the line (column) on the LCD panel.
  • the dot inversion driving method a data signal is supplied to each liquid crystal cell of the LCD panel, wherein the data signal has a polarity contrary to the data signal supplied to adjacent liquid crystal cells along vertical and horizontal directions.
  • the polarity of the data signals supplied to all the liquid crystal cells on the LCD panel is inverted for each frame.
  • US2008001890 shows changing the inversion method of the LCD panel depending on the specific pattern of the image data so as to prevent unwanted colors being displayed due to variation of the common voltage.
  • An object of the invention is to provide improved apparatus or methods.
  • the invention provides:
  • a liquid crystal display device arranged to receive an incoming signal for display on a corresponding array of liquid crystal elements, the device having an inversion part for controlling an inversion of the signal, and a blanking insertion part for driving the liquid crystal elements to a different level to that indicated by the signal, the inversion part being adaptable according to operation of the black insertion part.
  • the problem of the blank insertion interfering with the inversion in certain modes can be avoided. For example if for any given liquid crystal element, the insertion occurs more often when the signal is inverted and less often when the signal is not inverted, then, over time a net DC field may become non zero and that element can show a wrong color or become damaged, or flicker may become apparent.
  • the inversion part can be adaptable to balance a duration a given one of the liquid crystal elements is driven by an inverted signal with a corresponding duration for a non inverted signal, the durations excluding periods of black insertions for that given liquid crystal element.
  • the insertion part can be arranged to operate to insert any of black frames, black fields, black lines, or black dots, or groups of any of these.
  • the inversion part can be arranged to determine a current amount of net DC offset for each of the liquid crystal elements and to adapt the inversion to reduce the net DC offset.
  • the inversion part can be arranged to alter an inversion frequency.
  • the device can have an output to indicate a current phase of the inversion. This can be useful to enable synchronization with external parts for example.
  • the device can have a backlight arranged to provide scanned black insertion. This can add to the black insertion provided by the black insertion part.
  • the display device can be arranged to scan in an interlace mode or a progressive mode.
  • the device can have a de-interlacer for receiving an interlaced video input signal and converting it to a non interlaced version.
  • the inversion part can have an external input to allow the inversion to be altered by an external control signal.
  • Another aspect provides a liquid crystal display device, arranged to receive an incoming signal for display on a corresponding array of liquid crystal elements, the device having a blanking insertion part for driving the liquid crystal elements to a different level to that indicated by the signal, and a response compensation part for compensating the signal for liquid crystal response time, to maintain intended light output.
  • a backlight is provided for a panel display. It may be desirable to measure colour output of the backlight, to enable it to be tuned for greater accuracy.
  • a spectrometer can be provided in a path of the light output by the backlight, for example in a gap between the backlight and a liquid crystal array.
  • Monochrome light sensors can also be provided for particular parts of the backlight.
  • the backlight can also be arranged as a scanning backlight, for example having a number of sections in the form of trays. In a scanning mode these trays are blanked in a predetermined sequence. This type of blanking insertion can help to reduce image persistence between frames or between interlace fields and thus reduce visible motion artifacts.
  • the backlight scanning should be synchronized with the scanning of the LCD drive circuitry for updating the display each frame or field.
  • Blanking in the form of black frame insertion can be carried out by the LCD drive circuitry, for the same purpose, either instead of, or as well as the black insertion by the backlight.
  • This can be scanned line by line rather than the tray by tray scanning of the backlight black insertion, and can therefore match the scanning of the image more closely.
  • BFI using the LCD can interfere with the inversion described above.
  • the interference can result if the blank insertion occurs more often during one polarity of the inversion than the other polarity.
  • Blanking can be defined as driving to black or to white or to any shade in between.
  • the inversion is fixed and so the BFI can be tailored to try to minimize such imbalance in the inversion. But for a display panel which should have a variety of scanning modes, or be able to handle inputs having different frame rates or different bit rates, it can be difficult to avoid such imbalance for all scanning modes.
