Display device including variable wavelength LEDs and method of calibrating and driving same

Abstract

A display device includes a variable wavelength LED that is a pixel of the device or one of a plurality of sub-pixels of the pixel of the device; and an absorptive color filter. The absorptive color filter is positioned between the variable wavelength LED and an exit of the display device such that light emitted by the variable wavelength LED passes through the absorptive color filter before exiting the exit of the display device. The variable wavelength LED or second LED sub-pixel may be driven to emit light at a first emission wavelength in response to a first electrical driving condition, and the variable wavelength LED may be driven to emit light at a second emission wavelength in response to a second electrical driving condition. The absorptive color filter is configured to attenuate a brightness of light emitted at the second emission wavelength, the second emission wavelength being shorter than the first emission wavelength. Methods of calibrating and driving the display device are also provided.

Description

TECHNICAL FIELD

[0001] The present invention relates to a display device comprising variable wavelength LEDs, a method of calibrating a display device comprising variable wavelength LEDs, and a method of driving a display device comprising variable wavelength LEDs. In particular, the present invention relates to a display device comprising variable wavelength LEDs and absorptive color filters. BACKGROUND

[0002] III-V semiconductor materials, and in particular Group III nitride semiconductor materials, are of great interest in semiconductor device design.

[0003] “Group III-V” semiconductors include binary, ternary, and quaternary alloys of Group III elements such as Ga, Al, and In, and Group V elements such as N, P, As, and Sb, and are of great interest in a wide range of applications including electronics and optoelectronics.

[0004] A class of semiconductor materials known as “Group III nitride” materials, which includes gallium nitride (GaN), indium nitride (InN), and aluminum nitride (AIN), as well as ternary and quaternary alloys thereof, is of great interest. (Al, In) GaN is a term encompassing AlGaN, InGaN, and GaN. Group III nitride materials have not only achieved commercial success in solid state lighting and power electronics, but also exhibit particular advantages in quantum light sources and light-matter interactions.

[0005] While various Group III nitride materials are of commercial interest, gallium nitride (GaN) is widely regarded as one of the most important new semiconductor materials, and is of interest in a wide range of applications.

[0006] The present invention will be described primarily with reference to GaN and InGaN, but can be advantageously applied to alternative Group III nitride material combinations.

[0007] It is known that the introduction of porosity into bulk Group III nitrides such as GaN can greatly affect its material properties (optical, mechanical, electrical, and thermal, etc.). Thus, the possibility of tuning a wide range of material properties of GaN and Group III nitride semiconductors by altering their porosity has made porous GaN of great interest in optoelectronic applications.

[0008] In a significant improvement over known trichromatic LED displays, the present applicant, Poro Technologies Ltd, has developed variable wavelength LEDs formed from Ill-nitride semiconductor material grown over a porous region of Ill-nitride material. Instead of emitting in a single colour as known LEDs, these variable wavelength LEDs can emit a broad spectrum of different emission wavelengths in response to varying the drive conditions provided to the variable wavelength LED. These variable wavelength LEDs and methods of their manufacture are set out below.

[0009] As the peak emission wavelength of these variable wavelength LEDs can be tuned by varying the drive conditions, a single variable wavelength LED can replace multiple "single colour" sub-pixels. In particularly preferred embodiments, a variable wavelength LED can be tuned to emit any of red, green or blue light by tuning the drive conditions provided to the same 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 sub-pixel in a sub-pixelated display.

[0010] The ability of the variable wavelength LEDs to emit this broad range of emission wavelengths makes the display driving and calibration process much more complex than is the case for known RGB sub-pixel displays. The peak emission wavelength of a variable wavelength LED depends largely on the magnitude of the electrical drive signal provided to a given LED, and longer wavelength emission colours require lower magnitude drive currents / voltages which inherently produce lower luminance. This presents a challenge because the white balance must be maintained across the primary colour emission wavelengths which have inherently different luminances. An additional challenge is presented by the fixed limit on how short a drive duty cycle can be made for the shorter, brighter emission wavelengths. SUMMARY

[0011] The present application relates to a display apparatus comprising variable wavelength light emitting diodes (LEDs), a method of calibrating a display apparatus comprising variable wavelength light emitting diodes (LEDs), and a method of driving a display apparatus comprising variable wavelength light emitting diodes (LEDs). The display apparatus can preferably comprise a plurality of such variable wavelength LEDs, for example a variable wavelength micro-LED array.

[0012] The present application is defined in the independent claims which are present hereinafter. Preferred or advantageous features of the present application are defined in the dependent claims which are present hereinafter.

[0013] Display device

[0014] In a first aspect of the present application, there is provided a display apparatus comprising:

[0015] a variable wavelength LED, the variable wavelength LED being one of a pixel of the device, or a plurality of sub-pixels of a pixel of the device; and an absorptive color filter. The absorptive color filter is positioned between the variable wavelength LED and an exit of the display device, such that light emitted by the variable wavelength LED passes through the absorptive color filter before exiting from the exit of the display device.

[0016] The variable wavelength LED or a second LED sub-pixel can be drivable to emit light at a first emission wavelength in response to a first electrical driving condition provided to the variable wavelength LED or the second LED sub-pixel. The variable wavelength LED can be drivable to emit light at a second emission wavelength in response to a second electrical driving condition.

[0017] Preferably, the variable wavelength LED can be drivable to emit light at a first emission wavelength in response to a first electrical driving condition, and to emit light at a second emission wavelength in response to a second electrical driving condition. The first emission wavelength and the second emission wavelength are preferably both within an emission wavelength range of the variable wavelength LED.

[0018] The second emission wavelength is preferably shorter than the first emission wavelength, and the absorptive color filter is configured to attenuate the luminance of light emitted at the second emission wavelength.

[0019] Since the first emission wavelength has a longer wavelength than the second primary color emission wavelength, if the duty cycles of the first driving condition and the second driving condition are equal, then in use the first wavelength is emitted at a first luminance that is lower than the second luminance by which the second primary color emission wavelength is emitted, due to the correlation of luminance with driving current density. The absorptive filter compensates for this difference in luminance of the different emission wavelengths by absorbing some of the light emitted at the second emission wavelength.

[0020] The absorptive color filter is a wavelength selective filter, such that the absorptive color filter absorbs light having certain wavelengths, while allowing light having other wavelengths to be transmitted through the filter without being absorbed.

[0021] The absorptive color filter can be configured to attenuate the luminance of light within an absorption wavelength range. The absorptive color filter can be designed to absorb light at specific wavelengths, as is known in photography and optics technology.

[0022] A display device according to the present invention can comprise an absorptive color filter configured to absorb a portion of the intensity of light emitted by the variable wavelength LED at the second emission wavelength. Thus, the absorptive color filter can be configured to partially block the transmission of light at or having the second emission wavelength. In practice, the filter can be configured to partially block the transmission of light within an absorption wavelength range of the filter, the absorption wavelength range of the filter containing the second emission wavelength.

[0023] Examples of variable wavelength LEDs that can be used in the present application are known as Dynamic Pixel Tuning (DPT®) variable wavelength LEDs from Poro Technologies Ltd, and are disclosed in International Patent Application No. PCT / GB2022 / 051997 published as WO2023 / 007174.

[0024] While continuous different emission wavelengths can be emitted directly from a variable wavelength LED by varying the driving conditions supplied to the LED, additional chromaticities can be displayed by driving the LED to emit multiple "primary" wavelengths, such that the overall output signal observed by a viewer is a temporal combination of the emitted primary colors.

[0025] In a sub-pixelated display, different primary color emission wavelengths can be emitted by separate sub-pixels. The sub-pixels can all be variable wavelength LEDs, or some of the sub-pixels can be non-variable wavelength LEDs.

[0026] In one preferred embodiment, the display device is a sub-pixelated display, in which the variable wavelength LED is a first sub-pixel that is drivable to emit light at a second emission wavelength in response to a second driving condition provided to the first sub-pixel, and in which the display includes a second LED sub-pixel that is drivable to emit light at a first emission wavelength in response to a first driving condition provided to the first sub-pixel. The second LED sub-pixel is preferably a second variable wavelength LED sub-pixel. In this embodiment, the first sub-pixel and the second sub-pixel are sub-pixels of a display pixel, and the first sub-pixel and the second sub-pixel are independently drivable from one another.

[0027] In a sub-pixelated embodiment of the display device, due to the proximity of the sub-pixels to one another, the light emitted by the sub-pixels at the first primary color emission wavelength and the second primary color emission wavelength combine to form an output signal having an output chromaticity and an output luminance L O A viewer observing the display device will perceive only a single output signal from the sub-pixelated pixel within the display frame, and that perceived output signal will have a single output chromaticity and luminance that is the temporal average of the emission wavelengths emitted by the variable wavelength LED sub-pixel in a field-sequential manner.

[0028] In another preferred embodiment, the 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 multiple discrete emission wavelengths by driving the LED pixel with multiple different driving conditions. The variable wavelength LED is drivable to emit light at a first emission wavelength in response to a first electrical driving condition provided to the variable wavelength LED, and the variable wavelength LED is drivable to emit light at a second emission wavelength in response to a second electrical driving condition provided to the variable wavelength LED.

[0029] In a sequential field display, by supplying different drive conditions to the same variable wavelength LED in sequential sub-frames of a display frame, the LED can be controlled to emit a plurality of emission wavelengths. In this way, the same variable wavelength LED can be a pixel that emits a plurality of selected “primary emission wavelengths” one after the other within the duration of a single display frame.

[0030] In use, by varying the drive conditions provided to the variable wavelength LED, the variable wavelength LED of the display device is preferably driven to emit light at both the first emission wavelength and the second emission wavelength in turn.

[0031] The display device is preferably driven by field sequential control, such that the first emission wavelength and the second emission wavelength are emitted by the same variable wavelength LED during discrete display sub-frames within a single display frame. In other words, the same LED emits the first emission wavelength and the second emission wavelength one after the other during a short time interval (display frame). The light emitted at the first primary emission wavelength and the second primary emission wavelength combine to form an output signal having an output chromaticity and an output luminance L O A viewer observing the display device will only perceive a single output signal from the variable wavelength LED within the display frame, and that perceived output signal will have a single output chromaticity and luminance, which is the time average of the emission wavelengths emitted by the variable wavelength LED field sequentially.

[0032] Particular features of a Dynamic Pixel Tuning (DPT®) variable wavelength LED designed by Pronto Technology Limited and described in International Patent Application No. PCT / GB2022 / 051997 published as WO2023 / 007174, are that the emission wavelength of the variable wavelength LED is determined by a drive condition, such as a drive current density, provided to the LED. The luminance or brightness of the emission of this wavelength is determined by the duty cycle of the drive condition (the length of time for which the drive condition is supplied to the variable wavelength LED).

[0033] Thus, the luminance of the emission wavelength is controlled by pulse width modulation (PWM) of the drive signal, i.e. the width of the pulses of drive current density provided to the variable wavelength LED.

[0034] If the combination of light pulses at different emission wavelengths is made to produce a perceived output signal having a desired output chromaticity and output luminance, both the chromaticity of the individual pulses and the luminance of the individual pulses are relevant. For example, if the output signal is to become brighter without changing the output chromaticity, it is necessary to increase the luminance of all individual emission pulses that make up the output signal. This is done by increasing the length of the pulses of the respective drive conditions, while maintaining the relative ratios between the duty cycles of the pulse width modulated pulses. If the luminance of one emission wavelength is changed more than the other emission wavelengths and deviates from the desired color balance ratio, the perceived color balance and brightness balance of the display will be negatively affected.

[0035] These displays are therefore brightness limited when the duty cycle of any given primary color emission color reaches 100%. At this point, the duty cycle of this emission wavelength cannot become longer, and therefore the luminance of the given emission wavelength cannot be increased further. This limits the overall brightness of the display, because the other emission wavelengths cannot be made brighter after this point either in order to maintain the color balance.

[0036] For dynamic pixel tuning (DPT®) variable wavelength LEDs, the longer wavelength emission colors require lower magnitude drive currents / voltages, which inherently produce lower luminance. For example, the low current density required to drive a variable wavelength LED to emit red light means that the luminance of the red emission is inherently lower, which can be a limiting factor because the duty cycle of the red drive condition cannot be increased beyond 100%.

[0037] In one preferred embodiment, a set of two primary colors (primary emission wavelengths and their associated drive conditions) can be selected for display control. The output signal produced by the variable wavelength LED then has an output chromaticity and output luminance L O .

[0038] In one preferred embodiment, a set of three primary colors (primary emission wavelengths and their associated drive condition subfields) can be selected: a first primary emission wavelength, a second primary emission wavelength, and a third primary emission wavelength.

[0039] In one exemplary embodiment, a set of three primary colors is selected for a given display frame. To achieve a wide displayable color gamut, the three primary colors can be selected to be nominally red (R), green (G), and blue (B).

[0040] In one exemplary embodiment, the red primary emission wavelength, the green primary emission wavelength, and the blue primary emission wavelength can be selected to be 639 nm, 533 nm, and 466 nm, respectively.

[0041] While variable wavelength LEDs can be driven to emit all primary color wavelengths, the white balance condition is selected so that the relative luminosity of the three primary color emission wavelengths can be set. The drive conditions of the display device can then be calibrated so that the variable wavelength LEDs can be controlled to emit an "white" output signal on command.

[0042] The primary color emission wavelengths and their drive conditions can be set so that the combined wavelengths and luminosity of the primary colors are complementary and sum to a standard illuminant white point, such as CIE standard illuminant point D65.

