Method and apparatus for controlling image dimming on a display device

Abstract

Apparatus and methods for controlling image reproduction on a colour display having a pixel array are disclosed. The methods enable global brightness control / dimming of using one of the following: current control with variable gamut lookup table mapping to compensate for the wavelength shift of emitted light, scrolling update duty cycle control, pulse width modulation, and / or active control using a liquid crystal layer.

Description

[0001] Method and Apparatus for Controlling Image Dimming on a Display Device

[0002] The present invention relates to an apparatus and method for controlling image reproduction on a colour display. In particular for providing global dimming in a micro LED display.

[0003] Background lll-V semiconductor materials are of particular interest for semiconductor device design, in particular the family of Ill-nitride semiconductor materials.

[0004] “Ill-V” semiconductors include binary, ternary and quaternary alloys of Group III elements, such as Ga, Al and In, with Group V elements, such as N, P, As and Sb, and are of great interest for a number of applications, including electronics and optoelectronics.

[0005] Of particular interest is the class of semiconductor materials known as “Ill-nitride” materials, which includes gallium nitride (GaN), indium nitride (InN) and aluminium nitride (AIN), along with their ternary and quaternary alloys. (AI n)GaN is a term encompassing AIGaN, InGaN and GaN. Ill-nitride materials have not only achieved commercial success in solid-state lighting and power electronics, but also exhibit particular advantages for quantum light sources and light-matter interaction.

[0006] While a variety of Ill-nitride materials are commercially interesting, Gallium nitride (GaN) is widely regarded as one of the most important new semiconductor materials, and is of particular interest for a number of applications.

[0007] The present invention may be realised primarily using LEDs formed from GaN and InGaN, but may advantageously be applicable to LEDs containing alternative Ill-nitride material combinations.

[0008] It is known that the introduction of pores into bulk Ill-nitrides, such as GaN can profoundly affect its material properties (optical, mechanical, electrical, and thermal, etc.). The possibility of tuning a wide range of material properties of GaN and Ill-nitride semiconductors by altering its porosity therefore makes porous GaN of great interest for optoelectronic applications.

[0009] In a significant improvement over conventional three-colour LED displays, Poro

[0010] Technologies Ltd, the present applicant, has developed variable-wavelength LEDs formed from Ill-nitride semiconductor materials grown over a porous region of Ill-nitride material. Instead of a conventional LED which emits at a single colour, these variable-wavelength LEDs can emit a broad spectrum of different emission wavelengths in response to variations in the driving conditions provided to the variable-wavelength LED. These variable-wavelength LEDs, and their method of manufacture, are disclosed in international patent application no. PCT / GB2022 / 051997, published as W02023 / 007174.

[0011] As the peak emission-wavelength of such variable-wavelength LEDs can be tuned by varying the driving conditions, a single variable-wavelength LED can take the place of multiple “single-colour” subpixels. In particularly preferred embodiments, the same variablewavelength LED can be tuned to emit any one of red, green or blue light by tuning the driving conditions provided to that LED. This means that a colour display can be formed from an array of variable-wavelength LEDs, with each variable-wavelength LED acting as a colour-variable pixel, or alternatively as a colour-variable subpixel in a subpixellated display.

[0012] The ability of variable-wavelength LEDs to emit such a broad range of emission wavelengths makes display driving and calibration processes significantly more complex than they would be for a conventional RGB subpixel display.

[0013] Summary of Invention

[0014] The invention is defined in the independent claims, to which reference should now be made. Preferred or advantageous features of the invention are defined in the appended sub-claims.

[0015] The present invention is preferably carried out with a display device comprising a plurality of variable-wavelength LEDs. In preferred embodiments, such display devices comprise an array of variable-wavelength LEDs, with each variable-wavelength LED forming a pixel of the display device.

[0016] An example of variable-wavelength LEDs usable in the present invention are known as dynamic-pixel-tuning (DPT®) variable-wavelength LEDs from Poro Technologies Ltd, which are disclosed in international patent application no. PCT / GB2022 / 051997, published as WG2023 / 007174.