  • embodiments of the invention provide a dynamic inversion scheme adapted to a scanning mode of the display. For example if successive lines have different inversions, this could be adapted to provide different inversions of pairs of lines, if this provides a more balanced inversion scheme. Alternatively an inversion scheme which changes each frame, could be changed every two frames if this provides a more balanced inversion for any given BFI scheme and scanning mode.
  • the inversion can be adapted according to an input indicating a scanning mode, such as software selectable interlace or progressive scan modes.
  • the BFI scheme may be software selectable and again the inversion mode can be adapted accordingly.
  • An input can be provided to enable the inversion scheme to be controlled or adapted by an external signal.
  • An output can be provided indicating a current inversion polarity. This could be used for adapting other parts, or for monitoring a degree of balance for example.
  • an LCD display device has a compensation system that corrects the drive level of the LCD according to the LCD response times, ensuring correct perception of the calibrated colors in a BFI system.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 An illustration of an exemplary embodiment of the present disclosure.
  • FIG. 1 shows an embodiment of an LCD device 40 in schematic form showing some of the principal features.
  • a video input signal is fed to drive circuitry 50.
  • This drive circuitry 50 has a blanking insertion part 10 and an adaptable inversion part 20.
  • the adaptable inversion part 20 is arranged to control an inversion of the video signal, to avoid a cumulative build up of a net DC field on any individual one of the liquid crystal elements.
  • the blanking insertion part is arranged to cause a blank display by a given one or more of the liquid crystal elements, in between successive fields or frames of the video signal.
  • the inversion part is adaptable according to an operation of the blanking insertion part.
  • the drive circuitry 50 outputs a signal or signals to drive an array of liquid crystal display elements 30.
  • the blanking level for example a black level can be written into the frame buffer, to overwrite the video data after a given time.
  • the output of the drive circuitry 50 could be generated by reading out the frame buffer according to established practice.
  • the output could be inverted under the control of the adaptable inversion part 20.
  • the inversion could be carried out before the blanking insertion, for example by inverting selected values in the frame buffer, or by inverting parts of the video input before it is written to the frame buffer.
  • FIG. 2 shows graphically how the adaptation of the inversion can improve the balance of the inversion for a given pixel and its corresponding liquid crystal element. Time is represented by the x axis.
  • FIG. 1 a concrete example of blanking insertion is shown, in the form of black frame insertion, causing a problem with a standard LCD inversion scheme.
  • a sequence of six frames at a frame rate of 120Hz is shown. Only the top left 4x2 pixels are shown, and are shown in shading if they are driven by the video input and in black if they are subject to black insertion.
  • Their inversion polarity is shown by a + or a - symbol.
  • the backlight is always on, the LCD uses progressive driving and standard dot inversion.
  • the blanking be regular, or that it drives to the DC field to zero.
  • Alternative embodiments can have an irregular, non periodic inversion speed.
  • the speed at which the polarity of a LC cell could be changed can be slower; provided only that the net DC level over a period of time should remain zero.
  • FIG. 2 A simple way to do this is shown in FIG. 2 above, using a predetermined, regular pattern.
  • An alternative would be to provide a store for past inversion information and the accumulated field levels of every cell in a big framestore. Then a processor can be used to determine an optimal inversion scheme to continuously optimize the inversion of every separate pixel, with these two aims:
  • This system can therefore use inversion schemes completely different from the known predetermined inversion schemes.
  • FIG. 3 shows four examples of driving schemes. In each case, a sequence of six frames at a frame rate of 120Hz is shown. Only the top left 4x2 pixels are shown, and are shown in grey shading if they are driven by the video input and in black if they are subject to black insertion. Their inversion polarity is shown by a + or a - symbol.
  • a first line shows using a scanning backlight, and using interlaced driving, with a standard dot inversion. Only the lines in the current field are driven, the previous field is blanked with black lines. So, only half of the lines of the LCD are driven: the odd lines when the upper video field needs to be displayed, the even lines when the lower video field needs to be displayed.