[0043] To make a D65 white output signal using three primary color wavelengths of 639 nm, 533 nm, and 466 nm, the luminosity of the red / green / blue primary colors must be set in a ratio of 1.6:3.6:1, respectively.

[0044] The low current density required to drive a variable wavelength LED to emit red light at 639 nm means that the luminosity of the red primary color is naturally the lowest of the three primary colors. The low luminosity of the longest wavelength of the selected primary color emission wavelength, red primary color in this example, is therefore the limiting factor for the overall luminosity of the output signal.

[0045] To maximize the luminosity of the red primary color, the duty cycle of the red drive condition can be set to 100% duty cycle.

[0046] To satisfy the required white balance ratio of 1.6:3.6:1, the duty cycles of the green and blue drive conditions must be reduced until the luminosity of the green and blue primary colors satisfies this ratio with the red luminosity. Even at a 100% duty cycle of the red drive condition, the inherent low luminosity of the long wavelength red emission means that to satisfy this luminosity ratio, the green and blue drive duty cycles must be made very short. In the case of the shortest primary color wavelength, blue, to get a blue luminosity that will balance with the red luminosity, the blue duty cycle must be close to zero.

[0047] Driving short wavelength emissions with very short duty cycles presents difficulties in display control. If the duration of the pulse width modulation (duty cycle) of the drive condition is too short for the blue and green primary color emission wavelengths, the least significant bit, LSB, of the PWM, i.e., the emission wavelength that requires the shortest duty cycle, time will be too short to achieve. The micro-LED legacy capacitance will also cause poor performance in low gray scales.

[0048] The present invention improves the problem of too short duty cycles for shorter, brighter emission wavelengths by providing a wavelength specific absorptive color filter. In the present invention, the absorptive color filter is configured to attenuate or reduce the transmission luminosity of one or more emission wavelengths of the variable wavelength LED. The attenuation provided by the absorptive color filter allows the use of longer duty cycles to drive an emission wavelength, such as a second emission wavelength.

[0049] By absorbing light with an absorptive color filter is meant that there is a difference between the luminance of the emitted light at the point at which it is emitted by the variable wavelength LED and the luminance of the same emitted light after it has been transmitted through the filter.

[0050] The absorptive color filter can be configured so that, in use, light emitted at the first emission wavelength has a first luminance Li before transmission through the absorptive color filter and a first transmitted luminance L 1T after transmission through the absorptive color filter, and light emitted at the second emission wavelength has a second luminance L2before transmission through the absorptive color filter and a reduced second transmitted luminance L 2T .

[0051] Because the absorptive color filter reduces the luminance of light at the second emission wavelength, rather than having to use an extremely short second duty cycle to produce the required second luminance L2of the second emission wavelength, a longer and more practical second duty cycle can be used for the second drive condition. This longer second duty cycle causes the second emission wavelength to be emitted at a second luminance L2that is higher than the required luminance of the output signal, but the absorptive color filter offsets this by partially absorbing the light at the second wavelength and reducing the luminance of that light to a second transmitted luminance L 2T . 2T Advantageously, the second transmitted luminance L 2T is the required output luminance of the light emitted at the second emission wavelength.

[0052] When a viewer observes the output signal perceived as the combination of the emitted wavelengths transmitted through the filter of the display device, the transmitted luminances of the first and second emission wavelengths (the luminance exiting the absorptive filter) are combined to produce the perceived output signal. The absorptive color filter can therefore be designed to absorb a desired proportion of the light at the second emission wavelength so that the transmitted first luminance and the transmitted second luminance satisfy the required luminance balance ratio.

[0053] The absorptive color filter can preferably be configured to reduce or modulate the luminance of light having the second emission wavelength so that, in use, light emitted at the second emission wavelength has a second luminance L2before transmission through the absorptive color filter and a reduced second transmitted luminance L 2T < L2.

[0054] Preferably, the absorptive color filter is configured not to reduce the luminance of light having the first emission wavelength so that the first transmitted luminance L 1Tthe magnitude of the first luminance L1 by at least 90%, or at least 92%, or at least 95%, or at least 98%. Since the first emission wavelength can be a long emission wavelength for which brightness is a limiting factor even at 100% duty cycle, it can be preferable for the first emission wavelength to be transmitted through the absorptive color filter at as high an intensity as possible.

[0055] Due to the inherent properties of the filter, it can not be avoidable that a small proportion of the light having the first emission wavelength is absorbed by the filter. The absorptive color filter can therefore be configured to absorb light emitted at the second emission wavelength to a greater extent than light emitted at the first emission wavelength, such that the ratio of L2:L1 is lower than the ratio of L2:L1. 2T :L 1T the ratio of L2:L1 is lower than the ratio of L2:L1.

[0056] As described above, control of the display device can be simplified by driving the variable wavelength LED to emit light at a plurality of selected "primary color" emission wavelengths. The output signal can therefore be generated by combining pulses of the primary color emission wavelengths and varying the mixing ratio of the primary color wavelengths within this combination. Thus, the "first emission wavelength", "second emission wavelength" and "third emission wavelength" described throughout this document can alternatively be defined as first, second and third "primary color emission wavelengths".

[0057] The variable wavelength LED is preferably controllable to emit light at two or three or more than three selected primary color emission wavelengths, such that in use light transmitted out of the display device during a display frame forms an observation signal which is a temporal combination of the primary color emission wavelengths emitted by the variable wavelength LED during the display frame.

[0058] The first and second emission wavelengths can be first and second primary color emission wavelengths.

[0059] The display device can comprise a controller programmed to drive the variable wavelength LED to emit the first and second emission wavelengths in response to first and second respective drive conditions, the first and second emission wavelengths being wavelengths within an emission wavelength range of the variable wavelength LED.

[0060] The controller can be programmed to drive the variable wavelength LED to emit the first and second emission wavelengths within discrete sub-frames of the same display frame in a field sequential mode.

[0061] The first emission wavelength has a longer wavelength than the second primary color emission wavelength, and due to the correlation of brightness with current density, if the duty cycles of the first and second drive conditions are equal, the first wavelength has a lower luminance than the second primary color emission wavelength.

[0062] The first drive condition can be a first current density, and the second drive condition can be a second current density that is greater in magnitude than the first current density. Alternatively, the first drive condition can be a first voltage, and the second drive condition can be a second voltage that is greater in magnitude than the first voltage.

[0063] The absorptive color filter can be configured to reduce the luminance of the second emission wavelength emitted by the variable-wavelength LED, and the absorptive color filter can preferably be configured to not reduce the luminance of the first emission wavelength emitted by the variable-wavelength LED.

[0064] The absorptive color filter can be configured to reduce the luminance of the second emission wavelength emitted by the variable-wavelength LED, and the absorptive color filter can be configured to reduce the luminance of the first emission wavelength by a lower proportion than it reduces the luminance of the second emission wavelength.

[0065] The first emission wavelength or first primary color emission wavelength can be a red emission wavelength that is higher than 595 nm or higher than 630 nm. Preferably, the first emission wavelength is between 595 nm and 700 nm, or between 610 nm and 680 nm, preferably between 625 nm and 670 nm.

[0066] The second emission wavelength is preferably less than 595 nm, or less than 570 nm.

[0067] In a preferred embodiment, the second emission wavelength is a green emission wavelength between 500 nm and 590 nm or between 510 nm and 580 nm, preferably between 525 nm and 570 nm. In an alternative preferred embodiment, the second emission wavelength is a blue emission wavelength that is lower than 495 nm, preferably between 400 nm and 490 nm, or between 410 nm and 480 nm, preferably between 430 nm and 470 nm.

[0068] The first and second emission wavelengths or first and second primary color emission wavelengths are preferably separated by at least 40 nm, or by at least 50 nm, within the emission wavelength range of the variable-wavelength LED.

[0069] The display can comprise a plurality of absorptive color filters, each absorptive color filter configured to attenuate the luminance of light emitted at a respective (primary) emission wavelength.

[0070] The variable-wavelength LED or the second LED sub-pixel or the third LED sub-pixel is preferably drivable to emit light at a third emission wavelength in response to a third electrical drive condition. Preferably, the variable-wavelength LED is drivable to emit light at a third emission wavelength in response to a third electrical drive condition, the first, second, and third emission wavelengths being within the emission wavelength range of the variable-wavelength LED.

[0071] The absorptive color filter is preferably configured to attenuate the lightness of light emitted at the third emission wavelength and the lightness of light emitted at the second emission wavelength.

[0072] The absorptive color filter can be configured to reduce or attenuate the lightness of light having the third emission wavelength such that, in use, light emitted at the third emission wavelength has a third lightness L3 prior to transmission through the absorptive color filter and a reduced third transmitted lightness L 3T .

[0073] Alternatively, the absorptive color filter can be a second emission wavelength absorptive filter configured to attenuate the lightness of the second emission wavelength, and the device can include an additional third emission wavelength absorptive filter configured to attenuate the lightness of the third emission wavelength.

[0074] The first, second, and third wavelengths can be first, second, and third primary color emission wavelengths such that the display device is controlled to drive the variable wavelength LED to emit light at the discrete first, second, and third primary color emission wavelengths rather than at a continuous emission wavelength.

[0075] The display device can include a controller programmed to drive the variable wavelength LED to emit the first, second, and third emission wavelengths in response to respective first, second, and third drive conditions.

[0076] The controller can be programmed to drive the variable wavelength LED to emit the first, second, and third emission wavelengths within discrete subframes of the same display frame in a field sequential mode. For example, the controller can be programmed to drive the variable wavelength LED to emit three or more primary color emission wavelengths within an emission range of the variable wavelength LED by supplying the first, second, and third drive conditions to the variable wavelength LED in discrete sequential subframes of a display frame such that the variable wavelength LED emits a first primary color emission wavelength λ P1 , a second primary color emission wavelength λ P2 , and a third primary color wavelength λ P3 .

[0077] The first emission wavelength preferably has a longer wavelength than the second and third primary color emission wavelengths, and if the first, second, and third duty cycles of the first, second, and third drive conditions are equal, then light emitted by the variable wavelength LED at the first emission wavelength can inherently have a lower lightness than light emitted at the second and third primary color emission wavelengths.

[0078] The absorptive color filter can be configured to reduce the luminance of light emitted at the second emission wavelength, and to reduce the luminance of light emitted at the third emission wavelength, while the absorptive color filter is configured not to reduce the luminance of light emitted at the first emission wavelength. As discussed above, the luminance of the shortest primary emission wavelength is the limiting factor for the overall display brightness, so it can be preferable to filter only the shorter emission wavelengths, so that longer duty cycles can be used to produce the shorter emission wavelengths while still maintaining the required color balance and brightness balance at the display exit.

[0079] The absorptive color filter can be configured to reduce the luminance of light emitted at the second emission wavelength by the variable wavelength LED, and to reduce the luminance of light emitted at the third emission wavelength by the variable wavelength LED by a different proportion than it reduces the luminance of light emitted at the second emission wavelength.

[0080] The third emission wavelength can preferably be less than 595 nm, particularly preferably less than 570 nm, or less than 495 nm.

[0081] In a preferred embodiment, the second emission wavelength is a green emission wavelength between 500 nm and 590 nm, or between 510 nm and 580 nm, preferably between 525 nm and 570 nm, and wherein the third emission wavelength is a blue emission wavelength below 495 nm, preferably wherein the third emission wavelength is between 400 nm and 490 nm, or between 410 nm and 480 nm, preferably between 430 nm and 470 nm. The second and third emission wavelengths can thus be green and blue primary color wavelengths, respectively.

[0082] The first, second, and third primary emission wavelengths are preferably at least 40 nm apart from each other, or at least 50 nm apart, within the emission wavelength range of the variable wavelength LED, so that the highest and lowest of the first, second, and third emission wavelengths are at least 80 nm apart, or at least 100 nm apart. Having the first, second, and third primary emission wavelengths spread across a wide range of emittable wavelengths advantageously allows a larger color gamut to be displayed by combining the pulses of the three emission wavelengths in different mixing proportions.

[0083] The absorptive color filter can be configured to attenuate the luminance of the second emission wavelength from a second luminance L2 emitted by the variable wavelength LED to a transmitted second luminance L 2T so that when the calibrated first and second duty cycles are used, the wavelengths and transmitted luminances of the first and second emission wavelengths are complementary and sum to a standard illuminant point.

[0084] The display device can preferably be driven so that the wavelengths and transmitted luminances of the first and second emission wavelengths are complementary and sum to a standard illuminant point.

[0085] The absorptive color filter can be configured to attenuate a second luminance L2 of the second emission wavelength of the free variable wavelength LED emission to a second transmitted luminance L 2T , and to attenuate a third luminance L3 of the third emission wavelength of the free variable wavelength LED emission to a third transmitted luminance L 3T .

[0086] The display device can preferably be driven such that the wavelengths and transmitted luminances of the first, second, and third emission wavelengths are complementary and sum to a standard illuminant point.

[0087] The standard illuminant point can be a white point. Particularly preferably, the standard illuminant point is the CIE standard illuminant point D65.

[0088] The absorptive color filter can comprise a glass substrate, or a plastic substrate, or a semiconductor film substrate.

