Method and device for global gamma control

Abstract

A method and a device for gamma correction of a display device is disclosed. The method uses a curve for gamma correction that has a representative gamma value for each greyscale value, and that varies by greyscale with the gamma value increasing with increasing greyscale. This creates a hybrid gamma curve that advantageously provides an improved level of detail / quality at both the low and high greyscale ranges in comparison to the use of a conventional constant gamma curve, such as Gamma 2.2.

Description

[0001] Method and Device for Global Gamma Control

[0002] The present invention relates to an apparatus and method for gamma correction of a display. In particular for controlling the gamma correction of display devices intended for use in bright ambient conditions, such as an augmented reality device.

[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 using LEDs formed from GaN and InGaN, but may advantageously be applicable to LEDs containing alternative Ill-nitride material combinations. It will be appreciated that the present invention may also be realised using conventional LED display technologies.

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

[0009] 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.

[0010] 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.

[0011] SUMMARY OF THE INVENTION

[0012] The invention is defined in the independent claims, to which reference should now be directed. Preferred or advantageous features of the invention are defined in the appended dependent claims.

[0013] The present invention may be carried out with a display device comprising a plurality of variable-wavelength LEDs. In some embodiments, such display devices comprise an array of variable-wavelength LEDs, with each variable-wavelength LED forming a pixel of the display device.

[0014] 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. 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 colourspace, 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 colourspace is determined by the LED composition and structure, as described in W02023 / 007174.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] In one 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

[0028] Figure 1 is a CIE xy colour space plot showing an example of the wavelengths of light emittable by a variable-wavelength LED; Figure 2 is a CIE xy colour space plot showing an example of the wavelengths of light emittable by a variable-wavelength LED overlaid with example points representing emission wavelength primaries;

[0029] Figure 3 shows the CIE xy colour space plot of Figure 2 with dashed lines showing the linear mixing of pairs of emission wavelength primaries to create a reduced palette of displayable colours;

[0030] Figure 4 is a graph illustrating the emission luminance for a pair of uncalibrated emissive pixels by greyscale level;

[0031] Figure 5 is an illustration of three gamma curves for luminance verses greyscale level;

[0032] Figure 6 is a graph illustrating the emission luminance for the pair of uncalibrated emissive pixels of Figure 4 by greyscale level;

[0033] Figure 7 is a graph illustrating the three gamma curves of Figure 5 as well as two further gamma curves;

[0034] Figure 8 illustrates a method according to a first aspect of the invention for correcting a pixel-to-pixel non-uniformity of a display device comprising a plurality of LED pixels;

[0035] Figures 9A and 9B are example illustrations of a data set of pixel performances for a display device;

[0036] Figure 10A is a graph of normalised luminance across greyscale levels for three pixels;

[0037] Figure 10B is an illustration of the performance of the three pixels of Figure 10A after their brightness levels have been corrected in accordance with the first aspect of the present invention;

[0038] Figure 11 is a graph showing the impact of using black subframes in the correction of pixel brightness data;

[0039] Figures 12A and 12B are example pixel brightness corrections for display devices with a target gamma of Gamma 1 and Gamma 0.5 respectively;

[0040] Figure 13A is an illustration of the brightness sensitivity of the human eye;

[0041] Figure 13B is an illustration of a Gamma 2.2 curve;

[0042] Figures 14A and 14B are illustrations of Gamma 2.2 and Gamma 0.5 curves respectively;

[0043] Figure 15 is an illustration of an example hybrid gamma curve according to a second aspect of the present invention;

[0044] Figure 16 is an illustration of an example hybrid gamma curve according to a second aspect of the present invention in the gamma domain; Figure 17 is a graph showing the hybrid gamma curve of the second aspect of the present invention as a function of luminance verses greyscale level and in comparison to Gamma 2.2 and Gamma 1 curves; and

[0045] Figure 18 illustrates a method according to a second aspect of the invention for adjusting the global gamma of a display.

[0046] DETAILED DESCRIPTION

[0047] Figure 1 illustrates a CIE xy colour space plot 100, on which the wavelengths of light 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 .

[0048] 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.

[0049] 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 one 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. 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 use of only two of the available primaries during a given display frame can be advantageous, 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.

[0050] 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.

[0051] 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.

[0052] Display panels formed from variable wavelength pixels, monochrome (sub)pixels having fixed emission wavelengths, or indeed conventional pixel technologies, will include many different individual pixels. Micro LED displays can be expected to have an emitter-to- emitter variation in intensity - this may be due to variation in the micro LED device performance across the wafer, variations in the transistor thresholds across the display, or other manufacturing tolerances. This variation occurs from emitter-to-emitter (i.e. pixel-to- pixel), which causes a pixel-to-pixel non-uniformity across the display, for example these individual pixels may have differing brightnesses when driven by the same drive signal. This variation will lead to a noisy-looking display that cannot accurately reproduce images (whether colour or monochrome).

[0053] As such, it is desirable to calibrate the display in some manner to correct this pixel-to-pixel variability. Calibrating micro LED panels by correcting micro LED intensity variation is a process referred to as “demura”. However, this is traditionally a correction for low spatial- frequency variation across an LCD panel. The corrections required for micro LED displays may be expected to have a very high spatial frequency due to the use of individual emitters rather than backlight emitters.