[0017] A continuum of different emission wavelengths across an emission wavelength range can be emitted directly from a variable-wavelength LED by varying the driving conditions (the magnitude of the driving current and / or driving voltage) supplied to that LED. In CIE xy colour space, the variable-wavelength LED can emit any colour along a continuous curved line of wavelengths. The length and shape of the emittable-wavelengths line in CIE xy colour space is determined by the LED composition and structure, as described in W02023 / 007174.

[0018] One way of controlling the wavelength of light emitted by a variable-wavelength LED would be to provide a continuously-variable driving current to the LED. When the display device receives a signal identifying the target colour to be displayed by a given variablewavelength LED, the driving current to that pixel could then be delivered at whatever magnitude is required for the LED to emit a particular wavelength from its continuum of emittable wavelengths. Downsides of this approach, however, are the difficulty of precisely controlling the variable driving current at all times, and the limitation that the variablewavelength LED could only display chromaticities which are on the continuous line of emission wavelengths directly-emittable by that LED.

[0019] An alternative approach to driving variable-wavelength LEDs is to choose a set of predetermined “primaries” (primary wavelengths) from the continuum of emission wavelengths which are emittable by the variable-wavelength LED. Instead of controlling the emission wavelength to any point across the entire range of emittable colours, a limited set of operating points can be created by selecting a plurality of N primary wavelengths from the range of emittable colours. Each of the N primaries is then used as a pre-determined operating point for the variable-wavelength LED, and the N respective driving conditions required to generate those N primary wavelengths are programmed into a display device incorporating the variable-wavelength LED.

[0020] The N primary wavelengths which are selected from the LED’s range of emittable wavelengths form a palette of N available primaries, so that at any time the variablewavelength LED can be controlled to emit light at one of these N predetermined primary wavelengths. The N available primaries define a colour gamut of a plurality of displayable colours, as any colour within this gamut can be displayed by the variable-wavelength LED by mixing the available primary wavelengths.

[0021] When the display device receives a signal identifying the target colour to be displayed by a given variable-wavelength LED, the device can select, out of the palette of N available primaries, a smaller set of 2 or more primaries which are mixable to render the target colour. Which primaries are selected from the N available primaries will depend on the wavelength of the target colour to be displayed.

[0022] By driving the LED to emit light at only a plurality of “primary” wavelengths instead of any wavelength from the continuum of directly-emittable wavelengths, the display device can display additional chromaticities which are not directly-emittable by the variable-wavelength LED. Primary wavelengths can be mixed using colour-mixing techniques known for conventional displays, such that the overall colour observed by a viewer is a temporal and / or spatial combination of the emitted primaries, rather than a single colour which is emitted “directly” by a single variable-wavelength LED. Driving variable-wavelength LEDs to emit light at predetermined “primary” wavelengths may also advantageously simplify driving requirements, as instead of having to provide a continuously-variable driving current, the display device can be calibrated to deliver a more manageable set of N predetermined driving conditions which generate the N available primaries.

[0023] The number “N” of available primaries can be selected depending on the size of the colour gamut which is desired, and the desired complexity of the control system. The larger the number of N, the larger the displayable colour gamut, but the larger the number of required driving conditions and the more complex the control requirements. As the variablewavelength LED can emit a continuous range of wavelengths, N can be selected to be any number up to infinity (N = infinity being equivalent to continuous driving to any point within the range of emittable wavelengths). N is preferably greater than 3, so that the displayable colour gamut is sufficiently large to render a wide range of colours. Particularly preferably N is greater than or equal to 4, or 5, or 6, to encompass a large colour gamut while keeping the control requirements relatively straightforward. Preferably N may be less than or equal to 8, or 10, or 12, to prevent the control requirements from becoming overly complex.

[0024] In a preferred embodiment of a display device, each variable-wavelength LED is a pixel of the device, and the display device is a field-sequential display. In this embodiment, the same variable-wavelength LED pixel is driven to emit a plurality of discrete primary emission wavelengths in sequential subframes, by driving the LED pixel with the discrete driving conditions which correspond to those primary wavelengths.