  • the inversion is balanced in this mode, as can be seen since for any given pixel, there are a similar number of positive and negative inversions in the sequence. Hence there is no net DC field.
  • the second, third and fourth sequences of FIG. 3 show using the backlight in continuous mode, and doing BFI using the LCD.
  • the second sequence there is interlace driving and standard dot inversion. This shows unbalance in the inversions, as for any given pixel there are inversion symbols of only one polarity.
  • every scanning mode will have its optimal inversion scheme.
  • the LCD can have improved or optimal performance and need not suffer from the Black Frame Insertion or interlaced driving concepts.
  • An LCD does not have an infinite response speed. Response times are in the range 6 - 8 ms. If the optical response of a LCD pixel is measured, it will have an upward curve and a downward decay curve. This delay in making a transition means that blanking insertion and other causes of transitions can cause the luminance to differ from that indicated by the video signal.
  • the sensor can be a front face sensor, built into a front of the LCD for example in an area near an edge of the LCD.
  • LCD Response Lookup Table This table is continuously updated with the characteristics of the LCD: which drive level is needed to have a correctly perceived light output.
  • the video signal can be fed through this lookup table, and this way, the area under the response curve can be maintained in a 1-to-1 relation with the intended light output.
  • Using a front sensor which continuously measures the response times of the LCD's transitions can be better than relying on predetermined values. As these change over time and temperature, it makes perfectly sense to continuously measure the response times for all transitions, and continuously change the compensation tables with the latest measured information.
  • the digital video always passes through some FPGA or ASIC to do the necessary video processing (e.g. gamma lookup tables, scaling, OSD insertion, ...) before reaching the display device.
  • some FPGA or ASIC to do the necessary video processing (e.g. gamma lookup tables, scaling, OSD insertion, ...) before reaching the display device.
  • FIG. 4 shows another example of an LCD device.
  • the video is input to a frame buffer 80
  • the blanking insertion part 10 writes the black level to appropriate parts of the frame buffer.
  • the adaptable inversion part is coupled to an output of the frame buffer, and feeds an output signal with the inversions to drive the liquid crystal elements.
  • a mode select input is fed to the drive circuitry 50 to choose the mode in the sense of interlace or otherwise, frame rate, line rate and so on, of the input, and of the output of the frame buffer.
  • Light sensors 90 are shown which can be used to feed back measured light levels to the drive circuitry to control a level compensation part which can adapt the levels at the input or output to the frame buffer for example, or be used to adapt the drive level of a backlight 70.
  • FIG. 5 shows a side view of a display device having a backlight.
  • a number of trays of the backlight are arranged horizontally in rows, each with its own monochrome light sensor.
  • Each tray is formed of a horizontal PMMA light guide coupled to an LED source at one or both ends.
  • a reflective foil is arranged at the back of the backlight.
  • a diffuser is arranged at the front.
  • a spectrometer is placed at the top to sense light in the gap between the diffusor and the trays.
  • This example of a scanning backlight, with edge-lit topology can have LED light sources on left and right sides. It shows 6 light guides for scanning, though there can of course be other numbers of such guides.
  • Each light guide can be illuminated with 2 LED PCB's: left and right.
  • Every LED PCB can be driven with independent PWM.
  • every color on the LED PCB also has independent PWM (4 controls: R, G, G, B).
  • 6 monochrome light sensors are shown, one mounted on the back of every tray, in the middle. The spectrometer looks at the output of all trays together by looking in the cavity between the LED light guides and the diffusor.
  • FIG. 6 shows the backlight and the array of liquid crystal elements.
  • Table 1 Mode Latency added by LCD driving mode
  • Motion blur reduction Backlight dimming range Scanning backlight with deinterlacer Dependent on dimming range: good 10-100 cd/m 2 @ 120Hz : 8 to 16 ms @ 100Hz : 10 to 20 ms (highest latency at lowest dimming) Scanning backlight with black line insertion for "CRT interlaced mode emulation" Same best 5 - 50 cd/m 2
  • Always-on backlight LCD response time: 8ms No reduction 20-200 cd/m 2
  • FIG. 7 shows for an individual pixel in an LCD array a pixel response curve indicating a pixel transmission level over time.