[0054] For conventional (i.e. single-color) micro LEDs, there are two possible methods to compensate for intensity variations, namely: 1) varying the pixel current and 2) varying the pixel duty cycle. Method (1) is difficult because it requires very accurate (around microampere (uA)) current control of the micro LED current on a per-pixel basis. As such, method (2) is generally preferred because then only the greyscale values that are sent to the display are required to be changed.

[0055] For displays formed from monochrome pixels having fixed emission wavelengths, greyscale is typically controlled by pulse width modulation (PWM). 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. This means that the 8 bits of data commonly produce a greyscale range having 256 steps, i.e. a relative brightness from 0 (i.e. the pixel is off) up to 255 (i.e. the pixel is being driven at maximum brightness). These steps may be referred to as greyscale levels, and the shorthand GLO to GL255 will be used herein to refer to these greyscale levels.

[0056] As discussed above a pair of pixels in the display may emit a different amount of light when driven by a drive signal corresponding to GL255. This would reduce the accuracy with which the display can reproduce an input image and so the display panel of LED pixels are typically calibrated in order to overcome this defect. The conventional method for adjusting the brightness of individual pixels to calibrate the display is to first measure a relative intensity non-uniformity uxyfor the array of pixels in the display using a camera, then a correction term u'xymay then be evaluated to linearly compensate for the non-uniformity uxyof the display panel. An image, Ixy, to be displayed on the display panel would then be processed to produce a new image I'xy, based on the following equations, with this new image being sent to the display for reproduction: I xy ~ ^xy ’uxy

[0057] Since the panel brightness is reduced from the maximum value, the global image brightness is reduced by the most non-uniform pixel, i.e. by the factor min (uxy). The min (uxy) factor could be represented by a demura look up table that sets out the appropriate greyscale to drive each individual pixel at for any given input greyscale level. The global gamma of the display could then be calibrated as desired by the manufacturer or end user.

[0058] However, intensity compensation by greyscale control has a number of drawbacks. For example, as discussed, the global brightness has to be reduced by the most non-uniform pixel, i.e. by the factor min . This is undesirable and may be particularly problematic for implementations of displays intended for use in augmented reality devices / systems where high brightness levels are important for overcoming ambient lighting conditions. Also, the greyscale dynamic range is reduced from [0; 255] to [0; 255 ■ min ], as will be explained in greater detail below.

[0059] Figure 4 is a graph illustrating the emission luminance for a pair of uncalibrated emissive pixels by greyscale level. The graph has been normalised according to the luminance of pixel 1 , and accordingly a plot line 410 is shown between a luminance of 0 and 1 for pixel 1 . Pixel 2 is 20% brighter than pixel 1 , and so has a plot line 420 from a luminance of 0 to 1 .2. A dashed line is shown to illustrate that driving both pixel 1 and pixel 2 at a greyscale of GL160 would result in a difference in their respective luminance of AL. In order for the two pixels to emit the same luminance, the brighter pixel 2 could be driven at a greyscale level that is AGL less than that of pixel 1 . However, this type of greyscale correction would effectively cap the total brightness output of pixel 2 as well as reducing the total number of greyscale levels between the maximum and minimum (i.e. GLO) brightnesses for pixel 2, which will make it more difficult for pixel 2 to provide an accurate representation of an input image pixel.

[0060] Gamma Correction

[0061] Gamma describes the relationship between a colour value and its brightness as emitted by a display panel. For images described in an RGB colour space to appear visually correct, the display panel should generate an output brightness that varies linearly with the colour value of the input image / pixel. For most displays, a gamma correction a technique is required to compensate for non-linear characteristics of a display panel in order to provide this linear relationship. Gamma correction uses a correction function that is tailored to the characteristics of the display panel to adjust the values of input pixel data for before they are sent to the display panel itself. This mapping function is often implemented using a lookup table, and typically a separate table is used for each of the different colour components in an RGB display.

[0062] Three gamma curves are shown in Figure 5, which is a plot 500 of luminance vs greyscale level for gamma curves 510, 520, and 530. Plot 510 is a straight line from 0 to 1 and corresponds to a “curve” of Gamma 1 . Plot 520 is a curve corresponding to a gamma greater than 1 , for example Gamma 2.2, and plot 530 is a curve corresponding to a gamma less than 1 , for example Gamma 0.6. As can be seen from the plot 500, each of the three different gamma curves will result in a different luminance value for the same greyscale level. The inventors have appreciated that this feature of gamma curves could be used to calibrate the brightness of a / each pixel in a display without the need to use greyscale reduction / scaling.

[0063] Figure 6 is a graph 600 illustrating the emission luminance for the pair of uncalibrated emissive pixels of Figure 4 by greyscale level. Plot lines 410 and 420 have been reproduced, and a further curve 610 has been added to illustrate a correction of plot line 410 using a gamma curve of Gamma 0.6, i.e. curve 610 shows the luminance of pixel 1 when corrected by the Gamma 0.6 curve. As can be seen, the maximum brightness of pixel 1 remains the same, but the brightness initially increases faster with greyscale level such that the brightness of pixel 1 at GL160 matches that of the inherently brighter pixel 2 at GL160 (and Gamma 1). In this manner, the two pixels can be calibrated to remove the non-uniformity at a greyscale level GL160. Plots 420 and 610 are closer together than plots 420 and 410 for a given range of greyscale values, and accordingly the AGL would be lower using gamma correction than using the conventional greyscale correction across this range while maintaining higher average luminance values. The full range of 256 greyscale levels has also been maintained for each of the pixels, thus improving the greyscale reproduction accuracy of the pixels in comparison to conventional methods.