[0089] The absorptive color filter can be fabricated from a glass, plastic, or film substrate by exploiting the inherent absorption or transmission properties of the material, rather than interference effects from thin film coatings. The absorptive color filter can comprise a combination of semiconductor thin film materials based on the wavelengths the filter is intended to attenuate. The absorptive color filter can comprise one or more layers of absorptive material, and the layer thickness can be varied to provide a color filter that attenuates transmitted light of different wavelengths. By controlling the thickness of the material, precise filtering across a large wavelength bandwidth can be achieved.

[0090] Suitable absorptive color filters are widely used in photography, optical instruments, and lighting devices, and are available to those skilled in the art.

[0091] The absorptive color filter can be a long pass filter. Alternatively, the absorptive color filter can be a band pass filter. Alternatively, the absorptive color filter can be a low pass filter.

[0092] The absorptive color filter is preferably configured to not alter the wavelength of the light emitted by the variable wavelength LED. The absorptive color filter is preferably not a wavelength conversion layer. For example, the filter is preferably configured to absorb a proportion of certain emitted wavelengths, but preferably does not include color conversion materials for re-emitting the absorbed light.

[0093] The variable wavelength LED is preferably a micro-LED. Particularly preferably, the variable wavelength LED is a Ill-nitride LED.

[0094] A variable wavelength LED can have an emission wavelength range with a width between 100 nm and 200 nm, or between 125 nm and 185 nm, or between 150 nm and 175 nm. The variable wavelength LED is preferably continuously controllable over the emission wavelength range by varying the electrical drive conditions provided to the variable wavelength LED. However, instead of continuously varying the emission wavelength over this range, the display device can instead be controlled by driving the variable wavelength LED to emit light at only a particular selected wavelength (a primary color emission wavelength). An output signal having a desired chromaticity and luminance can then be generated as a perceived temporal combination of multiple discrete primary color emission wavelengths emitted during a display frame.

[0095] The emission wavelength range preferably extends from a lower limit that is preferably below 500 nm, or below 450 nm, to an upper limit that is preferably above 595 nm, or above 630 nm.

[0096] The lower limit of the emission wavelength range can preferably be between 440 nm and 495 nm, or between 440 nm and 485 nm, preferably between 450 nm and 470 nm.

[0097] The upper limit of the emission wavelength range can preferably be between 595 nm and 700 nm, or between 610 nm and 680 nm, preferably between 625 nm and 670 nm.

[0098] The FWHM of the variable wavelength LED can be 50 nm or less, or 40 nm or less, or 30 nm or less, or 20 nm or less.

[0099] The display device can comprise an array of variable wavelength LEDs, each variable wavelength LED being a pixel of the device, or one of a plurality of sub-pixels of a pixel of the device.

[0100] An absorptive color filter is preferably positioned over the plurality of variable wavelength LEDs between the variable wavelength LEDs and an exit of the display device, such that light emitted by the plurality of variable wavelength LEDs passes through the absorptive color filter before passing out of the exit of the display device. For example, the filter can extend across the entire array of variable wavelength LEDs in the device.

[0101] Calibration method

[0102] In a second aspect of the application, there is provided a method of calibrating a display device, the display device comprising: a variable wavelength LED, the variable wavelength LED being one of: a pixel of the device; or a plurality of sub-pixels of a pixel of the device; and an absorptive colour filter, the absorptive colour filter being positioned between the variable wavelength LED and an exit of the display device, such that light emitted by the variable wavelength LED passes through the absorptive colour filter before emerging from the exit of the display device.

[0103] The calibration method can comprise the steps of:

[0104] driving the variable wavelength LED or the second LED sub-pixel with a first drive condition to emit a first emission wavelength λ1 of the device having a first luminance L1 for a first duty cycle, wherein the first emission wavelength is transmitted with a first transmitted luminance L 1T from the absorptive colour filter;

[0105] driving the variable wavelength LED with a second drive condition to emit a second emission wavelength λ2 of the device having a second luminance L2 for a second duty cycle, the second emission wavelength being shorter than the first emission wavelength;

[0106] wherein the absorptive colour filter is configured to attenuate the second luminance L2 of the second emission wavelength such that the second emission wavelength is transmitted with a second transmitted luminance L 2T from the absorptive colour filter, wherein L 2T < L2;

[0107] setting the second duty cycle to a shorter duration than the first duty cycle, and controlling the duration of the second duty cycle such that the first emission wavelength λ1 and the second emission wavelength λ2 and the transmitted first luminance L 1T and the transmitted second luminance L 2T are complementary and sum to a standard illuminant point.

[0108] The duration of the second duty cycle is preferably controlled to find a calibrated second duty cycle at which the first emission wavelength λ1 and the second emission wavelength λ2 and the transmitted first luminance L 1T and the transmitted second luminance L 2T are complementary and sum to a standard illuminant point.

[0109] The first and second emission wavelengths emitted by the variable wavelength LED are transmitted through the absorptive filter, and a viewer observing the exit of the display device perceives the device to emit an output signal which is a combination of the light transmitted through the absorptive filter at the first and second emission wavelengths. The output signal transmitted through the filter and observable by the viewer has: an output chromaticity which is a time average of the emission wavelengths emitted by the variable wavelength LED; and an output luminance L Owhich is the time average of the transmitted luminance of the emitted wavelength emitted by the variable wavelength LED.

[0110] The calibration method is preferably a method of calibrating a display device according to the first aspect of the application described above. The display device that can be calibrated using the method of the second aspect can have any of the features described in relation to the first aspect of the application.

[0111] The method of the application can be a method of calibrating a display device to display an output signal having a desired output chrominance and output luminance by transmitting two or more emitted wavelengths through an absorptive colour filter configured to attenuate the luminance of at least one of the emitted wavelengths.

[0112] The second duty cycle is preferably controlled to compensate for the attenuation of the luminance of the second emitted wavelength by the absorptive colour filter. As the filter attenuates the second luminance, the second duty cycle can be increased (relative to the same display without the absorptive filter) so that the combined wavelengths and the combined transmitted luminances are complementary and sum to a standard illuminant point.

[0113] Preferably, the standard illuminant point is a white point, such as the CIE standard illuminant point D65. In this case, the display device can be calibrated to emit white light from the combination of light pulses at the first and second emitted wavelengths.

[0114] In one embodiment, the display device is a sub-pixelated display so that the variable wavelength LED is a first variable wavelength sub-pixel, and the method comprises the steps of: driving a second LED sub-pixel (preferably a second variable wavelength LED sub-pixel) to emit light at the first emitted wavelength; and driving the first variable wavelength sub-pixel to emit light at the second emitted wavelength.

[0115] In other preferred embodiments, the variable wavelength LED is a pixel of the device that can be driven to emit both the first and second emitted wavelengths by providing first and second drive conditions at different times. In this embodiment, there is no second LED sub-pixel, and the method preferably comprises the steps of: driving the variable wavelength LED with a first drive condition for a first duty cycle to emit a first emitted wavelength λ1 of the device; and driving the variable wavelength LED with a second drive condition for a second duty cycle to emit a second emitted wavelength λ2.

[0116] The absorptive colour filter is preferably configured to not reduce the first luminance of the first emitted wavelength so that the magnitude of the first transmitted luminance L 1T is at least 95%, or at least 98%, or at least 99% of the magnitude of the first luminance.

[0117] Preferably, the first duty cycle is a 100% duty cycle, such that the first drive condition provides to the variable wavelength LED for 100% of the duration of a display frame (in a sub-pixelated display) or display sub-frame (in a field sequential display). Thus, preferably, a standard light source point is obtained in the case where the first duty cycle is a 100% duty cycle and the second duty cycle is a calibrated second duty cycle shorter than the first duty cycle.

[0118] As described above in relation to the first aspect, the display apparatus can be controlled to emit a desired output signal by driving the variable wavelength LED to emit light at a plurality of discrete "primary color" emission wavelengths. Instead of driving the variable wavelength LED with a continuously variable drive condition to emit light at any emission wavelength within its emission wavelength range, by driving the variable wavelength LED to emit light at only two or three or more "primary color emission wavelengths", the display apparatus can be operated more similarly to a known three color display, in which the desired output signal is formed by the temporal combination of three discrete emission colors. At any given time, the variable wavelength LED can be driven to emit one of the primary color emission wavelengths.

[0119] The first and second emission wavelengths can thus be first and second primary color emission wavelengths selected for driving of the apparatus.

[0120] In known display apparatuses, it is always necessary to use three or more primary color emission wavelengths to obtain a 2D color gamut of observable output signals. However, by using a variable wavelength LED according to the present application, different sets of primary colors can be used in different display frames, so that a wide range of observable output chromaticities can be achieved using only a set of two complementary primary color emission wavelengths at the same time.

[0121] The method can comprise the step of selecting a set of two or more discrete primary color emission wavelengths λ P within the emission wavelength range of the variable wavelength LED. The set of primary color emission wavelengths can correspond to a set of two or more respective drive conditions; wherein the first emission wavelength λ1 is the longest of the selected primary color emission wavelengths, and wherein the second emission wavelength λ2 is another of the selected primary color emission wavelengths.

[0122] The step of controlling the duration of the second duty cycle controls the second transmission luminance L 2T of light emitted when the second drive condition is applied. Since the absorbing filter absorbs a portion of the light transmitted through the filter at the second emission wavelength, the second transmission luminance L 2T is also determined by the duration of the second duty cycle. A longer second duty cycle results in a higher second luminance L2 and thus a higher second transmission luminance L 2T .

[0123] In the calibration method, the second duty cycle is controlled so that the second transmittance is balanced with the first transmittance so that the first transmittance L 1T and the second transmittance L 2T are complementary and sum to a standard illuminant point. The standard illuminant point can be selected, such as the D65 white point, to define the illuminant point that the display device should be able to display to a viewer. This calibration method then balances the first and second duty cycles with the absorption characteristics of the absorptive color filter to ensure that the transmittance of these wavelengths will be balanced to meet the color balance or white balance condition at the standard illuminant point.

[0124] The method can include the steps of using the first luminance L1 as a reference luminance and using the first duty cycle as a reference duty cycle; and setting the second duty cycle to a shorter duration than the first duty cycle so that the second transmittance is balanced with the first transmittance L 1T to meet a standard illuminant point metamerism ratio at which the emission wavelengths and transmittances of the first and second emission wavelengths are complementary and sum to a standard illuminant point.

[0125] The method can include the step of driving the variable wavelength LED for a third duty cycle with a third drive condition to emit a third emission wavelength λ3 of the device having a third luminance L3, the third emission wavelength being shorter than the first emission wavelength.

[0126] The absorptive color filter can be configured to attenuate the third luminance L3 so that the third emission wavelength is transmitted from the absorptive color filter with a third transmittance L 3T where L 3T < L3.

[0127] The method can include the steps of setting the third duty cycle to a shorter duration than the first duty cycle; and controlling the duration of the third duty cycle so that the first emission wavelength, the second emission wavelength, and the third emission wavelength and the first transmittance L 1T , the second transmittance L 2T , and the third transmittance L 3T are complementary and sum to a standard illuminant point.

[0128] The duration of the third duty cycle is preferably controlled to find a calibrated third duty cycle at which the first emission wavelength, the second emission wavelength, and the third emission wavelength and the transmitted first luminance L 1T , the transmitted second luminance L 2T , and the transmitted third luminance L 3T are complementary and sum to a standard illuminant point.

[0129] the first, second and third emission wavelengths and the first, second and third transmission luminances summing to a standard illuminant point. 1T 2T 3T Preferably the sum is the same standard illuminant point as described above, for example a white point, such as D65 white.

[0130] The second and third emission wavelengths are preferably shorter than the first emission wavelength. Thus the second and third drive conditions will inherently require a higher current density than the first drive condition, which will mean that the second and third emission wavelengths will be emitted at a higher luminance than the first luminance, given an equal duty cycle. To reduce this effect so that the first, second and third luminances sum to a standard illuminant point, the second and third duty cycles are made shorter than the first duty cycle, and in addition, the absorbing colour filter absorbs a portion of the light at the second and third emission wavelengths. These effects reduce the second and third transmission luminances to the level required for the first luminance of the first emission wavelength to sum to a standard illuminant point.

[0131] The method can comprise the steps of using the first luminance L1 as a reference luminance and using the first duty cycle as a reference duty cycle; and setting the second and third duty cycles to a shorter duration than the first duty cycle so that the second and third transmission luminances sum to the reference luminance with the first transmission luminance L 1T The first, second and third emission wavelengths satisfy a standard illuminant point metamerism ratio at which the emission wavelengths and transmission luminances of the first, second and third emission wavelengths are complementary and sum to a standard illuminant point.

[0132] The first, second and third emission wavelengths can be first, second and third primary colour emission wavelengths selected for use as discrete primary colours for device control. The calibration method can comprise the step of selecting the first, second and third primary colour emission wavelengths for driving the variable wavelength LED.

[0133] The method can comprise the step of selecting a set of three discrete primary colour emission wavelengths λ P corresponding to a set of three respective drive conditions; wherein the first emission wavelength λ1 is the longest of the selected primary colour emission wavelengths, and wherein the second emission wavelength λ2 and the third emission wavelength λ3 are the other selected primary colour emission wavelengths.

[0134] The wavelengths of the primary colour emission wavelengths can be variable or varied between display frames, so that different colour output signals can be emitted in discrete display frames.

[0135] The three or more primary colour emission wavelengths are preferably variable over the emission wavelength range of the variable wavelength LED.

[0136] ​​The wavelengths of the three or more primary color emission wavelengths in any given display frame are selected to be complementary such that the sum of two primary color emission wavelengths in a given display frame is a standard illuminant point.