[0025] In a sequential-field display, the same variable-wavelength LED may be controlled to emit multiple discrete primary emission wavelengths one after the other, by supplying different driving conditions to the LED in sequential subframes of a display frame. In this way, the same variable-wavelength LED may be a pixel that emits a plurality of selected “primary emission wavelengths” one after another within the duration of a single display frame. During each individual subframe, only one primary emission wavelength is emitted by the pixel. Thanks to the persistence of vision of human observers, the primary wavelengths emitted during sequential subframes temporally-average, so that the resulting colour observed by a person viewing the display at a normal viewing distance is the mixture of the colours emitted during a display frame.

[0026] In a subpixellated display, different primary emission wavelengths can be emitted by separate subpixels. The subpixels may all be variable-wavelength LEDs, or some subpixels may be non-variable-wavelength LEDs.

[0027] The brightness, or greyscale, of the emitted light can be varied by shortening or lengthening the duty cycle (pulse width) of the driving current pulses supplied to the LEDs in the display, which varies the “on time” of the LEDs. In a field-sequential display, the maximum duty cycle is achieved when the driving current is supplied to a variablewavelength LED for 100% of the duration of the display frame, or subframe, allocated to the colour being emitted. While controlling the magnitudes of the driving currents provided to the variable-wavelength LEDs determines the emitted wavelengths, to achieve variable display brightness the duty cycles of the pulses of driving current must also be variable, which adds an additional layer of complexity to device control.

[0028] In Poro Technologies Ltd’s dynamic-pixel-tuning (DPT®) variable-wavelength LEDs, the peak emission wavelength of variable-wavelength LEDs is strongly dependent on the magnitude of the electrical driving signal provided to a given LED, and longer-wavelength emission colours require lower-magnitude driving currents / voltages which inherently produce a lower luminance. This presents a challenge for those situations where a high display brightness is desired, and also a challenge for colour-balance when naturally-bright shorter wavelengths must be mixed with naturally-dimmer longer wavelengths.

[0029] Detailed Description

[0030] Figure 1 illustrates a CIE xy colour space plot 100, on which the wavelengths emittable by a variable-wavelength LED are indicated as a continuous black line 101. By varying the driving conditions, such as the magnitude of the driving current or the driving voltage, provided to the LED, the wavelength of light which is directly emitted by the LED may be tuned to any wavelength along line 101. Figure 2 is a CIE xy colour space plot 200 which illustrates the same emission line 101 as shown in Figure 1 , overlaid with 6 points representing “primaries” P1 , P2, P3, P4, P5, P6. These six primaries are specific emission wavelengths which have been selected from the continuous line of emittable-wavelengths 101. The six respective driving conditions required to be supplied to the variable-wavelength LED to result in these six emission wavelengths can be easily found, and those six driving conditions can be programmed into a display device. During operation, those six driving conditions can be supplied to the variable-wavelength LED, leading to the LED emitting light at any one of the six primary wavelengths. The space bounded by the six primaries is the colour gamut of colours which are displayable using the variable-wavelength LED and these six primary wavelengths.

[0031] By driving the variable-wavelength LED using field-sequential driving, different driving conditions can be supplied to the LED during discrete subframe time periods. Thus, during a first subframe time period, a first driving condition can be supplied to the LED so that the LED emits light at primary wavelength P1 . During a subsequent subframe time period, a different driving condition can be supplied to the LED so that the LED emits light at a different primary wavelength (one of P2-P6). In a preferred embodiment, each display frame contains only two subframes, each of which is assigned to the emission of a different primary. The overall colour which will be observed by an observer viewing the variablewavelength LED will be a temporal average of the two primary wavelengths emitted during the display frame.

[0032] It is possible to divide a display frame into any number of shorter subframes. For example, one display frame could be divided into six subframes, so that each of the six primaries could be emitted one after the other within a display frame. However, the present inventors have found that it is desirable to use only two of the available primaries during a given display frame, as doing this simplifies colour mixing and reduces the power that is consumed in switching between driving conditions. In order to render a given target colour, two primary wavelengths are selected from the available palette of 6 primaries P1 -P6, and the two selected primaries are then mixed proportionally to give a temporal average which appears to an observer to be the target colour.