[0064] Display devices are made up of many different pixels and accordingly a wide variety of different pixel performances / maximum pixel brightnesses can be expected across the extent of a display panel. In one embodiment, one pixel may be identified as being the target pixel, and gamma curve corrections may be determined and fine-tuned for the remaining pixels in the display panel to bring their performance closer into line with that of the target pixel. It will be appreciated that storing and processing m ■ n individual (and potentially unique) gamma curve corrections for each pixel of an m x n array of pixels would be resource intensive; however, the present inventors have appreciated that the desired gamma curve corrections for each pixel could instead be constructed from a reduced set of gamma correction curves, or combinations thereof.

[0065] Figure 7 is a graph 700 illustrating the three gamma curves 510, 520, and 530 of Figure 5 as well as two further gamma curves 710 and 720. Plot 710 has been defined as the combination of (GammaA + GammaA + GammaB) / 3, while plot 720 has been defined as the combination of (GammaB + GammaC + GammaC) / 3. In this manner, the three curves GammaA, GammaB, and GammaC have been used to produce a set of five different gamma curves that could be used for gamma calibration. It will be appreciated that, depending on the resources available and the specific implementation, a desired number of individual gamma curves could be selected and the resulting gamma combinations identified. In general, the gamma curve for the first pixel in the array of the display could be defined as: where A + B + C = 1. While combinations of three gamma curves have been illustrated above, it will be appreciated that different numbers of gamma curves could be selected for each combination, and indeed that the individual gamma curves could be weighted either unequally or equally.

[0066] The present inventors have appreciated that an efficient means for combining a plurality of different individual gamma curves for a pixel in a display would be to divide the pixel output frame into a plurality of subframes, with an individual gamma curve being used to correct the input greyscale level for that pixel for that frame during each of the subframes. In this manner, a gamma correction according to plot 720 could be achieved by dividing the frame into three subframes and correcting the pixel’s input greyscale level with a GammaB curve during the first subframe, and then correcting the pixel’s input greyscale level with a GammaC curve during the second and third subframes. When the pixel frame is observed by the human eye, the three subframes will be averaged in the time domain into a single frame having the combined gamma correction, which may be referred to as a multi-gamma brightness integration. It will be appreciated that for some pixels, the gamma correction curve may be represented by the same individual / constituent gamma curve in each of the plurality of subframes if appropriate.

[0067] To identify the appropriate combination of gamma curves to correct each pixel, an algorithm may be executed to solve for the gamma combination that provides the best fit for that pixel across the greyscale range, potentially according to one or more boundary conditions. For example, if certain ranges of the greyscale spectrum are expected to be most commonly used in a particular application, then the fitting algorithm can be tuned to prioritise this range rather than trying to fit across the whole greyscale spectrum / range. Similarly, the permissible percentage of brightness deviation for each greyscale level may be defined as a boundary condition for the fitting algorithm.

[0068] Figure 8 illustrates a method 800 for correcting a pixel-to-pixel non-uniformity of a display device comprising a plurality of LED pixels. In a first step 810, brightness data is received for each pixel of the plurality of LED pixels of the display device when driven by one or more common drive signals. This brightness data for each pixel may be obtained by imaging / capturing an image of the plurality of LED pixels of the display device using a high resolution camera. The image may be captured when the pixels are being driven at a single common greyscale level, for example GL255 for the maximum brightness of each pixel, or a plurality of images may be captured for a respective number of steps along the greyscale range. In one example, an image is captured of the pixels at every greyscale level such that the brightness data includes the brightness of every pixel at every greyscale level.

[0069] At step 820, a target brightness characteristic is identified based on the brightness data, where the brightness characteristic defines the brightness of a target pixel of the plurality of LED pixels when driven by the one or more common drive signals. In conventional methods, this target pixel would have been the pixel having the lowest brightness output / performance; however, in the first aspect of the present invention the target pixel is preferably one that performs better than that having the lowest brightness output. In one example, the brightness characteristic may correspond to the brightnesses of the group of pixels having the most common brightness data, i.e. the biggest group of pixels that have similar brightness data / brightness performance across the greyscale range. This is advantageous because it minimises the number of pixels that need correcting whilst also avoiding the conventional technique of scaling down to the lowest performance common denominator.

[0070] An example illustration of a data set of pixel performances is shown in Figure 9A, with the corresponding normalised distribution of pixel performance being illustrated in Figure 9B. The dashed line in Figure 9B shows the largest group of pixels having similar brightness performance that is selected as the “target pixel”. The remaining pixels are adjusted to converge upon the performance of these target pixels as closely as possible. In step 830, a gamma correction is determined for each pixel having brightness data that does not match the target brightness characteristic. The gamma corrections are configured to alter the brightness data of the respective pixels (that are not target pixels) to reduce the disparity between their brightness data and the target brightness characteristic.

[0071] In one example, the brightness data of the pixels is normalised to that of the target pixel, or group of target pixels, and an algorithm may be configured to iteratively refine the combinations of gamma curves for each pixel until a desired plot of pixel brightness against greyscale level is achieved. The thresholds for this iterative refinement may be controlled by compensation criteria / the boundary conditions of the refinement algorithm.