[0137] The method of the second aspect can be a method of calibrating a display device to display an output signal having a desired output chromaticity and output luminance by emitting two or more or three or more primary color emission wavelengths.

[0138] The method can comprise the step of finding or calculating a standard illuminant point metamerism ratio in which light emitted at the selected primary color emission wavelengths and their respective luminances are combined such that the combined output signal reaches a standard illuminant point.

[0139] The method can comprise the steps of;

[0140] Using the standard illuminant metamerism ratio and the first luminance LI at the first emission wavelength, the required transmittance luminances L T and its first luminance LI, and sum to a standard illuminant point; and

[0141] Calculating a duty cycle for each of the primary color emission wavelengths which will result in the required transmittance luminances of the variable wavelength LED at each primary color emission wavelength.

[0142] In one preferred embodiment, the first emission wavelength λ1 is a red (R) wavelength.

[0143] In one particularly preferred embodiment, the first primary color emission wavelength, the second primary color emission wavelength and the third primary color emission wavelength are selected to be a red (R) primary color emission wavelength, a green (G) primary color emission wavelength and a blue (B) primary color emission wavelength respectively, and the first drive condition, the second drive condition and the third drive condition drive the variable wavelength LED to emit R light, G light and B light respectively.

[0144] The variable wavelength LED emits red light in response to the first drive condition, green light in response to the second drive condition and blue light in response to the third drive condition.

[0145] The first drive condition can drive the variable wavelength LED to emit a red primary color emission wavelength, and the first duty cycle can have a duration of 100% of the duration of the display sub-frame, such that the first luminance LI is the luminance emitted at the red primary color emission wavelength at 100% duty cycle.

[0146] The method can comprise the step of reducing the duty cycles of the green and blue sub-fields until the transmittance luminances of the green and blue primary color emission wavelengths and the red first transmittance luminance LI 1T meet the required metamerism ratio.

[0147] In a preferred embodiment, the variable wavelength LED is a device pixel, and the method of the second aspect is a method of calibrating a display for field sequential driving of variable wavelength LED pixels to emit a plurality of primary color emission wavelengths in sequential sub-frames of a display frame. The variable wavelength LED is preferably a pixel of a display device, and the method can be a method of calibrating a field sequential display device comprising variable wavelength LED pixels.

[0148] In another preferred embodiment, the display device comprises a sub-pixelated display, wherein the pixels of the display each comprise two or more sub-pixels, and the variable wavelength LED is a first sub-pixel of a plurality of sub-pixels of a device pixel.

[0149] The calibration method can comprise the step of storing the calibrated duty cycle of each of the first, second and optionally third emission wavelengths or first, second and optionally third primary color emission wavelengths.

[0150] The method can comprise the step of programming a display controller with the calibrated duty cycles of the primary color emission wavelengths.

[0151] The method can comprise the step of calibrating the transmission luminance of the primary color emission wavelengths by setting the calibrated duty cycles of each set of primary color emission wavelengths such that the sum of the complementary wavelengths and their respective transmission luminances is a standard illuminant point.

[0152] The method can comprise the step of calibrating the transmission luminance of the complementary primary color emission wavelengths by measuring the transmission luminance transmitted by the absorptive color filter and emerging from the exit of the display device when driving the variable wavelength LED pixel across the emission wavelength range of the variable wavelength LED pixel.

[0153] The driving condition associated with a particular emission wavelength of the variable wavelength LED is a particular condition of driving current and voltage which causes the variable wavelength LED to emit light of the corresponding wavelength. The first driving condition can thus refer to a first driving current or first driving current density, while the second driving condition can be a second driving current or second driving current density which differs in magnitude from the driving current or driving current density of the first driving condition.

[0154] The duty cycle of a driving condition refers to the duration for which the driving condition is supplied to the LED (the "on time" of the LED) relative to the duration of a display frame or a display sub-frame allocated to this sub-field. The driving condition is preferably provided in a pulse width modulation (PWM) fashion, and the duty cycle of the driving condition defines the width of the driving current pulse relative to the duration of the frame / sub-frame for driving the device.

[0155] The method is preferably a method of calibrating a display device for emitting a desired output signal using a combination of two or three primary color wavelengths.

[0156] The method can comprise repeating the steps of the method for a plurality of sets of two primary emission wavelengths or a plurality of sets of three primary emission wavelengths.

[0157] The display device can comprise a plurality of variable wavelength LEDs configured in an array, each variable wavelength LED being a pixel of the device, or one of a plurality of sub-pixels of a pixel of the device. The calibration method can comprise performing the steps of the second aspect on each of the variable wavelength LEDs in the display device.

[0158] Method of driving a display

[0159] In a third aspect, the present invention can provide a method of driving a display device, the display device comprising: a variable wavelength LED, the variable wavelength LED being a pixel of the device, or one of a plurality of sub-pixels of a pixel of the device; and an absorptive colour filter positioned between the variable wavelength LED and an exit of the display device, such that light emitted by the variable wavelength LED passes through the absorptive colour filter before emerging from the exit of the display device.

[0160] The method of driving a display device can comprise the steps of:

[0161] driving the variable wavelength LED or the second LED sub-pixel with a first drive condition to emit a first emission wavelength λ1 of the device having a first luminance L1 for a first duty cycle, wherein the first emission wavelength transmits through the absorptive colour filter with a first transmitted luminance L 1T from the absorptive colour filter;

[0162] driving the variable wavelength LED with a second drive condition to emit a second emission wavelength λ2 of the device having a second luminance L2 shorter than the first emission wavelength for a second duty cycle;

[0163] wherein the absorptive colour filter is configured to attenuate the second luminance L2 of the second emission wavelength such that the second emission wavelength transmits through the absorptive colour filter with a second transmitted luminance L 2T from the absorptive colour filter, wherein L 2T < L2;

[0164] wherein the duration of the first and second duty cycles are controlled such that an output signal emitted from the exit of the display device has an output chrominance and an output luminance L O .

[0165] In one embodiment, the display device is a sub-pixelated display such that the variable wavelength LED is a first variable wavelength sub-pixel, and the method comprises the steps of: driving a second LED sub-pixel, preferably a second variable wavelength LED sub-pixel, to emit light at the first emission wavelength; and driving the first variable wavelength sub-pixel to emit light at the second emission wavelength.

[0166] In other preferred embodiments, the variable wavelength LED is a pixel of the device, which is drivable to emit both the first and second emission wavelengths by providing the first and second drive conditions at different times. In this embodiment, the method preferably comprises the steps of driving the variable wavelength LED with the first drive condition for a first duty cycle to emit the first emission wavelength λ1 of the device; and driving the variable wavelength LED with the second drive condition for a second duty cycle to emit the second emission wavelength λ2.

[0167] The step of driving the pixel or sub-pixel to emit light at a single emission wavelength within a display frame is achieved by providing an electrical drive condition, such as an electrical drive current, to the pixel or sub-pixel. The nature of the drive condition supplied to the variable wavelength LED at any given time will determine the wavelength and associated luminance emitted by the variable wavelength LED.

[0168] The step of driving the pixel to emit light at the first and second emission wavelengths within a display frame can be achieved by driving the variable wavelength LED in a field sequential manner, such that discrete drive conditions are supplied in sequential sub-frames of the display frame, and the variable wavelength LED emits both emission wavelengths in a sequence within the display frame.

[0169] Alternatively, the step of driving the sub-pixel to emit light at a plurality of emission wavelengths within a display frame can be achieved by driving the variable wavelength LED sub-pixel to emit the second emission wavelength, and driving separate sub-pixels, preferably separate variable wavelength LED sub-pixels, to emit the first and / or third emission wavelengths.

[0170] The output signal emitted from the exit of the display device is comprised of light that has been emitted by the variable wavelength LED and transmitted through the absorptive colour filter. The output signal observed by a viewer is a time average of the light pulses emitted by the variable wavelength LED during the display frame. It will be appreciated by those skilled in the art that the frame rate and duration of individual display frames can vary as is known in the art.

[0171] The duration of the first and second duty cycles, during which the first and second drive conditions are supplied to the variable wavelength LED, is preferably controlled to determine the output chrominance and output luminance of the output.

[0172] The luminance of each pulse of emitted light can be controlled by varying the duty cycle (width) of the drive condition pulse supplied to the variable wavelength LED. The output chrominance that will be perceived by a viewer can be controlled by varying the ratio between different emission wavelengths emitted within the same display frame.

[0173] The output signal can need to have an output chrominance and an output luminance matching a standard illuminant point, such as a white point, for example a D65 white. By calibrating the display device to produce this standard illuminant point, the device can be controlled to output D65 white light on demand. To do this, the second duty cycle can have a shorter duration than the first duty cycle, and the duration of the second duty cycle can be controlled so that the first emission wavelength λ1 and the second emission wavelength λ2 and the transmitted first luminance L 1T and the transmitted second luminance L 2T are complementary and sum to the standard illuminant point.

[0174] The method of driving a display device is preferably the method of driving a display device according to the first aspect of the application described above. The display device that can be driven using the method of the third aspect can have any of the features described in relation to the first aspect of the application, and can preferably have been calibrated using the method of the second aspect described above.

[0175] The second duty cycle of the variable wavelength LED with the second drive condition is preferably a calibrated second duty cycle obtained from the calibration method described above.

[0176] The output chrominance and output luminance are preferably determined by an input signal provided to the display device. The display device can receive an input signal determining the chrominance and luminance to be displayed at the outlet of the display device, and the variable wavelength LED can then be driven to produce the desired output chrominance and luminance by controlling the duration of the first and second duty cycles.

[0177] The method of driving a display device can be the method of driving a display device of the first aspect of the application, and / or the method of driving a display device following the calibration method of the second aspect of the application, and can comprise the steps of:

[0178] driving the variable wavelength LED for the first duty cycle with a first drive condition to emit a first emission wavelength λ1 of the device having a first luminance L1, wherein the first emission wavelength is transmitted from the absorptive colour filter with a first transmitted luminance L 1T ;

[0179] driving the variable wavelength LED for the calibrated second duty cycle with a second drive condition to emit a second emission wavelength λ2 of the device having a second luminance L2, the second emission wavelength being shorter than the first emission wavelength;

[0180] wherein the absorptive colour filter attenuates the second luminance L2 so that the second emission wavelength is transmitted from the absorptive colour filter with a second transmitted luminance L 2T , wherein L 2T < L2;

[0181] And control the duration of the first and second working cycles so that the output signal emitted from the output of the display device has output chroma and output brightness L. O .

[0182] The method may include the following steps: driving a variable wavelength LED or a second LED sub-pixel (if present) with a third driving condition for a third operating cycle to emit a third emission wavelength λ3 with a third brightness L3, the third emission wavelength being shorter than the first emission wavelength. An absorptive color filter may preferably attenuate the third brightness L3, such that the third emission wavelength has a third transmission brightness L3. 3T The light transmitted through the self-absorbing color filter, where L 3T < L3.

[0183] The method may include the following steps: controlling the duration of the first, second, and third working cycles, such that the output signal emitted from the output of the display device has output chroma and output brightness L. O .

[0184] The method may include the following steps: setting a third duty cycle to a duration shorter than that of the first duty cycle; and controlling the duration of the third duty cycle such that the first emission wavelength, the second emission wavelength, the third emission wavelength, and the first transmittance L are such that... 1T Second transmittance L 2T and third transmittance L 3T They are complementary and their sum equals the standard light source point.

[0185] Preferably, the second and third emission wavelengths are shorter than the first emission wavelength. The relative durations of the first, second, and third operating cycles will depend on the output chromaticity and output brightness produced by the variable wavelength LEDs driven in a given display frame.

[0186] The first, second, and optionally third emission wavelengths are preferably discrete primary color emission wavelengths that drive variable wavelengths, such that the output signal is formed by a combination of emitted pulses having these primary color emission wavelengths.

[0187] The method for driving a display device may be the method for driving a display device according to the first aspect of the present invention, and / or the method for driving a display device that has been calibrated using the calibration method of the second aspect of the present invention, and may include the following steps:

[0188] For the first working cycle, a variable wavelength LED is driven by a first driving condition to emit a first primary color emission wavelength λ1 with a first brightness L1, wherein the first primary color emission wavelength has a first transmission brightness L1. 1T Self-absorbing color filters transmit light;

[0189] for a second duty cycle, the variable wavelength LED is driven with a second drive condition to emit a second primary color emission wavelength λ2of the device having a second luminance L2, the second primary color emission wavelength being shorter than the first emission wavelength;

[0190] wherein the absorptive color filter attenuates the second luminance L2such that the second primary color emission wavelength is transmitted from the absorptive color filter with a second transmitted luminance L 2T from the absorptive color filter, wherein L 2T < L2;

[0191] for a third duty cycle, the variable wavelength LED is driven with a third drive condition to emit a third primary color emission wavelength λ3of the device having a third luminance L3, the third emission wavelength being shorter than the first emission wavelength;

[0192] wherein the absorptive color filter attenuates the third luminance L3such that the third primary color emission wavelength is transmitted from the absorptive color filter with a third transmitted luminance L 3T from the absorptive color filter, wherein L 3T < L3;

[0193] and controlling the duration of the first, second and third duty cycles such that the output signal emitted from the exit of the display device has an output chromaticity and an output luminance L O .

[0194] In one embodiment, the display device is driven such that the exit chromaticity and luminance corresponds to a standard illuminant point, such as D65 white. For a standard illuminant point for which the display device has been calibrated, the respective duty cycles of the subfields of the three primary color emission wavelengths are preferably the calibrated duty cycles obtained from the method of the second aspect.