[0033] As illustrated in Figure 3, the palette of 6 available primaries P1 -P6 can be divided into a plurality of different pairs. Each pair of primaries can then be linearly mixed with one another to display any colour along the straight lines 301 between the two primary wavelengths. As is also evident from Figure 3, however, using a limited set of primaries (N = 6 for example) means that not all colours inside the colour gamut are positioned on a linear mixing line 301 between a pair of primaries. Thus, with such a small palette of available primaries, not all of the colours in the gamut can be formed from only a pair of primaries.

[0034] Display panels formed from variable wavelength pixels, or monochrome (sub)pixels having fixed emission wavelengths will have differing requirements for both greyscale control (controlling the brightness of a given pixel channel) and global brightness control (scaling the brightness of the display panel as a whole). It is desirable for micro LED displays to be able to provide global brightness control while minimising any impact this may have on greyscale control and the ability to mix colours using a plurality of available primaries.

[0035] Global Brightness Control By Current

[0036] For displays formed from monochrome pixels having fixed emission wavelengths, greyscale is typically controlled by pulse width modulation (PWM) and the global brightness may then be set by current. There is typically, a limitation of ~8 bits per pixel due to the data load time associated with the process node, which means that it is not feasible to add a dimming capability by PWM alone.

[0037] It is straightforward, however, to control the dimming range by modulating the applied current to provide a global brightness control. For a micro LED display, this is normally avoid because of a strong interaction between dominant wavelength and applied current - reducing the current to provide dimming also shifts the wavelength emitted, thus affecting the colour reproduction of the display. Advantageously, porous GaN has been found to exhibit a comparatively relatively weak change in the dominant wavelength with current compared to conventional GaN or AllnGaP.

[0038] This wavelength shift is illustrated in the colour space plot 400 of Figure 4. In particular, it can be seen that, for a given current variation, the shift in wavelength 410 for a red LED is much wider than the wavelength shift 420 for a green LED or the wavelength shift 430 for a blue LED. Regardless of the magnitude of the wavelength shift, the impact will be that the colour gamut of the display changes with global dimming level.

[0039] Display devices typically employ a mapping from a source gamut (usually sRGB) to a display gamut. This is often achieved by a 3D look-up table (3D LUT) that is prepopulated with source gamut to display gamut mappings. In the case of micro LEDs, the 3D LUT would need to change with dimming level to compensate for the change in colour gamut at differing dimming levels. Preferably, the corrections to the 3D LUT would ensure that the mapping between source and gamut is at a constant-hue, regardless of the dimming level, such that the change in colour gamut volume does not affect hue appearance.

[0040] Global Brightness Control Using Scrolling Updates

[0041] Alternatively, the global brightness can be controlled by controlling the number of lines of pixels in the array of the display that are “on” simultaneously. This is often referred to as a “scrolling update” where a band of rows (for example) may be switched on and this band may then progress across the array of the display. The fewer rows that are on simultaneously, the shorter the duty cycle of each pixel will be and therefore the dimmer the light output will be for the display.

[0042] This advantageously means that the current supply can be kept constant while pixels are “on”, and therefore that wavelength shift need not be considered. Depending on the resolution of the display, this can provide very granular global brightness control as turning on all lines at the same time will provide a 100% global brightness output whereas having a scrolling band of just a single line turned “on” can provide a global brightness of approximately 0.1%.

[0043] Global Brightness Control For Variable Wavelength LEDs

[0044] Variable wavelength LEDs, such as those described above, typically use PWM to control greyscale and use the current applied to pixels to control the colour of light emitted by each pixel. Global colour channel brightness (and hence the white point) is controlled by the number of rows updated simultaneously. The minimum greyscale time depends upon the number of rows updated simultaneously in a given colour channel.