[0072] In one example, criteria may define a greyscale range over which the fitting algorithm is configured to operate as a boundary condition, i.e. the fitting thresholds are only applied to brightness values within that grayscale range. This may be useful for example if lower brightness parts of the greyscale range may be of limited importance due to high levels of ambient light in a particular use case, which would make this greyscale range hard to make out. Alternatively or additionally, a criterion may define a maximum acceptable percentage difference between the brightness of the target pixel at a given greyscale level and the brightness of the pixel (for which the gamma correction is being calculated) at the given greyscale level.

[0073] In this manner, the fitting algorithm divides the frame of data for each pixel having brightness data that does not match the target brightness characteristic into the plurality of subframes and iteratively refines the gamma curves for each of the subframes until each of the one or more criteria have been satisfied by the resultant gamma correction. Then at step 840 the respective gamma corrections are stored for each pixel so that they can later be used for operating on input image pixel data to be displayed by the display device. This may be stored in a lookup table for the display device, which may be referred to as a demura factor table.

[0074] From the above method, a standard brightness performance level has been defined that is above the performance of the pixel with the minimum brightness levels and those pixels that have performance levels below the standard brightness performance level have been gamma corrected to raise their brightness in the middle to high greyscale range, thus converging their performance in this range with that of the standard performance level. In this manner, the global brightness of the display is maintained in comparison to the conventional techniques for demura and the conventional reduction in greyscale range has been avoided for a large number of the pixels in the display device. However, for pixels that have a performance level above (which may be by a certain percentage level above, for example this may be 10% in one example) the standard brightness performance level a degree of greyscale reduction may still be required.

[0075] Accordingly, for pixels having a higher performance in this range, a greyscale compensation value may be defined and stored in the demura factor lookup table for those pixels of the display device. This greyscale compensation value may be an absolute greyscale reduction, or it may be a percentage reduction to scale the greyscale level. As such, for the pixels in this performance range the greyscale level to be output by the pixel may be the input greyscale level, adjusted by the gamma correction (for example in the multiple subframes), and with each of the frames / subframe greyscale levels being reduced by the greyscale compensation value.

[0076] Figure 10A is a graph of normalised luminance across greyscale levels for three pixels, the pixel represented by curve 1010 is the target pixel and has a maximum luminance of L, the pixel represented by curve 1020 has a higher performance with a maximum luminance of 1 .2L, and the pixel represented by curve 1030 has a lower performance with a maximum luminance of 0.9L. In this example the display is calibrated globally to Gamma 2.2 as can be seen from the respective curves. Figure 10B is an illustration of the performance of the three pixels of Figure 10A after their brightness levels have been corrected in accordance with the first aspect of the present invention. In particular, the pixel with a maximum luminance of 1 .2L is now represented by curve 1025, and the pixel with a maximum luminance of 0.9L is now represented by curve 1035. As can be seen from the results, the performance of the 0.9L and 1 .2L pixels has converged with that of the target pixel for the majority of the greyscale range. In the highest greyscale ranges, the performances will continue to diverge to some extent since the maximum brightness of the 0.9L pixel cannot be increased. However, it can be seen that the maximum brightness of the 1 .2L pixel has been reduced to approximately 1 .1 L, for example by greyscale correction using a greyscale compensation value as above.

[0077] In an example where the gamma correction is implemented using three subframes, this greyscale compensation value may be implemented for pixels having a performance that is approximately 10% to 30% higher than that of the defined standard brightness performance level. In this example, for any pixels that have a performance that is more than 30% higher than that of the defined standard brightness performance level, it may be desirable for the pixel to be turned off for one of the subframes. It will be appreciated that this would lead to a 33% reduction in the luminance output of the pixel. For example, a pixel with an intrinsic maximum brightness of 1 .35L in comparison to the target pixel would then have a maximum brightness output of 0.9L. This is illustrated in Figure 11 , which shows a graph of normalised luminance across greyscale levels for a target pixel represented by curve 1110 and a high performing pixel represented by curve 1120, with curve 1125 representing the high performing pixel implemented with three subframes in which one of these is a black / blank subframe.

[0078] It will be appreciated that the threshold at which the use of a blank / black subframe (e.g. during which the pixel in question is driven at a greyscale level of GLO and thus is turned off) may become desirable will depend on the number of subframes each frame is divided into for any given implementation of the above invention. In one example, turning the pixel off in this manner could be achieved by selecting a constituent gamma curve of Gamma 0 for that subframe.

[0079] In the above description, gamma corrections have been discussed for individual pixels. It will be appreciated that in certain implementations, it may be desirable to apply the analysis and resulting gamma corrections to a group of neighbouring pixels, for example a block of pixels, of the display device based on their averaged brightness data. While this would reduce the accuracy of the correction / calibration, it would also reduce the processing required and may be suitable in implementations having a particularly high density of pixels for example.

[0080] While the above describes the use of gamma curves at the pixel level to correct pixel-to- pixel non-uniformity, it will be appreciated that display devices are often calibrated on a global level with a global target gamma curve (which may differ for each of the colour channels in a colour display). In such a use case, the group of target pixels would be calibrated with the target gamma curve and the processing of the other pixels in the display device would be to converge upon the performance of the target pixels as processed by the target gamma curve. As such, the gamma correction determined for each pixel having brightness data that does not match the target brightness characteristic would additionally be based on the target gamma curve for the display device. In some of the above examples, a gamma of Gamma 2.2 has been illustrated. Figure 12A illustrates a graph of normalised luminance across greyscale levels for a group of pixels that have been calibrated globally to a target of Gamma 1 . Line 1210 represents the target pixel and has a maximum luminance of L. Line 1220 represents a pixel with a lower performance having a maximum luminance of 0.9L that has been processed by using the subframe gamma correction methods describes above. Line 1230 represents a pixel with a higher performance having a maximum luminance of 1 .2L that has been processed by using the subframe gamma corrections methods described above in combination with greyscale reduction using a greyscale compensation value. As can be seen the first aspect of the present invention also advantageously reduces pixel-to-pixel non-uniformity across a wide range of greyscale levels for a Gamma 1 output.