[0195] In a preferred embodiment, the method is a field sequential driving method for driving a display device comprising variable wavelength LED pixels, the method comprising the steps of:

[0196] driving the variable wavelength LED pixels to emit light at three or more primary color emission wavelengths in response to respective drive conditions in respective sequential subframes of a display frame.

[0197] The method can comprise the step of driving the variable wavelength LED pixels by supplying one of three drive conditions to the variable wavelength LED during each subframe, the three drive conditions corresponding to emission of the three primary color emission wavelengths.

[0198] The method can comprise the step of repeating the driving step for each subsequent display frame.

[0199] The method can comprise the step of increasing the duty cycle of a drive condition to increase the luminance of a respective primary color emission wavelength emitted during this subframe.

[0200] The method can comprise the step of reducing the duty cycle of the driving conditions to reduce the luminance of the respective primary colour emission wavelengths emitted during the sub-frame.

[0201] The method can comprise the step of selecting, for each display frame, the set of three or more primary colour emission wavelengths to be emitted during the display frame.

[0202] The method can comprise the step of selecting different sets of primary colour emission wavelengths for different display frames.

[0203] The method can comprise the step of dividing the display frame into a plurality of sub-frames, wherein the number of sub-frames is three or more.

[0204] Each primary colour emission wavelength is preferably displayed in one or more display sub-frames during the display frame.

[0205] In a given display frame, the variable wavelength LEDs are preferably driven to emit light at three or more discrete primary colour emission wavelengths during discrete sub-frames.

[0206] The display device can comprise a plurality of variable wavelength LEDs configured in an array, each variable wavelength LED being a pixel of the device, or one of a plurality of sub-pixels of a pixel of the device. The method of driving the display device can comprise separately driving each of the variable wavelength LEDs in the display device such that each variable wavelength LED emits its own output signal. The output signal can be the same for individual variable wavelength LEDs such that two or more pixels / sub-pixels display the same output colour and luminance, or the output signal can differ between variable wavelength LEDs.

[0207] It will be appreciated by persons skilled in the art that features described above in relation to the first aspect of the application are equally applicable to the second and third aspects, and vice versa.

[0208] Opto-electric display device

[0209] The display device used in the present application is preferably an optoelectronic display device comprising:

[0210] a drive wafer comprising drive circuitry; and

[0211] a matrix of pixels formed from an array of variable wavelength LEDs, each variable wavelength LED being connected to the drive circuitry and configured to receive a tunable drive signal.

[0212] The display device according to the present application preferably has a display screen made up of an array of variable wavelength LEDs. Each variable wavelength LED acts as a pixel or sub-pixel such that the array of variable wavelength LEDs forms a matrix of pixels. Each pixel of the display is preferably a variable wavelength LED that is responsive to different drive current densities or different drive voltages by emitting different peak emission wavelengths.

[0213] The drive circuit can be configured to control the power or current or voltage to the power supply of each variable wavelength LED in the pixel matrix. The drive circuit can be configured to provide pulsed or CW or quasi-CW power supply to the variable wavelength LEDs.

[0214] Each variable wavelength LED has an epitaxial structure that preferably includes an n-doped region, a p-doped region, and a light emitting region disposed between the n-doped region and the p-doped region.

[0215] Each variable wavelength LED can include a porous region of Ill-nitride material epitaxially connected to the variable wavelength LED epitaxial structure.

[0216] It is particularly preferred that each variable wavelength LED in the pixel matrix has the same device epitaxial structure. Since an array of variable wavelength LEDs can be formed by etching a single epitaxial structure into multiple individual LED device mesas, the composition and layer configuration of each of the device mesas will be the same.

[0217] While the epitaxial layer design of each variable wavelength LED is the same, different LEDs can optionally have different lateral sizes, such that different variable wavelength LEDs have light emitting regions of different sizes.

[0218] It is preferred that each variable wavelength LED in the pixel matrix is electrically coupled to its own CMOS driver in the drive circuit, such that each pixel (each variable wavelength LED) can be independently driven by the drive circuit.

[0219] The LED epitaxial device structure of a variable wavelength LED includes a light emitting region that preferably includes a multiple quantum well (MQW) containing a plurality of quantum wells (QWs) or quantum dots, quantum wires, or other quantum nanostructures.

[0220] The LED includes an n-doped portion, a p-doped portion, and a light emitting region between the n-doped portion and the p-doped portion, the light emitting region including a light emitting layer that emits light at a peak emission wavelength under an electrical bias across it.

[0221] Variable wavelength LED

[0222] In one particularly preferred embodiment, each variable wavelength LED in the display has the same variable wavelength LED device epitaxial structure, such that each variable wavelength LED in the display device is an individual variable wavelength LED configured to emit a variable peak emission wavelength in response to a change in drive current provided to the LED. By driving the variable wavelength LED pixels separately through the drive circuit, it is advantageously possible to individually tune the emission wavelength of each pixel by providing different drive currents to the variable wavelength LED pixels.

[0223] A variable wavelength LED is configured to emit a variable peak emission wavelength in response to a change in an electrical drive signal provided to the LED (a change in drive current or drive voltage). By varying the drive current to the LED, the peak emission wavelength of the LED can be continuously controlled preferably over an emission wavelength range of at least 40 nm. By varying the drive current, the peak emission wavelength can be varied preferably over an emission wavelength range of at least 50 nm or at least 60 nm or at least 70 nm or at least 80 nm, preferably over a range of up to 100 nm or 110 nm or 120 nm or 140 nm or 160 nm or 180 nm or 200 nm or 400 nm or 450 nm.

[0224] The display is preferably connected to a drive circuit configured to supply a variable magnitude drive current or a variable magnitude drive voltage to the variable wavelength LED pixel to vary the peak emission wavelength of the variable wavelength LED device mesa.

[0225] A variable wavelength light emitting diode (LED) that can be used in a display device and that can be driven and calibrated using the method of the present application preferably comprises:

[0226] an n-doped portion;

[0227] a p-doped portion;

[0228] a light emitting region between the n-doped portion and the p-doped portion, the light emitting region comprising a light emitting layer that emits light at a peak emission wavelength under an electrical bias across it;

[0229] wherein the LED is configured to receive an electrical power supply, wherein by varying or controlling the electrical power supply, the peak emission wavelength of the LED can be continuously controlled over an emission wavelength range. By varying or controlling the electrical power supply, the peak emission wavelength of the variable wavelength LED can be continuously controlled or continuously variable over an emission wavelength range of at least 40 nm.

[0230] As the peak emission wavelength of the variable wavelength LED is continuously controllable or continuously variable over an emission wavelength range, the LED can be described as a variable wavelength LED.

[0231] The variable wavelength emission behavior of the LED structure is achieved by the fact that the LED structure (n-doped portion, light emitting region, and p-doped portion) is grown over a template that contains a porous region. The inventors have discovered that the presence of a porous region of Group III nitride material in the template structure prior to overgrowth of the LED structure results in higher quality crystal growth and thus significant benefits, including the possibility of changing the emission wavelength of the LED light emitting region. The mechanism by which the porous region enables variable wavelength emission of the LED is the subject of ongoing research. The benefits provided to the LED by the porous region include strain relaxation, lattice parameter increase, wafer bow reduction, and mechanical and thermal effects during growth of the light emitting region at high temperature.

[0232] The variable wavelength LED is configured to receive a supply of power or a drive current from a power supply or LED driver. The term "power supply" is used herein to refer to the power or current or voltage supplied to drive the LED during use.

[0233] By varying or controlling the magnitude of the drive current provided to the variable wavelength LED, the peak emission wavelength of the LED can be continuously controlled or continuously variable, preferably over a range of emission wavelengths.

[0234] In conventional LED devices, changes in the drive current provided to the LED result in a very small shift in the emission wavelength, but the inventors have discovered that the wavelength shift can be widened and controlled compared to conventional LED materials. Instead of the few nm emission range of prior art devices, the LED of the present invention can be controlled to emit over a very wide emission range, for example over a range of at least 40 nm. Since the LED of the present invention is tunable to emit over a wide range of wavelengths, it can be referred to as a variable wavelength LED.

[0235] The LED can be a dynamic color tunable LED, in which the peak emission wavelength of the LED can be tuned by varying the drive conditions provided to the LED by the drive circuit.

[0236] The LED is preferably drivable to emit at a single peak emission wavelength in response to a steady supply of power, but to emit at a different peak emission wavelength in response to a change in the supply of power. Thus, the LED can be used to emit a particular color over an extended period of time, or alternatively, the LED can be driven to emit a plurality of different wavelengths by providing varying drive conditions.

[0237] Preferably, the n-doped portion, the p-doped portion, and the light emitting region all comprise or consist of a Group III nitride material, which is preferably GaN, InGaN, AlGaN, or AlInGaN.

[0238] The variable wavelength LED preferably contains a single epitaxially grown diode structure that contains an n-doped portion, a p-doped portion, and a light emitting region. Thus, the variable peak emission wavelength of the LED is produced by the same LED diode structure and composition emission.

[0239] The LED preferably includes a porous region of Ill-nitride material. The light emitting region of the LED is preferably formed over the porous region of Ill-nitride material. In some embodiments, one of the n-doped portion or the p-doped portion can contain the porous region of Ill-nitride material. In other embodiments, the n-doped portion, the p-doped portion, and the light emitting region are disposed on a substrate that includes a porous region of Ill-nitride material. The light emitting region is preferably overgrown after the porous region has been formed during epitaxial growth of the LED.

[0240] The inventors have discovered that a porous region of Ill-nitride material enables the same LED to emit over a range of peak emission wavelengths rather than at one particular wavelength. The peak emission wavelength of the LED can be varied across the range of emission wavelengths by varying the power supply provided to the LED. The present invention thus provides a variable wavelength LED that can be controlled to emit at any wavelength across a continuous range of emission wavelengths. By varying the drive conditions provided to the LED by the power supply, the LED is capable of emitting at any wavelength within the emission wavelength range of the LED rather than simply at a discrete peak emission wavelength.

[0241] The inventors have discovered that the ability of the LED to emit at a tunable wavelength across a wide emission range can be imparted by incorporating a porous region of Ill-nitride semiconductor material into or forming the LED diode structure over a porous region of Ill-nitride semiconductor material. The benefits provided to the LED by the porous region include strain relaxation, lattice parameter increase, wafer bow reduction, and beneficial mechanical and thermal effects during growth of the light emitting region at high temperature.

[0242] During fabrication, the light emitting region of the LED is preferably formed over the porous region of Ill-nitride material such that the porous region influences the structural and mechanical properties of the layers of semiconductor epitaxially deposited over the porous region. The layers of semiconductor material deposited over the porous region during growth bring benefits such as reduced strain, increased lattice parameter, and reduced wafer bow that are imparted to the LED light emitting region and influence its structure and its light emitting behavior.

[0243] Once the LED light emitting (active) region has been epitaxially grown over the porous region and the quality of the active region has been enhanced by affecting the porous region, the beneficial effects of the porous region on the emission properties are permanently imparted to the LED active region. Thus, the LED diode structure can be left on the porous region, in which case the variable wavelength LED comprises a porous region of III-nitride material, or alternatively, the porous region can be removed from the LED structure after epitaxial growth, during processing of the LED into a device.

[0244] The width of the emission wavelength range can vary depending on the structure and composition of the LED structure (n-doped portion, light emitting region and p-doped portion) as well as the structure and porosity of the porous region. The width of the emission wavelength range can also vary depending on the size and shape of the LED (pixel size and shape).

[0245] In preferred embodiments, the peak emission wavelength can be controlled by varying the power supply across an emission wavelength range of at least 40 nm or at least 50 nm or at least 60 nm or at least 70 nm or at least 80 nm. Preferably, the peak emission wavelength can be controlled across an emission wavelength range of up to: 100 nm, or 110 nm, or 120 nm, or 130 nm, or 140 nm, or 150 nm, or 160 nm, or 170 nm, or 180 nm, or 190 nm, or 200 nm, or 400 nm, or 450 nm. Thus, the size of the emission wavelength range that can be obtained with the inventive LED is much larger than the emission range that can be achieved with prior art LEDs.

[0246] The variable wavelength LED can advantageously be controlled to emit at any peak emission wavelength within its emission wavelength range. By varying the characteristics of the power supply as well as the LED pixel size and shape, the variable wavelength LED can thus be controlled to emit light at any selected peak emission wavelength within this range.

[0247] The emission wavelength of the variable wavelength LED can preferably continuously vary across its emission wavelength range in response to continuously varying the driving conditions provided by the power supply across its driving condition range.

[0248] The location of the emission wavelength range in the electromagnetic spectrum can also vary depending on the design of the variable wavelength LED structure (n-doped portion, light emitting region and doped portion). For example, the wavelengths contained within the emission wavelength range can depend on the number and composition of the light emitting layers in the variable wavelength LED. A variety of LED active regions for emitting at different wavelengths in the visible spectrum are known in the art, thus by forming the light emitting region of the inventive LED with different light emitting regions, emission wavelength ranges covering different portions of the spectrum can be obtained.

[0249] The variable wavelength LED emission wavelength range can be between 400 nm to 850 nm, or 400 nm to 800 nm, or 400 nm to 690 nm, or 400 nm to 675 nm. The emission wavelength range can be a sub-range within the range of 400 nm to 750 nm. The emission wavelength range can be tuned to cover any portion of this range by selecting different LED active regions and controlling the size and shape of the LED pixels.