[0045] In such a configuration, PWM may be used for global brightness control if it is acceptable to reduce the number of levels of greyscale control afforded by the PWM control. The reduction in the level of greyscale control may be compensated for be introducing spatial dithering to synthesise the missing greyscale control. With high quality dithering algorithms, this can be made substantially imperceptible to the human eye. The finite MTF of the projection optics in a display device may also help to hide dithering artifacts by providing a level of blurring. In one example, of a PWM scheme that provides 8 bit (256 levels of) greyscale control, 255 of these levels may be dedicated to providing global brightness control, with a single level (i.e. “off” or “on”) providing greyscale. Using spatial dither, the number of pixels that are on in a given area can be used to control the grayscale brightness of the colour in that area. This can be seen in the comparison between the image of Figure 5, which is rendered using 8 bits of greyscale for each of the red green and blue channels, and Figure 6, which is rendered using 1 bit of greyscale for each of the red green and blue channels. Similarly, the test image of Figure 7 is rendered using 8 bits of greyscale for each of the red green and blue channels, while Figure 8 is rendered using 1 bit of greyscale for each of the red green and blue channels. As the pixel size in a display becomes smaller and smaller, (i.e. the number of pixels per inch increases) the spatial dithering will become less and less perceptible. As an example, displays having pixel sizes of ~1 um are particularly advantageous in such scenarios.

[0046] Active Global Brightness Control For Variable Wavelength LEDs

[0047] A further approach to provide global brightness control for variable wavelength LEDs is to provide active dimming using liquid crystals. Since approximate response times in the order of ~ 100ms would be acceptable for global dimming applications, a ~1mm nematic liquid-crystal (NLC) with crossed polarizers could be added close to the display to provide a variable level of dimming.

[0048] The control of the optical throughput of a LC cell (i.e. its dimming ratio) can be controlled in a simple manner, for example by appropriately setting an applied voltage. However, the use of crossed polarizers in an NLC cell results in a significant reduction in brightness of the display due to the net transmission of the pair of polarizers, which can be expected to be in the region of around 40%.

[0049] An alternative approach would be to use a liquid crystal material that is configured to scatter light, such as a guest-host mode LC cell. Because this brightness control uses scattering rather than polarization control, the addition of crossed polarizers is no longer required and a much higher net transmission can be achieved. The transmission of such scattering devices can be modulated by UV light or by voltage. As an example, the transmittance at reasonable drive voltages may be around 70%, which is a significant improvement over using an NLC cell with polarizers. As the scattered light is effectively wasted energy, active global dimming using a liquid crystal layer could be combined with and supplemented by global dimming control using one of the other techniques described above. For example, in one embodiment 6 bits of liquid crystal dimming could be combined with 2 bits of greyscale dimming.

Claims

CLAIMS:1 . A method for controlling image reproduction on a colour display having a pixel array in which each pixel comprises a plurality of light emitting diodes, the method comprising: receiving pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; controlling a global brightness of the pixel array, based on the global dimming data, by modulating the current magnitude applied to each light emitting diode of each pixel of the pixel array to one of a plurality of current magnitude levels; and processing the colour brightness data for each pixel to produce greyscale data in each of a plurality of colour space primaries for each pixel using one of a plurality of lookup tables, wherein the lookup table of the plurality of lookup tables is selected based on the current magnitude to be applied to each light emitting diode.

2. The method of claim 1 , further comprising controlling a brightness of each light emitting diode in each pixel of the pixel array, based on the greyscale data in each of a plurality of colour space primaries for each pixel, by driving respective light emitting diodes with a pulse width modulated current.

3. The method of claim 1 or 2, wherein the plurality of lookup tables comprise fewer lookup tables than the number of current magnitude levels, and wherein processing the colour brightness data for each pixel to produce greyscale data in each of the plurality of colour space primaries for each pixel further comprises interpolating between two or more of lookup tables.

4. The method of claim 1 or 2, wherein the plurality of lookup tables comprise a lookup table for each of the plurality of current magnitude levels5. The method of any of claims 1 to 4, wherein the plurality of lookup tables are a plurality of 3D lookup tables.

6. The method of any of claims 1 to 4, wherein each lookup table further comprises a sub lookup table for each of the plurality of colour space primaries, and wherein the relevant sub lookup table is further selected based on the colour space primary associated with a given light emitting diode.