[0081] Figure 12B illustrates a graph of normalised luminance across greyscale levels for a group of pixels that have been calibrated globally to a target of Gamma 0.5. Line 1250 represents the target pixel and has a maximum luminance of L. Line 1260 represents a pixel with a lower performance having a maximum luminance of 0.9L that has been processed by using the subframe gamma correction methods describes above. Line 1270 represents a pixel with a higher performance having a maximum luminance of 1 .1 L that has been processed by using the subframe gamma corrections methods described above in combination with greyscale reduction using a greyscale compensation value. As can be seen the first aspect of the present invention also advantageously reduces pixel-to-pixel non-uniformity across a wide range of greyscale levels for a Gamma 0.5 output.

[0082] Global Gamma Configuration

[0083] The following discussion addresses control of the target gamma for the display as a whole rather than pixel non-uniformities.

[0084] Display devices are often calibrated to Gamma 2.2, and this is due to the relationship between real brightness and the brightness perceived / sensed by a human eye. Figure 13A illustrates a plot 1300 of the variation in brightness sensed by a human eye as a function of the real brightness of a light source. An area 1310 of the plot 1300 in which the curve has a steep gradient has been highlighted. This area 1310 represents the high sensitivity of the human eye to relative changes in brightness a low brightness levels. This means that very small changes in the brightness of an object can be perceived by the observing human in this area 1310. By contrast, at high brightness levels the slope reduces and it will take a larger increase in real brightness for the brightness change to be sensed / perceived by the observing human.

[0085] As discussed above, greyscale levels for displays are communicated using a limited amount of data for each pixel, and this is commonly 8 bits of data to provide 256 levels of dimming, i.e. from 0 to 255. In order to make the most effective use of this data, the real brightness increment between greyscale levels is configured to be smaller at lower brightness levels and larger by comparison at high brightness levels. This is achieved by using a gamma curve such as Gamma 2.2 as illustrated in the plot 1350 of Figure 13B.

[0086] The use of Gamma 2.2 for a display device presumes that the display device is the primary source of brightness being observed by the user, which effectively assumes that the display device is being observed in a relatively dim environment. For some use cases of display technology, for example for augmented reality devices, the user may be in bright environments, such as outdoors on a sunny day, for a significant duration of use. In such bright environments, the user’s eyesight will adjust based on the ambient lighting and it will be difficult for the user to see the fine steps of brightness at lower levels that Gamma 2.2 correction is usually directed to.

[0087] In such use cases the brightness of the display will need to be above a minimum threshold brightness in order to compete with the ambient light levels and the detail of image content in areas of the display having a brightness lower than this threshold will be lost / difficult for the user to perceive. For outdoor applications, this minimum brightness threshold may be taken to be around 1 k nits; however, it will be appreciated that this will depend on the prevailing weather conditions.

[0088] For augmented reality applications, the light emitted by the display’s light source is directed through a waveguide before reaching the surface on which the content is viewed by the user. If an efficiency of this optical system is assumed to be around 1%, then the brightness that will be viewable for the user for a display having a light source of around 0.8 million to 3 million nits would be around 8k to 30k nits (depending on the original brightness of the light source).

[0089] Taking the example of an 8k nit maximum brightness viewable by a user, the bottom 12.5% of the luminance generated by the display would be below the 1 k nit threshold and thus difficult to perceive by the user. If the display was calibrated using Gamma 2.2 and an 8 bit colour depth, then this would correspond to 100 levels of greyscale being difficult to perceive and conversely meaning that only around 61% of the greyscale range was usable. While this can be mitigated by using a brighter display, this example of a 30k nit maximum brightness display would still result in only around 78.5% of the greyscale range being usable in effect (the bottom 3.3% of the luminance generated by the display would be below the 1 k nit threshold and this would correspond to 55 levels of greyscale).

[0090] From the above it can be seen that while traditional Gamma 2.2 correction of displays is well matched for human eyesight in low brightness conditions, it suffers severe grayscale loss in high ambient brightness conditions, such as when using augmented reality devices outdoors. Severe greyscale loss in this context is intended to mean that a large number of greyscale levels at the bottom end of the scale will simply not be perceptible to the user due to the significant amount of ambient light.

[0091] Figure 14A illustrates a Gamma 2.2 curve and Figure 14B illustrates a Gamma 0.5 curve side-by-side. Box 1410 in Figure 14A highlights the low slope section of the Gamma 2.2 at low greyscale levels - as discussed above, this would cause a large number of grayscale levels in this range to fall below the minimum brightness threshold and be difficult for a user to perceive.

[0092] The Gamma 0.5 curve has the benefit of a fast increase in brightness level at the low end of the grayscale range, which means that fewer greyscale levels will be used up prior to exceeding the minimum brightness threshold. However, the Gamma 0.5 curve increases brightness very slowly in the high end of the greyscale range as shown in box 1420. Since the human eye perceives comparatively small changes in luminosity across this greyscale range, it may not be possible for the user to distinguish between several adjacent greyscale levels in this high brightness range and thus this would also be an inefficient use of the available greyscale.