[0250] Preferably, the emission wavelength range of the variable wavelength LED extends from a lower end below 410 nm or 430 nm or 450 nm or 470 nm or 500 nm or 520 nm or 540 nm or 560 nm to an upper end above 570 nm or 580 nm or 600 nm or 610 nm or 630 nm or 650 nm or 675 nm. As described above, the first and second ends of the emission wavelength range can be tuned depending on the LED structure as well as the selection of LED shape and size.

[0251] For example, in a preferred embodiment, the lower end of the emission wavelength can be between 400 nm to 450 nm (violet) or between 450 nm to 500 nm (blue) or between 500 nm to 570 nm (green), and the upper end of the emission wavelength can be between 570 nm to 590 nm (yellow) or between 590 nm to 610 nm (orange) or between 610 and 700 nm (red).

[0252] In one preferred embodiment, the variable wavelength LED emission wavelength range can extend from a lower end below 500 nm to an upper end above 610 nm, such that the peak emission wavelength of the LED can be changed by varying the power supply to emit at any wavelength from blue (below 500 nm) to red (above 610 nm). It is highly advantageous to provide a single LED design that can be controlled to emit at blue wavelengths (450 nm to 500 nm), green (500 nm to 570 nm), and also at yellow (570 nm to 590 nm), orange (590 nm to 610 nm), and red (610 nm to 760 nm), and can provide significant advantages for LED displays.

[0253] In other preferred embodiments, the variable wavelength LED emission wavelength range can extend between 520 nm to 660 nm or between 550 nm to 650 nm by varying the power supply to the LED.

[0254] In particularly preferred embodiments, the peak emission wavelength can be controlled between 540 nm to 680 nm or 560 nm to 675 nm by varying the power supply. Thus, the same LED can be controllable to emit at a peak emission wavelength anywhere between green at 540 nm to red at 680 nm. Green and red LEDs have historically been more difficult to manufacture than shorter wavelength blue LEDs due to problems such as difficulty incorporating the required indium content into the light emitting region. Thus, it is highly advantageous to provide a single LED design that can be controlled to emit at green wavelengths (500 to 570 nm) and also at yellow (570 to 590 nm), orange (590 to 610 nm), and red (610 to 760 nm), and can provide significant advantages for LED displays.

[0255] In another preferred embodiment, the peak emission wavelength can be controlled between 520 nm to 675 nm or 550 nm to 650 nm by varying the power supply.

[0256] While a variable wavelength LED can emit across a continuous emission wavelength range, in some embodiments, it can be desirable to control the LED to function in multiple discrete emission modes, for example, in response to a power supply having multiple drive modes. For example, by driving the LED in multiple different modes corresponding to discrete emission colors, a simplified color display can be provided in which the discrete emission colors are mixed in known methods to achieve a desired visual effect.

[0257] A variable wavelength LED can preferably be controlled to emit at least two discrete peak emission wavelengths by varying a drive condition provided by the power supply between two discrete drive conditions, such as two discrete values of drive current. The LED can be controllable to emit at a first peak emission wavelength in response to a first drive condition provided by the power supply, which can be a drive current having a first value, and to emit at a second peak emission wavelength in response to a second drive condition provided by the power supply, which can be a drive current having a second value different from the first value.

[0258] A variable wavelength LED can preferably be controlled to emit at least three discrete peak emission wavelengths by varying a drive condition provided by the power supply. The peak emission wavelength of the variable wavelength LED can thus be variable over at least three "colors" in the EM spectrum.

[0259] A variable wavelength LED can be controllable to emit at a first peak emission wavelength in response to a first drive condition provided by the power supply, to emit at a second peak emission wavelength in response to a second drive condition provided by the power supply, and to emit at a third peak emission wavelength in response to a third drive condition provided by the power supply.

[0260] The variable wavelength LED can preferably be controllable to emit a blue peak emission wavelength in response to a first drive condition provided by the power supply, a green peak emission wavelength in response to a second drive condition provided by the power supply, and a red peak emission wavelength in response to a third drive condition provided by the power supply.

[0261] The variable wavelength LED can be controllable to emit a first peak emission wavelength in the range of 400 nm to 500 nm in response to a first drive condition provided by the power supply, a second peak emission wavelength in the range of 500 nm to 550 nm in response to a second drive condition provided by the power supply, and a third peak emission wavelength greater than 600 nm in response to a third drive condition provided by the power supply.

[0262] Preferably, the variable wavelength LED is controllable to emit a first peak emission wavelength in the range of 430 nm to 460 nm in response to a first drive condition provided by the power supply, a second peak emission wavelength in the range of 510 nm to 560 nm in response to a second drive condition provided by the power supply, and a third peak emission wavelength in the range of 600 nm to 660 nm in response to a third drive condition provided by the power supply.

[0263] The first, second, and third drive conditions can be first, second, and third current densities, or the first, second, and third drive conditions can be first, second, and third power densities.

[0264] The morphology of quantum wells (QWs) in the active light emitting region can be varied. For example, the light emitting region can contain uniform QWs with well-defined interfaces, or fragmented QWs with less well-defined interfaces, fragmentation of QW well width / composition, or quantum dots (e.g., localized centers). This control of QW morphology can determine the range of variable emission wavelengths to be controlled and manipulated.

[0265] The light emitting region preferably includes a plurality of quantum wells (QWs). The quantum wells can be continuous. The quantum wells can be fragmented or non-continuous.

[0266] The variable wavelength LED can include a current confinement layer or a current restriction layer, which is a dielectric layer configured to confine the lateral area of the LED through which current is conducted. The use of a current confinement layer can advantageously allow for further control of the current density in order to better control the peak emission wavelength of the LED.

[0267] Advantageously, the current confinement layer can enable manipulation of the power density provided to the variable wavelength LED in order to control the peak emission wavelength.

[0268] The current confinement layer is preferably a layer of dielectric material. By way of example, the current confinement layer can be any dielectric, such as SiO2, SiN or SiN X .

[0269] The current confinement layer can be positioned in a variety of locations in the variable wavelength LED, so long as it restricts the lateral area of the LED through which current is conducted. The current confinement layer can be positioned in the LED between the electrical n- and p- contacts.

[0270] The current confinement layer can be positioned adjacent to the n- or p-doped portion of the LED. By way of example, the current confinement layer can be positioned between the n-doped portion and the light emitting region. Alternatively, the current confinement layer can be positioned between the light emitting region and the p-doped portion. The current confinement layer can be positioned between the electrical contacts and the LED structure (n-doped portion, p-doped portion and light emitting region).

[0271] The current confinement layer preferably includes an aperture extending through the current confinement layer or one or more voids extending through the current confinement layer. The aperture can preferably be positioned in the center of the current confinement layer. By way of example, the current confinement layer can include a circular opening in the center of the LED structure.

[0272] The variable wavelength LED can be configured such that the electrical contacts contact the LED structure via the aperture in the current confinement layer, such that the area of the aperture defines a contact area over which the contacts and LED structure touch.

[0273] The lateral dimension of the aperture or apertures is preferably much smaller than the lateral dimension of the LED. By providing an aperture through the dielectric current confinement layer, a high local current density can be achieved, which can advantageously enable improved control over the power through the LED.

[0274] By way of example, the lateral width (or diameter) of the aperture can be equal to or less than 50% of the lateral width of the LED structure (LED mesa). The width of the aperture can be equal to or less than 45%, or 40%, or 35%, or 30%, or 25%, or 20% of the width of the LED structure.

[0275] The relative area of the aperture compared to the total area of the current confinement layer (blocking region) can be varied to modify the local current density.

[0276] The light emitting region preferably includes a multiple quantum well (MQW) containing a plurality of quantum wells (QWs) or quantum dots, quantum wires or other quantum nanostructures.

[0277] In some embodiments, the light emitting region includes a plurality of quantum wells (QWs), and the quantum wells are continuous.

[0278] The inventors have discovered that non-uniformity in the light emitting region has a significant effect in widening the emission wavelength range over which the light emitting region can emit light in response to changes in the power supplied to the LED. In the prior art, non-uniformity in the light emitting region is generally viewed as a problematic defect that is undesirable and should be avoided in any way possible, as the goal is generally a high quality, low defect semiconductor wafer. The inventors have bucked this trend in the art and have discovered that intentionally creating non-uniformity in the light emitting region can advantageously widen the emission wavelength range and result in a variable wavelength LED that can emit over a wavelength range much wider than possible in the prior art.

[0279] In alternative embodiments of the invention, the light emitting region is non-uniform, fragmented, or non-continuous. The light emitting region can be intentionally introduced to achieve effects of carrier localization centers in the InGaN quantum wells, such as multiple types of QW regions with different indium composition and well width and quantum barriers, non-uniform or fragmented or broken or gapped or non-continuous quantum wells that will result in fluctuations in well width, InGaN quantum dots or nanostructures, quantum wells formed on polar, semipolar, or non-polar facets.

[0280] In a preferred embodiment, the light emitting region includes a plurality of quantum wells (QWs), and the quantum wells are non-uniform, fragmented, or non-continuous.

[0281] The plurality of QWs can include fluctuations in well width. For example, the well width of the QWs can fluctuate by at least 2%, 5%, 10%, 20%, 25%, or 50%, or 75%. The fluctuations in well width can be changes between quantum wells (vertically) and within one quantum well (laterally).

[0282] The plurality of QWs can include fluctuations in alloy composition. For example, the indium composition of the QWs can change across the light emitting region by at least 2%, 5%, 10%, 20%, 25%, or 50%, or 75%.

[0283] The inventors have discovered that fluctuations in well width and / or alloy composition can induce carrier localization centers in the upper or lower interface of the QWs. Any carrier localization center will induce a variable wavelength in the variable wavelength LED of the invention. The greater the density of such carrier localization centers, the greater the variable wavelength range that can be achieved.

[0284] The variable wavelength LED can include v-shaped pits that extend or spread through the light emitting region. Preferably, the LED includes a plurality of v-shaped pits that extend through the light emitting region.

[0285] Preferably, the variable wavelength LED can include at least 1 x 10 7 / cm 2 , for example at least 5 x 10 7 / cm2 or at least 1 x 10 8 / cm 2 v-pit density (measured when looking down on the LED structure), for example 1 x 10 7 / cm 2 to 5 x 10 9 / cm 2 v-pit density.

[0286] The variable wavelength LED can comprise a v-pit density of less than 5 x 10 9 / cm 2 , for example less than 1 x 10 9 / cm 2 or less than 5 x 10 8 / cm 2 v-pit density.

[0287] V-pits are an epitaxial semiconductor growth phenomenon known in the art, and methods of growing v-pits in semiconductor structures are known in the art. For example, v-pits and their growth are described in the prior art in The effect of nanometre-scale V-pits on electronic and optical properties and efficiency droop of GaN-based green light-emitting Diodes; Zhou et al; Scientific Reports | (2018) 8: 11053 | DOI: 10.1038 / s41598-018-29440-4.

[0288] The v-pits are v-shaped when viewed in cross-section, but in reality, they are conical or funnel-shaped voids formed in the semiconductor structure, which are grown from the bottom up using known epitaxial growth methods. While the cross-section of the pits is v-shaped, when viewed from above, the pits are typically hexagonal. The point of the v-pit always points down to the early deposited layers of the semiconductor structure, as the pit widens as subsequent epitaxial growth layers are deposited on top of the structure.

[0289] While v-pits are known in the art, they are generally considered to be problematic defects in semiconductor structures, which are unwanted, as the goal is generally a high quality, low defect semiconductor wafer.

[0290] In unusual cases where v-pits have been incorporated into semiconductor structures in the past, v-pits have been used as a masking mechanism to create a higher bandgap region that prevents current carriers from following a threading dislocation as a leakage path.

[0291] However, in some preferred embodiments of the present application, v-pits are intentionally incorporated into the variable wavelength LED structure. The v-pits extend far enough down into the semiconductor structure that they terminate in a layer below the active light emitting region. This means that the v-pits must extend through the thickness of the active light emitting region.

[0292] The present inventors have discovered that v-pits extending through the light emitting region of an LED structure can advantageously widen the emission wavelength range over which the variable wavelength LED can emit.

[0293] When the v-pits extend through the active region of the LED, during the bottom-up epitaxial growth, quantum well (QW) layers that grow across the rest of the structure, planar, grow on the sloped sidewalls of the v-pits. The QWs deposited on the pit sidewalls distort and stretch around the sides of the pit, and thus end up with a different thickness and composition than the planar QWs across the bulk of the structure.

[0294] Around the v-pits, the QW layers of semiconductor material grow as planar layers. Thus, the active light emitting region is planar around the v-pits. However, in the location of the v-pits, the active layers distort and stretch down the sidewalls into the v-pits. This stretching effect changes the thickness of the QWs on the sidewalls of the pits, such that they are different than the planar QW layers formed over the rest of the LED structure.

[0295] The present inventors have discovered that v-pits can create a local strain relaxation, and the MQWs deposited on the sidewalls of these v-pits will have a different thickness and composition than the rest of the MQWs, and thus the MQWs in the v-pit region will produce a different emission wavelength.