7. An apparatus for controlling image reproduction on a colour display having a pixel array in which each pixel comprises a plurality of light emitting diodes, the apparatus comprising: an input configured to receive pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; and a processor configured to control the global brightness of the pixel array, based on the global dimming data, by modulating the current magnitude applied to each light emitting diode of each pixel of the pixel array to one of a plurality of current magnitude levels; the processor further configured to process the colour brightness data for each pixel to produce greyscale data in each of a plurality of colour space primaries for each pixel using one of a plurality of lookup tables, wherein the lookup table of the plurality of lookup tables is selected based on the current magnitude to be applied to each light emitting diode.

8. The apparatus of claim 7, wherein the processor is configured to control a brightness of each light emitting diode in each pixel of the pixel array, based on the greyscale data in each of a plurality of colour space primaries for each pixel, by driving respective light emitting diodes with a pulse width modulated current.

9. The apparatus of claim 7 or 8, wherein the plurality of lookup tables comprise fewer lookup tables than the number of current magnitude levels, and wherein processing the colour brightness data for each pixel to produce greyscale data in each of the plurality of colour space primaries for each pixel further comprises interpolating between two or more lookup tables.

10. The apparatus of claim 7 or 8, wherein the plurality of lookup tables comprise a lookup table for each of the plurality of current magnitude levels11 The apparatus of any of claims 7 to 10, wherein the plurality of lookup tables are a plurality of 3D lookup tables.

12. The apparatus of any of claims 7 to 10, wherein each lookup table further comprises a sub lookup table for each of the plurality of colour space primaries, and the processor is configured to select the relevant sub lookup table based on the colour space primary associated with a given light emitting diode.

13. A method for controlling image reproduction on a colour display having a pixel array in which each pixel comprises at least one light emitting diode, the method comprising: receiving pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; processing the colour brightness data for each pixel to produce greyscale data for each of a plurality of colour space primaries for each pixel; determining a positive integer value n based on the global dimming data; and controlling a brightness of each pixel of the pixel array by, for each colour space primary: iteratively loading and outputting for display the greyscale data for the colour space primary corresponding to a plurality of rows of the pixel array such that n rows of the pixel array are progressively activated to simultaneously output greyscale data for the colour space primary; and further iteratively loading and outputting for display the greyscale data for the colour space primary in any remaining rows of the pixel array such that, for the colour space primary, no more than n rows are activated to simultaneously output greyscale data and each row is activated for the same duration of time.

14. The method of claim 13, further comprising controlling a brightness of each light emitting diode of the pixel array, based on the greyscale data in each of the plurality of colour space primaries for each pixel, by driving respective light emitting diodes using pulse width modulation.

15. The method of claim 14, wherein the respective light emitting diodes are driven by a pulse width modulated current.

16. The method of any of claims 13 to 15, wherein each pixel comprises a plurality of light emitting diodes, the plurality of light emitting diodes being respectively associated with the plurality of colour space primaries.

17. The method of any of claims 13 to 15, wherein each pixel comprises a light emitting diode that has a peak emission wavelength that is dependent on the drive current such that the light emitting diode can reproduce light with a peak wavelength at two or more of the plurality of colour space primaries.

18. The method of claim 17, wherein the light emitting diode is configured to sequentially emit light at the two or more of the plurality of colour space primaries during respective subframes.

19. An apparatus for controlling image reproduction on a colour display having a pixel array in which each pixel comprises at least one light emitting diode, the apparatus comprising: an input configured to receive pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; and a processor configured to process the colour brightness data for each pixel to produce greyscale data for each of a plurality of colour space primaries for each pixel; the processor further configured to determine a positive integer value n based on the global dimming data; and the processor further configured to control a brightness of each pixel of the pixel array by, for each colour space primary: iteratively loading and outputting for display the greyscale data for the colour space primary corresponding to a plurality of rows of the pixel array such that n rows of the pixel array are progressively activated to simultaneously output greyscale data for the colour space primary; and further iteratively loading and outputting for display the greyscale data for the colour space primary in any remaining rows of the pixel array such that, for the colour space primary, no more than n rows are activated to simultaneously output greyscale data and each row is activated for the same duration of time.