  • a first addressing frame time there is an upward curve labeled as the pixel response time, followed by a flat region for the rest of the frame time, marked as the illumination frame.
  • This unshaded region can indicate a time when the backlight is illuminated.
  • the next frame can be a black insertion time, in which case the pixel is driven to a black level during an addressing frame and remains there for the subsequent illumination frame.
  • part or all of the rise or fall portion of the response may be in the illumination period. In this case, some compensation may provided, so that the area under the response curve can be maintained in a 1-to-1 relation with the intended light output, as described above.
  • FIG. 8 shows a graph of frame delays relative to vertical position of a given pixel. It shows how with vertical position of the pixel, the illumination frame and addressing frames are delayed by different amounts in accordance with a scanning scheme.
  • FIG. 9 shows a similar view for delay relative to vertical position of different trays of a backlight.
  • the lines show a response of an individual pixel at the top of the respective tray.
  • the rectangles show times when the tray is lit. It shows how there is a corresponding relative delay between trays according to a vertical position of the tray.
  • a colour image is generated by sequentially generating multiple primary colour images that together form the colour image.
  • the backlight will switch continuously between for instance red, green and blue.
  • the backlight will be red, and during that frame the LCD pixels will be driven as to represent the red component in the colour image that is to be displayed.
  • the backlight will be set to green, and the LCD pixels will be driven as to represent the green component in the colour image that is to be displayed.
  • the backlight will be set to blue and the LCD pixels will be driven as to represent the blue component. If the frame rate is high enough then the human eye will integrate these images and the combination of these three individually monochrome frames will be perceived as a colour image. In the case of an LCD with LED backlight all red LEDs will be driven in frame one and no green or blue LEDs will be driven, in frame two only the green LEDs will be driven and in frame three only the blue LEDs will be driven. In the case of the LCD display with LED backlight, the colour point of the "primary colours" is extra modulated with a longer period. In other words: if one takes the example of a colour sequential LCD display that uses three frames Red, Green and Blue.
  • the combination with backlight modulation to adjust colour will mean that the red colour itself is also modulated over time.
  • a 2-frame two-level dither scheme for instance would mean that there are two (slightly) different variations on the red colour and that the luminance value of those two red colours can be different.
  • the same concept is valid for the green and blue colours.
  • the three frames from the colour sequential display system can also be compared to a single frame on a colour display, and that "colour display frame" can be modulated in colour and/or luminance over time in order to have a working implementation.
  • a starting point can be a normal colour sequential system could have backlight values for sequential frames as follows: R, G, B, W, R, G, B, W, ...
  • R represents a red-alike colour with specific colour point and luminance and also G,B,W represent light with a specific colour point and luminance value. If for example a two-level dither scheme is used on this colour sequential display system then the backlight values for sequential frames could look like this: R1, G1, B1, W1, R2, G2, B2, W2, R1, G1, B1, W1, ... where R1 represents a red-alike colour with specific colour point and luminance and R2 represents a red-alike colour with colour point and/or luminance value that is different from R1.
  • G1, G2; B1; B2; W1; W2 all represent pairs that have difference in colour point and/or luminance value (although it is not a requirement that all primaries are modulated, it is for example possible that R1 differs from R2, but that at the same time B1 is equal to B2).
  • optimise the embodiments to include spatial variations over the display system area. For instance: with LCD displays there is always some variation in luminance behaviour (transfer curve) and colour behaviour (transmission spectrum) over the display area. This could mean for instance that certain areas on the LCD are more bright or dark than other areas or that there is a significant difference in luminance transfer curve depending on the exact place on the LCD. The same problem is also present for colour behaviour. It is possible to optimise the two-level dither scheme by really taking into account the different luminance and/or colour behaviour of the display system over its complete display area.
  • a first parameter is the behaviour of the backlight: the luminance and colour behaviour of the backlight in function of the driving level of the backlight (typically a backlight can be driven between a minimum DAC-value zero and a maximum DAC-value for instance 4095.