[0296] The quantum wells grown on the sidewalls of the v-pits are thinner than the bulk planar QWs elsewhere in the structure, which can affect the QW bandgap and allow the QWs in this region to emit at a different wavelength than those emitted by the planar QWs elsewhere in the structure. In addition to this, the QWs on the pit sidewalls can also end up with a higher indium (In) content than the surrounding planar QWs, because the sidewalls expose the semi-polar facets of the QWs - these facets have more indium during epitaxial growth, so the QWs in the v-pit region can have a higher indium than the planar QWs around the pit. Higher indium incorporation generally results in a longer peak emission wavelength. Both the QW thickness and the indium content affect the emission wavelength produced by the light emitting region. Thus, the presence of v-pits in an LED structure can advantageously modify the composition and thickness of the QWs in the light emitting region in a way that extends the emission wavelength range over which the LED can be driven to emit light.

[0297] V-pits are typically grown from threading dislocations in a semiconductor structure. When additional layers are grown over a layer containing threading dislocations, the threading dislocations continue up through the structure, and at some point the dislocations widen into v-pits. Typically, those skilled in the art aim to keep the threading dislocation concentration low in order to produce "high quality" low defect wafers.

[0298] V-pits can alternatively be grown using a 3-dimensional epitaxial growth mode. 3D epitaxial deposition techniques are known in the art and are typically used to grow "islands" or "pyramids" of semiconductor material on a template. By using a 3D epitaxial deposition technique to control the deposition of the LED structure, v-pits can be grown artificially in desired locations, where there need not be a threading dislocation to "seed" the formation of the v-pit. By using such deposition control, the bottom (lowest point) of the pit can be created at a desired location in the structure (both a desired lateral location in the structure and a desired height).

[0299] The bottom of the v-pit can be located in a connecting layer of the semiconductor structure. The connecting layer can be located between the porous region and the n-doped portion.

[0300] The bottom of the v-pit can be located in a pre-strained layer of the semiconductor structure. The pre-strained layer can be located above the n-doped portion and below the light emitting region.

[0301] Preferably, the variable wavelength LED includes a plurality of v-pits extending through the active light emitting region.

[0302] Preferably, the variable wavelength LED includes a v-pit density (measured from above the LED structure) of at least 1 x 10 7 / cm 2 , for example at least 5 x 10 7 / cm 2 or at least 1 x 10 8 / cm 2 . The LED can include a v-pit density of less than 5 x 10 9 / cm 2 , for example less than 1 x 10 9 / cm 2 or less than 5 x 10 8 / cm 2 .

[0303] For example, the variable wavelength LED can include a v-pit density of 1 x 10 7 / cm 2 to 5 x 10 9 / cm 2 or 5 x 10 7 / cm 2 to 5 x 10 9 / cm 2 or 1 x 10 8 / cm 2 to 5 x 10 8 / cm 2 v-pit density.

[0304] The variable wavelength LED can include more than 0.1 v-pits per square micron, or more than 1 v-pit per square micron, or more than 2 v-pits per square micron.

[0305] The concentration of v-pits in the variable wavelength LED is preferably controlled, as too many v-pits can adversely affect the light emission of the LED by destroying the recombination of the radiation. For example, the LED can include less than 10 v-pits per square micron, or less than 8 v-pits per square micron, or less than 6 v-pits per square micron.

[0306] In a preferred embodiment, the LED structure can include no more than 10 9 threading dislocations per square centimeter. Preferably, the semiconductor structure below the active light emitting region (typically the substrate, the porous region, and the connecting layer) includes no more than 10 9 threading dislocations per square centimeter. The threading dislocation density is preferably limited to this level so that further epitaxial growth does not create too many v-pits in the light emitting region.

[0307] Both the density and size (depth) of the v-pits can be controlled. The size of the v-pits can be controlled by the position and growth conditions of the pre-strained layer and the low temperature nGaN layer where the pits start.

[0308] The morphology of the quantum wells (QWs) in the active light emitting region can be varied. For example, the light emitting region can contain uniform QWs with well defined interfaces, or fragmented QWs with less well defined interfaces, broken up QW well widths / compositions, or quantum dots (like localized centers). This control of the QW morphology can determine the range of variable emission wavelengths to be controlled and manipulated.

[0309] The light emitting region preferably includes a plurality of quantum wells (QWs). The quantum wells can be continuous. The quantum wells can be fragmented or non-continuous.

[0310] If the QWs are continuous and very uniform in thickness and composition, the recombination of the charge carriers can only occur in a regularly well defined manner. On the other hand, if the QWs are fragmented or non-continuous, this creates many nanostructures, which in turn creates different bandgaps that result in different colors being emitted.

[0311] The light emitting region and / or LED can have lateral dimensions (width and length) greater than 100 pm and less than 300 pm. In this case, the LED can be referred to as a "small LED". In preferred embodiments, the small LED can be square or circular or square with rounded corners and have dimensions such as 300 pm x 300 pm, 200 pm x 200 pm, 100 pm x 100 pm.

[0312] The light emitting region and / or LED can alternatively have lateral dimensions (width and length) less than 100 pm. In this case, the LED can be referred to as a "micro LED". The micro LED can preferably have lateral dimensions less than: 80 pm, or 70 pm, or 60 pm, or 50 pm or 30 pm, or 25 pm, or 20 pm, or 15 pm or 10 pm, or 5 pm or 3 pm or 2 pm.

[0313] In preferred embodiments, the micro LED can be square or circular or square with rounded corners and have dimensions such as: 75 pm x 75 pm, 50 pm x 50 pm, 40 pm x 40 pm, 30 pm x 30 pm, 25 pm x 25 pm, 20 pm x 20 pm or 10 pm x 10 pm, or 5 pm x 5 pm, or 2 pm x 2 pm, or 1 pm x 1 pm, or 500 nm x 500 nm or less.

[0314] The light emitting region and / or LED can alternatively have lateral dimensions (width and length) less than 1 pm. In this case, the LED can be referred to as a "nano LED". The nano LED can preferably have lateral dimensions less than or 500 nm or 200 nm or 100 nm or 50 nm.

[0315] The LED can be circular, triangular, rectangular, square, oval, diamond, hexagonal, pentagonal and any combination of such shapes. In the case of irregular shapes of the pixel design, at least one dimension should be within the dimensions defined above in order to classify the LED as a small or micro LED. For example, the width or diameter of the LED is preferably less than 100 pm so that the LED is classified as a micro LED.

[0316] Porous region

[0317] The variable wavelength LED preferably comprises one or more porous regions of Ill-nitride material.

[0318] The n-type region, the light-emitting region, and the p-type region (which may be referred to as the LED structure or LED diode structure) are preferably grown on a semiconductor template containing porous regions. The semiconductor template may also contain multiple layers of semiconductor material configured to provide a suitable substrate for the overgrowth of the LED structure.

[0319] The porous region can be a porous layer, such that the light-emitting diode comprises a porous layer of group III nitride material. Preferably, the porous region can be a continuous porous layer, for example, formed from a continuous layer of porous group III nitride material.

[0320] The porous region may include multiple porous layers and optionally multiple non-porous layers. In a preferred embodiment of the invention, the porous region is a stack of alternating porous and non-porous layers, wherein the top surface of the stack defines the top of the porous region and the bottom surface of the stack defines the bottom of the porous region. The luminescent region may be formed above the porous region comprising the stack of porous layers of a group III nitride material.

[0321] Each variable wavelength LED may include a porous region of its own group III nitride material, or alternatively, a shared porous region may be epitaxially connected to multiple variable wavelength LEDs. Attached Figure Description

[0322] Embodiments of the invention will now be described with reference to the figures in which:

[0323] Figure 1A The illustration shows the CIE xy color gamut that can be used in this invention and the wavelengths that can be directly emitted by an exemplary variable wavelength LED;

[0324] Figure 1B The diagram shows... Figure 1A The CEI xy color gamut overlaps with the emission wavelengths of the two primary colors that can be used for device control;

[0325] Figure 1C The diagram shows... Figure 1A The CEI xy color gamut overlaps with the emission wavelengths of the three primary colors that can be used for device control;

[0326] Figure 2 The diagram illustrates the current gain and duty cycle applied to a variable wavelength LED pixel within a display frame for three driving modes.

[0327] Figure 3a The schematic diagram illustrates three emission peaks emitted by the same variable wavelength LED pixel in response to three different drive currents;

[0328] Figure 3b The schematic diagram illustrates three emission peaks emitted by the same variable wavelength LED pixel in response to three different driving voltages;

[0329] Figure 4 a duty cycle in which three pulses of drive current are provided to the same variable wavelength LED in three discrete sub-frames in a field sequential manner;

[0330] Figure 5 schematically illustrates the emission luminance of three primary color emission wavelengths from a variable wavelength LED due to three pulses of drive current having different duty cycles;

[0331] Figure 6 schematically illustrates a display device according to the present application in which an absorptive color filter is positioned between the variable wavelength LED and the exit of the display device;

[0332] Figure 7 schematically illustrates the emission luminance and transmission luminance of three primary color emission wavelengths from a display device of Figure 6 ;

[0333] Figure 8A shows experimental results showing the transmission luminance measured when testing embodiments of the present application with a suitable long pass filter;

[0334] Figure 8B illustrates the expected absorption band of the filter tested in Figure 8A ;

[0335] Figure 9 is a series of five EL images of the same micro-LED pixel driven at different currents in constant wave mode (CW), showing five different emission colors;

[0336] Figure 10A is a plot of emission wavelength versus current density for a 25 μm x 25 μm 100 x 100 variable wavelength LED pixel array driven in pulsed mode with 100 μs pulses at 1% duty cycle;

[0337] Figure 10B is a plot of emission wavelength versus current density for a 30 μm x 30 μm 100 x 100 variable wavelength LED pixel array driven in pulsed mode with 100 μs pulses at 1% duty cycle;

[0338] Figure 11 is a plot of intensity versus wavelength for a single variable wavelength LED driven at different currents in pulsed drive mode with 100 μs pulses at 1% duty cycle;

[0339] Figure 12A to Figure 12G illustrates alternative embodiments of non-uniform, fragmented or non-continuous light emitting regions of a variable wavelength LED that can be used in preferred embodiments of the present application;

[0340] Figure 13ATEM image of the cross section of a known non-variable wavelength LED;

[0341] Figure 13B TEM image of the light emitting region of a variable wavelength LED including v-pits that can be used in embodiments of the present invention;

[0342] Figure 13C TEM image of a variable wavelength LED that can be used in a preferred embodiment of the present invention Figure 12B TEM image of a variable wavelength LED that can be used in a preferred embodiment of the present invention

[0343] Figure 14A Plot of peak emission wavelength versus drive current density for a known non-variable wavelength LED;

[0344] Figure 14B Plot of peak emission wavelength versus drive current density for a variable wavelength LED according to an embodiment of the present invention;

[0345] Figure 14C Plot of peak emission wavelength versus drive current density for a variable wavelength LED according to another embodiment of the present invention;

[0346] Figure 15A Plot of peak emission wavelength versus drive current density for another variable wavelength LED that can be used in embodiments of the present invention;

[0347] Figure 15B to Figure 15D Photograph of a variable wavelength LED of Figure 15A Photograph of a variable wavelength LED of where the inserted emission spectra show different peak emission wavelengths at different drive current densities. DETAILED DESCRIPTION

[0348] As described above, an optoelectronic display device containing variable wavelength LEDs can be operated to display any chromaticity within a displayable color gamut by selecting a plurality of primary color emission wavelengths and combining the emission of these primary color wavelengths to provide a combined output signal having a desired output chromaticity and output luminance.

[0349] The size of the color gamut that can be achieved depends on the wavelengths of the selected primary colors.

[0350] In the present invention, a variable wavelength LED is used to emit at least one of the primary color emission wavelengths, and preferably the same variable wavelength LED emits a plurality of the primary color emission wavelengths.

[0351] The variable wavelength LEDs that can be used in the present application are known as Dynamic Pixel Tuning (DPT®) variable wavelength LEDs from Poro Technologies Ltd, and are disclosed in International Patent Application No. PCT / GB2022 / 051997 published as WO2023 / 007174.

[0352] In a preferred embodiment, the same variable wavelength pixel is used to emit all the selected primary colours using field sequential emission. In this case, all the primary emission wavelengths within the emission wavelength range of the variable wavelength LED are selected.

[0353] Although the present application can be carried out using a sub-pixelated display in which a variable wavelength LED emits at least one of the emission wavelengths, and one or more other primary emission wavelengths are emitted by other sub-pixels, the present application will be described with reference to a field sequential control embodiment in which one variable wavelength LED forms a pixel of the display device and is driven to emit all the selected primary colours using field sequential emission.

[0354] Figure 1A The emission wavelength range of one such variable wavelength LED that can be used in the present application is illustrated (indicated by line 100), plotted in the CIE xy colour space. The variable wavelength LED can be driven to emit at any wavelength positioned on line 100 by controlling the drive conditions provided to the LED.

[0355] As described above, instead of driving a variable wavelength LED to emit a continuous emission wavelength by supplying a continuously varying drive current, it is convenient to drive such LEDs by selecting two or three or more than three “primary” emission wavelengths at which the variable wavelength LED operates. An output signal having a desired output colour and output luminance can then be generated from a temporal combination of light pulses at the primary emission wavelengths, similar to known field sequential displays.