[0093] It has been appreciated by the inventors that a more effective use of the 8-bit bit depth could be achieved by using hybrid gamma curves. In one embodiment, the hybrid gamma curve may be designed to have a larger brightness / luminance increase per greyscale level (or to put it another way, a brightness level that raises faster) at the low end of the greyscale range, while the gamma curve behaves in a similar manner to a traditional gamma curve (such as Gamma 2.2) above a given minimum brightness (i.e. within the brightness range that is expected to be perceptible to the user in the outdoor environment, which may be referred to as the effective brightness range).

[0094] In the example curve 1510 shown in the graph of Figure 15, a display system with a maximum 8k nit brightness viewable by the user would exceed the 1 k nit threshold at GL36 meaning that around 86% of the greyscale range would be usable, which is a significant increase in comparison to the corresponding 61% for Gamma 2.2. This results in the effective brightness range being that shown by the dashed box 1520. In the example of a display system with a maximum 30k nit brightness viewable by the user, this would increase to 98.8% of the greyscale range being usable as the 1 k nit threshold would be exceeded at just GL3. This significantly increases the granularity of the greyscale control within the effective brightness range. In turn, this means that an improved level of detail / quality can be provided at both the low and high greyscale ranges in comparison to the use of a conventional constant gamma curve like Gamma 2.2.

[0095] While the curve 1510 of Figure 15 is referred to here as a hybrid gamma curve, it will be appreciated that it is not a traditional gamma curve as the gamma value that could be associated with each greyscale level changes as the greyscale levels progress. Figure 16 shows a graph that illustrates the curve 1510 of Figure 15 in the gamma domain, i.e. the curve 1510 is expressed in terms of an equivalent gamma value for the points on the curve as a function of greyscale. As can be seen, this example hybrid gamma curve 1510 transitions from being equivalent to a Gamma 0.7 curve at low greyscale values, to being equivalent to a Gamma 2.5 curve as the greyscale level progressively increases to high greyscale values.

[0096] The graph of Figure 16 has been divided into three sections or regions, a low greyscale region (illustrated as greyscale levels GL1 to GL32), a middle greyscale region (illustrated as greyscale levels GL32 to GL192), and a high greyscale region (illustrated as greyscale levels GL192 to GL255). As illustrated in the graph for the example curve 1510, the rate of change in the gamma value stabilises at about GL5 in the low greyscale region and increases with a slope of roughly 0.0097. In the middle greyscale region this rate of increase slows to a slope of roughly 0.0073, and then in the high greyscale region this rate of increase slows again to a slope of roughly 0.0051 . The inventors have found that hybrid gamma curves in this form are particularly effective as a global gamma correction for display devices that are intended for use in bright environments, such as augmented reality display systems. In a display device, this hybrid gamma curve can be achieved by splitting a sampling frame of brightness data into subframes and using different gamma curves for each of the subframes as discussed above in relation to the gamma correction method. In the example where the method splits a frame into three subframes, the hybrid gamma curve could be expresses as:

[0097] GammaAGammaBGammachybrid gamma curve = - - - 1 - - - 1 - - -

[0098] Alternatively, the hybrid gamma curve could be achieved by using high resolution (e.g. more than 16-bit) PWM output control. In both cases, the result is that as the greyscale level changes from GLO to GL255, the effective gamma of the hybrid curve changes (rather than remaining at a constant gamma value as in a conventional gamma curves). Figure 17 is a graph showing the hybrid gamma curve 1510 as a function of luminance verses greyscale level, and in comparison to a Gamma 2.2 curve 1720 and a Gamma 1 curve 1730. As can be seen, the behaviour of the hybrid gamma curve 1510 closely follows that of the Gamma 2.2 curve 1720 in the higher greyscale levels, but provides improved brightness for details in the lower and middle greyscale ranges. In this manner, the hybrid gamma curve of the present invention provides more detail at low greyscale ranges than a conventional Gamma 2.2 curve while also providing more detail at high greyscale ranges than a conventional Gamma 0.5 curve or a conventional Gamma 1 curve.

[0099] Figure 18 illustrates a method 1800 for controlling the brightness of an image signal according to the second aspect of the present invention. In a first step 1810, image signal brightness data is received at a brightness controlling means. Then in step 1820, the image signal brightness data is processed by the brightness controlling means using a curve for gamma correction, the curve having a gamma value that varies by greyscale, and the gamma value increasing with increasing greyscale. Then in step 1830, image brightness data is output to a display device by the brightness controlling means.

[0100] Embodiments of the first aspect of the present disclosure may be described with reference to the following numbered clauses, with preferred features laid out in the dependent clauses:

[0101] Clause 1 . A method for correcting a pixel-to-pixel non-uniformity of a display device comprising a plurality of LED pixels, the method comprising: receiving brightness data for each pixel of the plurality of LED pixels of the display device when driven by one or more common drive signals; identifying a target brightness characteristic based on the brightness data, the brightness characteristic defining the brightness of a target pixel of the plurality of LED pixels when driven by the one or more common drive signals; determining a gamma correction for each pixel having brightness data that does not match the target brightness characteristic, each gamma correction being configured to alter the brightness data of the respective pixel to reduce the disparity between the respective pixel’s brightness data and the target brightness characteristic; and storing the respective gamma correction for each pixel for operating on input image pixel data to be displayed by the display device.