  • the DAC-value is related to the current sent to the backlight lamps or LEDs).
  • a second parameter is the behaviour of the display panel (LCD, DMD, DLP, ...) This can be regarded as the luminance and colour behaviour of the panel as a function of the DDL of the panel.
  • the table of digital driving values can consist of a one-dimensional array in case of a monochrome LCD, a multidimensional table in case of a monochrome LCD with each pixel consisting of multiple sub pixels or a multidimensional table in case of a colour LCD with each pixel consisting of a number of coloured sub pixels. This means that the optimal dither variables are depending on parameters that can be different for each display system.
  • the backlight behaviour can differ for each individual backlight (for instance a LED backlight where there is typically a lot of variance between luminance and colour behaviour between different batches of LEDs) or for each individual panel (for instance: the transmission spectrum of an LCD panel can differ significantly from panel to panel).
  • each individual display system can be characterized to determine an optimal dither scheme for each display system.
  • Another approach can be to select the exact dither scheme such that even when variation between the display systems is present the performance will still be more or less the same. For example: suppose that the backlight is based on LEDs. LEDs that are dimmed to deep will not emit any light anymore. The exact dimming range can differ between different batches of LEDs or even from LED to LED. Therefore a compromise would be not to use very deep dimming (so not optimal) but choose a value that will be safe for all display systems.
  • Embodiments can use a combination of backlight luminance and colour coordinates and panel behaviour to obtain an accurate reproduction of colour and luminance. If of course the behaviour of the backlight (luminance source of the display system) and/or the panel (modulation system of the display system) changes then the dither scheme might not be optimal anymore. Therefore it is possible that extra measurement devices are used to compensate for these behaviour changes.
  • a first example is that a sensor can monitor the luminance and colour behaviour of the backlight system. If the luminance and/or colour behaviour changes then a new dither scheme can be calculated based on the known original colour and luminance behaviour and the new measured colour and luminance behaviour of the backlight system. Suppose that after a few thousand hours of operation the backlight has a colour shift towards red, then this information can be used to make sure that the desired colour point of the backlight for the individual frames of the dither scheme is still correct.
  • a stabilization device typically measures parameters such as but not limited to luminance and/or colour point or contrast ratio in a specific situation and makes sure that for example (but not limited to) luminance and/or colour is always equal to a selected target value (by changing the backlight driving values or the pixel values). For instance: in medical imaging the white luminance (luminance output when fully white is displayed) of the display is very often kept stable at a selected level (for instance 500 cd/m ⁇ 2> ). It is of course possible to use such a stabilization system together with other features described above. In this situation the white luminance (and perhaps also the white colour point) will determine luminance output and the colour point of the display.
  • the two level dither scheme then can be configured so that both the luminance and colour point at full white do not change anymore. This can be done by making sure that the average luminance output over the dither period is equal to the target luminance and also that the average colour point over the dither period is equal to the target colour point.
  • the calculation method of the dither variables can give more accurate results in many situations if measurements are of the final output of the combination of backlight system and panel.
  • the backlight system produces light with a certain spectrum that is usually well spread over the visual spectrum range (380nm -800 nm).
  • the transmission spectrum of the panel is spread over the same visual spectrum range. For example: suppose a monochrome display system with a backlight is used and the colour shift measured when going from video level zero to video level maximum. It is then not a priori certain that proportionally the same colour shift will be seen if there are changes to the colour of the backlight. In other words: suppose that the x-coordinate of the measured light of the display system is 20% higher at maximum video level compared to minimum video level then it is not a priori certain that if the backlight colour is changed, that this will still be valid.