20. The apparatus of claim 19, wherein the processor is configured to control a brightness of each light emitting diode of the pixel array, based on the greyscale data in each of the plurality of colour space primaries for each pixel, by driving respective light emitting diodes using pulse width modulation.21 . The apparatus of claim 20, wherein the respective light emitting diodes are driven by a pulse width modulated current.

22. The apparatus of any of claims 19 to 21 , wherein each pixel comprises a plurality of light emitting diodes, the plurality of light emitting diodes being respectively associated with the plurality of colour space primaries.

23. The apparatus of any of claims 19 to 21 , wherein each pixel comprises a light emitting diode that has a peak emission wavelength that is dependent on the drive current such that the light emitting diode can reproduce light with a peak wavelength at two or more of the plurality of colour space primaries.

24. The apparatus of claim 23, wherein the light emitting diode is configured to sequentially emit light at the two or more of the plurality of colour space primaries during respective subframes.

25. A method for controlling image reproduction on a colour display having a pixel array in which each pixel comprises at least one light emitting diode, the method comprising: receiving pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; processing the colour brightness data for each pixel using spatial dithering to produce greyscale data for each of a plurality of colour space primaries for each pixel; and controlling a global brightness of the pixel array, based on the global dimming data, by driving respective light emitting diodes using pulse width modulation.

26. The method of 25, wherein the respective light emitting diodes are driven by a pulse width modulated current.

27. The method of 25 or 26, wherein processing the colour brightness data for each pixel using spatial dithering comprises selecting a respective dithering pattern for each image frame.

28. The method of any of claims 25 to 27, wherein each pixel comprises a plurality of light emitting diodes, the plurality of light emitting diodes being respectively associated with the plurality of colour space primaries.

29. The method of any of claims 25 to 27, wherein each pixel comprises a light emitting diode that has a peak emission wavelength that is dependent on the drive current such thatthe light emitting diode can reproduce light with a peak wavelength at two or more of the plurality of colour space primaries.

30. The method of claim 29, wherein the light emitting diode is configured to sequentially emit light at the two or more of the plurality of colour space primaries during respective subframes.31 . An apparatus for controlling image reproduction on a colour display having a pixel array in which each pixel comprises at least one light emitting diode, the apparatus comprising: an input configured to receive pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; and a processor configured to process the colour brightness data for each pixel using spatial dithering to produce greyscale data for each of a plurality of colour space primaries for each pixel; and to control a global brightness of the pixel array, based on the global dimming data, by driving respective light emitting diodes using pulse width modulation.

32. The apparatus of 31 , wherein the respective light emitting diodes are driven by a pulse width modulated current.

33. The apparatus of 31 or 32, wherein processing the colour brightness data for each pixel using spatial dithering comprises selecting a respective dithering pattern for each image frame.

34. The apparatus of any of claims 31 to 33, wherein each pixel comprises a plurality of light emitting diodes, the plurality of light emitting diodes being respectively associated with the plurality of colour space primaries.

35. The apparatus of any of claims 31 to 33, wherein each pixel comprises a light emitting diode that has a peak emission wavelength that is dependent on the drive current such that the light emitting diode can reproduce light with a peak wavelength at two or more of the plurality of colour space primaries.

36. The apparatus of claim 35, wherein the light emitting diode is configured to sequentially emit light at the two or more of the plurality of colour space primaries during respective subframes.

37. A method for controlling image reproduction on a colour display having a pixel array, in which each pixel comprises at least one light emitting diode, and a liquid crystal cell positioned in front of the pixel array, the method comprising: receiving pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; processing the colour brightness data for each pixel to produce greyscale data for each of a plurality of colour space primaries for each pixel; and controlling the optical transmission characteristic of the liquid crystal cell to control a global brightness of the colour display.

38. The method of claim 37, wherein the liquid crystal cell comprises a nematic liquid crystal layer between crossed polarisers, and the optical transmission characteristic of the liquid crystal cell is controlled by an applied electric or magnetic field.