[0102] Clause 2. The method of clause 1 wherein each determined gamma correction is formed from a combination of a plurality of constituent gamma curves.

[0103] Clause 3. The method of clause 2 wherein each of the plurality of constituent gamma curves are for operating on the input image pixel data during a respective subframe of a plurality of subframes to provide the gamma correction, the plurality of subframes representing a frame during which the input image pixel data is to be displayed by the pixel of the display device.

[0104] Clause 4. The method of clause 3 wherein each subframe lasts for an equal duration of time.

[0105] Clause 5. The method of clause 3 or 4 wherein there are three subframes in each frame.

[0106] Clause 6. The method of any one of clauses 2 to 5 wherein the plurality of constituent gamma curves are selected from a set of n gamma curves, where n is less than the number of pixels.

[0107] Clause 7. The method of any one of clauses 1 to 6 wherein the target pixel is selected based on the that target pixel having the most common brightness data.

[0108] Clause 8. The method of any one of clauses 1 to 7 wherein, for pixels having brightness data that is a first percentage range above the target brightness characteristic, the method further comprises determining and storing a greyscale compensation value for operating on input image pixel data to reduce the greyscale level for those pixels by the greyscale compensation value.

[0109] Clause 9. The method of any one of clauses 3 to 8 wherein, for pixels having brightness data that is a second percentage above the target brightness characteristic, the method causes the respective pixel to be turned off for one or more subframes of the plurality of subframes.

[0110] Clause 10. The method of clause 9 wherein the second percentage is based on the number of subframes that represent a frame.

[0111] Clause 11 . The method of clause 9 or 10 wherein the respective pixel is turned off by selecting a constituent gamma curve of Gamma 0 for that subframe.

[0112] Clause 12. The method of any one of clauses 1 to 11 wherein the gamma correction for each pixel having brightness data that does not match the target brightness characteristic is determined by a fitting algorithm based on one or more criteria.

[0113] Clause 13. The method of clause 12 wherein the one or more criteria define a boundary condition greyscale range over which the fitting algorithm is configured to operate.

[0114] Clause 14. The method of clause 12 or 13 wherein the one or more criteria define a maximum acceptable percentage difference between the brightness of the target pixel at a given greyscale level and the brightness of the pixel in question at the given greyscale level.

[0115] Clause 15. The method of any one of clauses 12 to 14 wherein the fitting algorithm iteratively refines the gamma correction for each pixel having brightness data that does not match the target brightness characteristic until each of the one or more criteria have been satisfied.

[0116] Clause 16. The method of any one of clauses 1 to 15 wherein the gamma correction determined for each pixel having brightness data that does not match the target brightness characteristic is based on a target gamma curve for the pixels of the display device. Clause 17. The method of any one of clauses 1 to 16 wherein storing the respective gamma correction for each pixel comprises storing a lookup table for each pixel.

[0117] Clause 18. The method of any one of clauses 1 to 17 wherein a determined gamma correction may apply to a group of neighbouring pixels of the display device based on their averaged brightness data.

[0118] Clause 19. The method of any one of clauses 1 to 18 wherein the brightness data for each pixel of the plurality of LED pixels of the display device is received from a high resolution camera configured to image the plurality of LED pixels of the display device.

[0119] Clause 20. A display device comprising a plurality of LED pixels and a processing unit, the processing unit being configured to correct a pixel-to-pixel non-uniformity by: retrieving a respective stored gamma correction for each pixel of the plurality of LED pixels of the display device; receiving input image data comprising input image pixel data for each pixel; and operating on input image pixel data with the gamma correction retrieved for the respective pixel; wherein the respective gamma corrections for each pixel are determined for pixels having brightness data that does not match a target brightness characteristic, each gamma correction being configured to alter brightness data of the respective pixel to reduce a disparity between the respective pixel’s brightness data and the target brightness characteristic when driven by one or more common drive signals; and wherein the brightness characteristic defines the brightness of the target pixel of the plurality of LED pixels when driven by the one or more common drive signals.

[0120] Clause 21 . The display device of clause 20 wherein each determined gamma correction is formed from a combination of a plurality of constituent gamma curves.

[0121] Clause 22. The display device of clause 21 wherein each of the plurality of constituent gamma curves are configured to operate on the input image pixel data during a respective subframe of a plurality of subframes to provide the gamma correction, the plurality of subframes representing a frame during which the input image pixel data is to be displayed by the pixel of the display device. Clause 23. The display device of clause 22 wherein each subframe lasts for an equal duration of time.

[0122] Clause 24. The display device of clause 22 or 23 wherein there are three subframes in each frame.

[0123] Clause 25. The display device of any one of clauses 21 to 24 wherein the plurality of constituent gamma curves are selected from a set of n gamma curves, where n is less than the number of pixels.

[0124] Clause 26. The display device of any one of clauses 20 to 25 wherein the target pixel is configured to be selected based on the that target pixel having the most common brightness data.

[0125] Clause 27. The display device of any one of clauses 20 to 26 wherein, for pixels having brightness data that is a first percentage range above the target brightness characteristic, the greyscale level of the input image pixel data for those pixels is configured to be reduced by a greyscale compensation value.