  • Another solution is to use a blinking backlight system. Indeed: it does not really matter where exactly the light transmission takes place in the frame. This can be equally distributed over the frame or concentrated in one or more parts of a frame period. If one uses a blinking backlight for instance, that concentrates most of the light energy at the end of each frame then the problem of slow LCDs can be reduced. This means of course that the backlight will need to be able to emit the energy in a more concentrated form (an equal amount of energy in a smaller part of time). If the energy is concentrated (for instance but not limited to) at the end of the frame, then the LCD has more time to complete the required transitions before the actual light is produced. This means that the problem is solved for all transitions of pixels (that would normally result in artefacts and/or wrong luminance and or colour point) and that take place before the backlight produces light.
  • Another method to cope with the response time of the panel is to actually take into account the response time of the panel when calculating the required dither scheme. If it is known in advance that a particular pixel transition requires a particular amount of time then it can be calculated what the light will be that is produced by the display system during that transition. Of course this requires that at all times the exact transition times are known. Note that the response time of LCD can change over time and with temperature.
  • the two-level dither has indeed the ability to avoid specific driving signals sent to the LCD by changing the backlight luminance and/or colour point for some or all frames of the dither period.
  • level 8 (rather dark level) with bad viewing angle behaviour
  • level 200 (rather high video level) with better viewing angle behaviour and make sure that the luminance output is still correct by changing the luminance value of the backlight for one or more frames.
  • a two-level dither scheme could introduce flicker on the display system. This is because the luminance intensity of the backlight is modulated from frame to frame and rather large differences between frames are possible. An easy solution to avoid flicker is increasing the frame rate of the display system, but unfortunately this is not always possible. Another solution is to keep the luminance value of the frame more or less constant by inserting a phase difference between the modulations of the different colour components. For instance: in a colour LCD system with a three-frame two-level dither scheme, with luminance intensities of the backlight being L1 for frame 1, L2 for frame 2 and L3 for frame 3, one could in frame one drive the red colour component to luminance value L1, the green to value L2 and the blue to value L3.
  • the luminance intensity of the three colours is normally not the same (green could have higher intensity than red and blue) but the general idea has been described here: by inserting a phase difference or scrambling the modulation scheme for the three colours in a well-chosen way, it is possible to reduce luminance flicker.
  • the same argument is valid for colour flicker: by inserting a phase difference or scrambling the modulation scheme of the three colours in a well-chosen way, it is possible to reduce the colour-point difference (average of the three main colours) between the three frames and therefore reduce colour flicker.
  • Another solution to avoid flicker is to also introduce a spatial shift in the modulation scheme. For instance: if there is a LED backlight or CCFL backlight with multiple elements that emit light, then it is possible to drive in frame one some part of the display area with (local) backlight luminance value L1, and drive other parts of the backlight with respective luminance values L2 and L3.
  • a backlight with LEDs organized in stripes and a two-frame two-level dither scheme one could drive in frame one the upper part of the display with local backlight value L1 and the lower part of display with local backlight value L2 and in the second frame one would then drive the upper part of the display with local backlight value L2 and the lower part of the display with local backlight value L1. This will cause the average luminance over the complete display are to be constant over all frames.
  • Another possible problem with embodiments of the present invention could be the existence of motion artefacts due to the multi-frame dither block. Indeed: if moving objects are shown on the display system then it is possible that flicker and motion artefacts are created because the actual image to be displayed changes in the middle of a "period" of the dither algorithm (temporal moiré artefacts between backlight and
  • a remarkable application of the two-level dither scheme is to improve spatial colour-uniformity on greyscale and/or colour display systems.
  • a greyscale LCD system and there is spatial colour non-uniformity over the display area.
  • the upper part of the display has a higher x-coordinate (colour coordinate) than the lower part of the display.
  • the pixels in the upper part of the display are driven with a higher pixel value in the first frame and a lower pixel value in the second frame, which will correct for the spatial colour non-uniformity in the greyscale display system.
  • this is example is not intended to be limiting; it is just given for clarity.
  • the principle is that by providing frames where the backlight colour and/or luminance is modulated and at the same time the pixel data is modulated, it is possible to improve colour non-uniformity on greyscale display systems. Note that the same principle can be applied to reduce colour non-uniformity on colour display systems. In that case there are even more degrees of freedom so it is easier to find an optimal solution.

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