39. The method of claim 37, wherein the liquid crystal cell is configured to scatter light incident upon the liquid crystal cell.

40. The method of claim 39, wherein the liquid crystal cell is configured in a guest-host mode.41 . The method of claim 39 or 40, wherein the liquid crystal cell comprises a polymer dispersed liquid crystal layer, and the optical transmission characteristic of the liquid crystal cell is controlled by one or more of an applied electric, an applied magnetic field, and / or incident ultraviolet light.

42. The method of any of claims 37 to 41 , further comprising controlling a brightness of each light emitting diode of the pixel array, based on the greyscale data in each of the plurality of colour space primaries for each pixel, by driving respective light emitting diodes using pulse width modulation.

43. The method of claim 42, wherein the respective light emitting diodes are driven by a pulse width modulated current.

44. The method of any of claims 37 to 43, wherein each pixel comprises a plurality of light emitting diodes, the plurality of light emitting diodes being respectively associated with the plurality of colour space primaries.

45. The method of any of claims 37 to 43, wherein each pixel comprises a light emitting diode that has a peak emission wavelength that is dependent on the drive current such that the light emitting diode can reproduce light with a peak wavelength at two or more of the plurality of colour space primaries.

46. The method of claim 45, wherein the light emitting diode is configured to sequentially emit light at the two or more of the plurality of colour space primaries during respective subframes.

47. The method of any of claims 42 to 46, further comprising controlling the global brightness of the colour display by controlling the pulse width modulation of each light emitting diode of the pixel array based on the global dimming data.

48. An apparatus for controlling image reproduction on a colour display having a pixel array, in which each pixel comprises at least one light emitting diode, the apparatus comprising: an input configured to receive pixel data corresponding to an image to be displayed by the pixel array, the pixel data comprising colour brightness data for each pixel, the brightness data further comprising global dimming data for the pixel array; a processor configured to process the colour brightness data for each pixel to produce greyscale data for each of a plurality of colour space primaries for each pixel; and a liquid crystal cell positioned in front of the pixel array and configured to control a global brightness of the colour display by varying the optical transmission characteristic of the liquid crystal cell.

49. The apparatus of claim 48, wherein the liquid crystal cell comprises a nematic liquid crystal layer between crossed polarisers, and the optical transmission characteristic of the liquid crystal cell is controlled by an applied electric or magnetic field.SUBSTITUTE SHEET (RULE 26)50. The apparatus of claim 48, wherein the liquid crystal cell is configured to scatter light incident upon the liquid crystal cell.51 . The apparatus of claim 50, wherein the liquid crystal cell is configured in a guesthost mode.

52. The apparatus of claim 50 or 51 , wherein the liquid crystal cell comprises a polymer dispersed liquid crystal layer, and the optical transmission characteristic of the liquid crystal cell is controlled by one or more of an applied electric, an applied magnetic field, and / or incident ultraviolet light.

53. The apparatus of any of claims 48 to 52, wherein the processor is configured to control a brightness of each light emitting diode of the pixel array, based on the greyscale data in each of the plurality of colour space primaries for each pixel, by driving respective light emitting diodes using pulse width modulation.

54. The apparatus of claim 53, wherein the respective light emitting diodes are driven by a pulse width modulated current.

55. The apparatus of any of claims 48 to 54, wherein each pixel comprises a plurality of light emitting diodes, the plurality of light emitting diodes being respectively associated with the plurality of colour space primaries.

56. The apparatus of any of claims 48 to 54, wherein each pixel comprises a light emitting diode that has a peak emission wavelength that is dependent on the drive current such that the light emitting diode can reproduce light with a peak wavelength at two or more of the plurality of colour space primaries.

57. The apparatus of claim 56, wherein the light emitting diode is configured to sequentially emit light at the two or more of the plurality of colour space primaries during respective subframes.

58. The apparatus of any of claims 53 to 57, wherein the processor is further configured to control the global brightness of the colour display by controlling the pulse width modulation of each light emitting diode of the pixel array based on the global dimming data.

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