[0126] Clause 28. The display device of any one of clauses 22 to 27 wherein, for pixels having brightness data that is a second percentage above the target brightness characteristic, the display device is configured to cause the respective pixel to be turned off for one or more subframes of the plurality of subframes.

[0127] Clause 29. The display device of clause 28 wherein the second percentage is based on the number of subframes that represent a frame.

[0128] Clause 30. The display device of clause 28 or 29 wherein the respective pixel is configured to be turned off by selecting a constituent gamma curve of Gamma 0 for that subframe.

[0129] Clause 31 . The display device of any one of clauses 20 to 30 wherein the gamma correction for each pixel having brightness data that does not match the target brightness characteristic is configured to be determined by a fitting algorithm based on one or more criteria. Clause 32. The display device of clause 31 wherein the one or more criteria define a boundary condition greyscale range over which the fitting algorithm is configured to operate.

[0130] Clause 33. The display device of clause 31 or 32 wherein the one or more criteria define a maximum acceptable percentage difference between the brightness of the target pixel at a given greyscale level and the brightness of the pixel in question at the given greyscale level.

[0131] Clause 34. The display device of any one of clauses 31 to 33 wherein the fitting algorithm iteratively refines the gamma correction for each pixel having brightness data that does not match the target brightness characteristic until each of the one or more criteria have been satisfied.

[0132] Clause 35. The display device of any one of clauses 20 to 34 wherein the gamma correction determined for each pixel having brightness data that does not match the target brightness characteristic is based on a target gamma curve for the pixels of the display device.

[0133] Clause 36. The display device of any one of clauses 20 to 35 wherein storing the respective gamma correction for each pixel comprises storing a lookup table for each pixel.

[0134] Clause 37. The display device of any one of clauses 20 to 36 wherein a determined gamma correction may apply to a group of neighbouring pixels of the display device based on their averaged brightness data.

[0135] Clause 38. The display device of any one of clauses 20 to 37 wherein the brightness data for each pixel of the plurality of LED pixels of the display device is received from a high resolution camera configured to image the plurality of LED pixels of the display device.

Claims

CLAIMS1 . A method for controlling the brightness of an image signal, the method comprising: receiving, at a brightness controlling means, image signal brightness data; processing, by the brightness controlling means, the image signal brightness data; and outputting, by the brightness controlling means, output image brightness data to a display device; wherein the brightness controlling means is configured to process the image signal brightness data using a curve for gamma correction, the curve having a gamma value that varies by greyscale, and the gamma value increasing with increasing greyscale.

2. The method of claim 1 wherein the gamma value of the curve increases at a first rate at low greyscale levels, increases at a third rate at high greyscale levels, and increases at a second rate at greyscale levels that are between the low and high greyscale levels, the first rate being higher than the second rate, and the second rate being higher than the third rate.

3. The method of claim 1 or 2 wherein the gamma value of the curve increases from approximately Gamma 0.7 at low greyscale levels to Gamma 2.5 at high greyscale levels.

4. The method of claim 1 or 2 wherein the gamma value of the curve approximates Gamma 2.2 at greyscale levels above a first threshold.

5. The method of claim 4 wherein the first threshold depends on an ambient lighting condition of the environment around the display device.

6. The method of any one of claims 1 to 5 wherein the curve for gamma correction is generated by dividing each frame of image signal brightness data into a plurality of subframes and applying a different gamma curve to the image signal brightness data for each subframe.

7. The method of claim 6 wherein each subframe lasts for an equal duration of time.

8. The method of claim 6 or 7 wherein there are three subframes in each frame.

9. The method of any one of claims 1 to 5 wherein the curve for gamma correction is generated by high resolution pulse width modulation output control.

10. The method of any one of claims 1 to 9 wherein the method is for controlling the brightness of an image signal for display in an augmented reality display device.

11. A display device comprising a plurality of pixels and a brightness controlling means, the brightness controlling means being configured to control the brightness of an image signal by: receiving image signal brightness data for each of the plurality of pixels; processing the image signal brightness data using a curve for gamma correction, the curve having a gamma value that varies by greyscale, and the gamma value increasing with increasing greyscale; and outputting output image brightness data to a display device.

12. The display device of claim 11 wherein the gamma value of the curve is configured to increase at a first rate at low greyscale levels, to increase at a third rate at high greyscale levels, and to increase at a second rate at greyscale levels that are between the low and high greyscale levels, the first rate being higher than the second rate, and the second rate being higher than the third rate.

13. The display device of claim 11 or 12 wherein the gamma value of the curve is configured to increase from approximately Gamma 0.7 at low greyscale levels to Gamma 2.5 at high greyscale levels.

14. The display device of claim 11 or 12 wherein the gamma value of the curve approximates Gamma 2.2 at greyscale levels above a first threshold.

15. The display device of claim 14 wherein the first threshold depends on an ambient lighting condition of the environment around the display device.

16. The display device of any one of claims 11 to 15 wherein the curve for gamma correction is configured to be generated by dividing each frame of image signal brightness data into a plurality of subframes and applying a different gamma curve to the image signal brightness data for each subframe.

17. The display device of claim 16 wherein each subframe lasts for an equal duration of time.

18. The display device of claim 16 or 17 wherein there are three subframes in each frame.

19. The display device of any one of claims 11 to 15 wherein the curve for gamma correction is configured to be generated by high resolution pulse width modulation output control.

20. The display device of any one of claims 11 to 19 wherein the display device is an augmented reality display device.

Images