[0356] Unlike prior art displays, a display device with variable wavelength LEDs can be operated using only two primary emission wavelengths. Figure 1BThe emission wavelength range 100 of a variable wavelength LED is shown, from which two primary color emission wavelengths 210, 220 have been selected. By varying the relative ratios of light emitted at the two primary color emission wavelengths 210, 220, output signals of any color along the mixture line 230 can be produced. The two selected primary color emission wavelengths 210, 220 are complementary, such that the mixture line 230 passes through the D65 white point 200, and D65 white light can be produced by mixing the two primary color emission wavelengths in the desired proportions. In order to emit output signals of different output chromaticities and luminances in different display frames, the two primaries can be moved to different locations on the emission wavelength range 100 between display frames. Moving the two primary color emission wavelengths to different wavelengths advantageously moves the mixture line 230, so that a variety of different colors can be produced.

[0357] Instead of using two primaries, a variable wavelength LED can be operated by driving the LED to emit three discrete primary color emission wavelengths at three discrete drive conditions, such as three drive currents having different magnitudes. Figure 1C The emission wavelength range 100 of a variable wavelength LED is shown, from which two primary color emission wavelengths 210, 220 have been selected. By varying the relative ratios of light emitted at the two primary color emission wavelengths 210, 220, output signals of any color along the mixture line 230 can be produced. The two selected primary color emission wavelengths 210, 220 are complementary, such that the mixture line 230 passes through the D65 white point 200, and D65 white light can be produced by mixing the two primary color emission wavelengths in the desired proportions. In order to emit output signals of different output chromaticities and luminances in different display frames, the two primaries can be moved to different locations on the emission wavelength range 100 between display frames. The emission wavelength range 100 of a variable wavelength LED is shown, overlapping three selected primary color emission wavelengths 250, 260, 270. By varying the relative ratios of light emitted at the three primary color emission wavelengths, output signals of any color within the color gamut triangle 280 can be produced. The three selected primary color emission wavelengths 250, 260, 270 are complementary, such that the color gamut triangle 280 contains the D65 white point 200, and D65 white light can be produced by mixing the three primary color emission wavelengths in the desired proportions. Depending on the desired chromaticity and luminance of the output signal, the locations of the three primaries in the emission wavelength range 100 can optionally be varied between display frames.

[0358] For simplicity, the following examples refer to a preferred embodiment in which three primary color emission wavelengths are used to drive a variable wavelength LED using field sequential driving. In order to obtain a large color gamut triangle 280 of displayable output chromaticities, in the preferred embodiment the LED is driven to emit first, second, and third primary color emission wavelengths that are respectively red (R), green (G), and blue (B).

[0359] Figure 2 Current gain and duty cycle of three drive conditions applied to a variable wavelength LED pixel within one display frame are illustrated. This diagram illustrates the way in which emission wavelength and luminance can be adjusted independently of one another in a display incorporating variable wavelength LEDs. The longer the duty cycle ("on time") of the drive signal, the higher the luminance of the emitted light. The higher the current density of the drive signal, the shorter the wavelength. Figure 2 How three different drive current values produce three different emission colors, while three different duty cycles produce three different luminances (brightnesses), is schematically illustrated.

[0360] Figure 3a Three emission peaks emitted by the same variable wavelength LED pixel in response to three different drive currents I1, I2, I3are schematically illustrated. Different magnitudes of drive current result in different peak emission wavelengths.

[0361] Figure 3b An alternative scenario of three emission peaks emitted by the same variable wavelength LED pixel in response to three different drive voltages V1, V2, V3is schematically illustrated. Different magnitudes of drive voltage result in different peak emission wavelengths.

[0362] Table 1 illustrates experimental data obtained for a variable wavelength LED that can be used in the present application. Table 1 shows the measured conditions for a 0.26" array at 1% APL, where the wavelength is in nm, and L is luminance.

[0363]

[0364] Table 1.

[0365] This shows that the variable wavelength LED emits longer wavelengths at much lower luminance than where the shorter wavelengths are emitted. When high display luminance is required, the low luminance of the longer wavelength primary colors can become a limiting factor.

[0366] When driving a display comprising variable wavelength LEDs, it is advantageous to calibrate the display device by choosing white balance conditions so that the relative luminance of the primary colors can be set, although the LEDs can be driven to emit all such wavelengths.

[0367] In one exemplary embodiment, a set of three primary color emission wavelengths is chosen to be red, green and blue at 639 nm, 533 nm and 466 nm, respectively.

[0368] In order for the output signal formed by the combination of these three primary colors to produce D65 white, the luminance of the red, green and blue primary colors must be set in the ratio 1.6:3.6:1, respectively.

[0369] In order to maximize the luminance of the red primary color, the duty cycle of the drive signal of the red primary color is set to 100% at 639 nm. This results in the red primary color emitting at 639 nm with a luminance of 68 cd / m 2 .

[0370] In order to satisfy the required color balance ratio, the luminance required for the green and blue primary colors is calculated as shown in Table 2. Knowing the required luminance of these primary colors, the duty cycles of the drive signals of the green and blue primary colors are calculated to satisfy the white point condition.

[0371] The duty cycles for the other wavelengths are free parameters and are calculated by interpolation.

[0372]

[0373] Table 2.

[0374] Now we have a wavelength (i.e. chromaticity coordinates) and luminance for all wavelengths in the set.

[0375] For this data set, the D65 white balanced luminance is 264 cd / m 2 .

[0376] This is limited by the luminance at the longest wavelength to be used as a primary color emission wavelength, with a duty cycle of 100%.

[0377] The blue duty cycle is very short (at or near 0%) because even at the duty cycle case of the red drive condition at 100% duty cycle, the inherent low luminance of the long wavelength red emission means that to satisfy this luminance ratio, the green and blue drive duty cycles must be made very short. In the case of the shortest primary wavelength (blue), to get a blue luminance that will balance the red luminance, the blue duty cycle must be close to zero.

[0378] If the duration of the pulse width modulation (duty cycle) of the drive condition is too short for the blue and green primary emission wavelengths, the least significant bit LSB (i.e. the emission wavelength requiring the shortest duty cycle) time of the PWM will be too short to achieve. The micro-LED parasitic capacitance will also cause poor performance in low gray levels.

[0379] Figure 4 An exemplary field sequential drive sequence is illustrated in which R, G, and B drive conditions are supplied as individual pulses of drive current I R、 I G、 I B to a variable wavelength LED within three sequential sub-frames within a single display frame. As set forth above, to balance the luminance of the three primaries, the duty cycle of the G and B drive pulses is shorter (narrower pulse width) than the duty cycle of the R drive pulse driving the LED to emit the red primary emission wavelength. In fact, the order and duration of the sub-frames can vary.

[0380] Figure 5 A schematic illustration of the relative luminance of the R, G, and B primary emission wavelengths is provided, which is required in order for the output signal to be perceived as D65 white light. To achieve this, the B duty cycle must be made very short while the R duty cycle is maintained at 100%.

[0381] The present invention provides a solution to the problem of too short duty cycle for short primary wavelengths (such as B) by incorporating an absorptive color filter into the display device in a location between the LED and the exit of the device that is visible to the viewer.

[0382] Figure 6 A display device according to the present application is illustrated, having an absorptive color filter 600 positioned over a variable wavelength LED pixel 650 that is drivable to emit light at three primary color emission wavelengths λ 1、 λ2 and λ3.

[0383] In this example, λ1 is the shortest of the primary color emission wavelengths and is preferably a red wavelength, while λ2 is a longer primary color emission wavelength, such as a green wavelength, and λ3 is the longest primary color emission wavelength, such as a blue wavelength.

[0384] To compensate for the inherently higher brightness of the green and blue emissions from the LED 650, the absorptive color filter 600 is configured to absorb a proportion of the light having wavelength λ2, and to emit an even higher proportion of the light having wavelength λ3.

[0385] The filter 600 can alternatively be provided as two filters 600, one of which absorbs light having wavelength λ2, and the other of which absorbs light having wavelength λ3.

[0386] Since the shortest wavelength λ1 is transmitted as far as possible from the device, the absorptive filter 600 is designed not to absorb light having wavelength λ1.

[0387] In use, when a first drive condition is supplied to the LED 650, the LED emits light having the first primary color wavelength λ1. The emitted light has a first luminance LI, which depends on the length of the first duty cycle of the first drive condition supplied to the LED. Since the absorptive filter 600 is configured not to absorb any light at wavelength λ1, this emitted light is transmitted through the filter 600 at or close to 100% transmission, so that the first transmitted luminance L 1T of the light that passes through the filter is close to or equal to the first luminance.

[0388] When a second drive condition is supplied to the LED 650, the LED emits light having the second primary color wavelength λ2. The emitted light has a second luminance L2, which depends on the length of the second duty cycle of the second drive condition supplied to the LED. Since the absorptive filter 600 is configured to absorb a proportion of the light at wavelength λ2, this emitted light is transmitted through the filter 600 at less than 100% transmission. This means that the second transmitted luminance L 2T of the light that passes through the filter is less than the second luminance L2.

[0389] When the third driving condition is supplied to LED 650, the LED emits light with a third primary color wavelength λ2. The emitted light has a third brightness L3, which depends on the length of the third duty cycle supplied to the LED. Because the absorptive filter 600 is configured to absorb a high proportion of light at wavelength λ3, the emitted light is transmitted through the filter 600 with significantly less than 100% transmission. This means that the third transmittance brightness L of the light emitted from the display device after passing through the filter is... 3T It is much lower than the third brightness level, L3.

[0390] The use of the absorptive filter 600 solves the problem of excessively short duty cycles for shorter, brighter emission wavelengths by providing a wavelength-specific absorptive color filter. This wavelength-specific absorptive color filter reduces the transmittance of the green and blue emission wavelengths of the variable wavelength LED without reducing the transmittance of the red primary color emission wavelength. The attenuation of the green and blue wavelengths by the absorptive color filter allows for the use of longer duty cycles to drive the green and blue emission wavelengths.

[0391] Because absorptive color filters reduce the brightness of transmitted green / blue light, instead of requiring extremely short duty cycles to produce suitable green and blue brightness, longer and more practical duty cycles can be used for green and blue emission. These longer green and blue duty cycles cause the green and blue emission wavelengths to be emitted at a brightness higher than required for the output signal, but absorptive color filters counteract this by partially absorbing light at these primary color wavelengths, reducing the brightness of this light to the transmitted brightness required for the green / blue components of the output signal.

[0392] When an observer of the display device perceives the output signal as a combination of the wavelengths emitted by the transmissive filter, the transmitted brightness of the first and second emitted wavelengths is combined to generate the perceived output signal. An absorptive color filter can therefore be designed to absorb light at the desired proportions of the second and third primary color emitted wavelengths, such that the transmitted brightness of the first, second, and third primary colors satisfies any brightness balance ratio required for the output signal.

[0393] Figure 7 The diagram illustrates the relationship between... Figure 5 It uses the same drive current and RGB primary color emission wavelengths, but the absorptive color filter 600 is positioned between one or more variable wavelength LEDs and the output of the display device. Because the filter 600 absorbs a certain proportion of the green and blue emitted light, the green and blue duty cycles can be relative to... Figure 5 The longer green and blue duty cycles make it easier to control precisely. The absorptive filter 600 compensates for these longer green and blue duty cycles and absorbs some of the additional light emitted at those wavelengths, resulting in an output signal transmitted from the filter 600 that has the same characteristics as... Figure 5The output signal produced in both cases is the same color balance and brightness balance. For example, the transmission wavelength and brightness can be combined to produce D65 white light in both cases, where Figure 7 The longer duty cycle in the case of the longer wavelength makes this embodiment more practical for driving.

[0394] Various types of absorptive filters used in photography, optical instruments and lighting devices can be adapted for use in the present invention.

[0395] Absorptive color filters 600 can be made from glass, plastic or film substrates by taking advantage of the inherent absorption or transmission properties of the materials. Absorptive color filters can be formed from combinations of semiconducting thin film materials.

[0396] Depending on the wavelength to be absorbed, suitable filters can be provided as long pass, band pass and low pass filters.

[0397] The wavelength absorbed by the filter can depend on the thickness of the absorbing layer in the filter. By controlling the thickness of the material in the filter, precise filtering across a large bandwidth can be achieved.

[0398] Figure 8A Experimental results are shown that illustrate the measured transmission brightness when testing embodiments of the present invention using an off-the-shelf ultra-long pass filter #35124 available from Edmund Optics. Figure 8B The expected absorption band of this filter is illustrated.

[0399] ProLite® variable wavelength LEDs from ProLite Technology were driven to emit white light from a combination of three primary color emission wavelengths at 650 nm, 560 nm and 460 nm, and it was found that the wavelengths 650 nm, 560 nm and 460 nm were about 92%, 50% and 10% transmitted by the absorptive filter, respectively.

[0400] In a particularly preferred embodiment of the present invention, the display device includes one or more absorptive color filters that transmit green and blue primary color emission wavelengths at a particular absorption to transmission ratio. Preferably, for example, the filters are configured to transmit green and blue filters in the relative proportions required to emit a white D65 output signal. This can be achieved using custom filters configured to transmit these wavelengths in the desired proportions or by stacking multiple absorptive filters having different absorption bands.

[0401] The display device of the present invention is preferably calibrated using a calibration method involving driving the variable wavelength LED for a first duty cycle with a first drive condition to emit a first emission wavelength λ1 of the device having a first luminance L1. The variable wavelength LED is then driven for a second duty cycle with a second drive condition to emit a second emission wavelength λ2 shorter than the first emission wavelength having a second luminance L2. The variable wavelength LED is then driven for a third duty cycle with a third drive condition to emit a third emission wavelength λ3 shorter than the first emission wavelength having a third luminance L3.

[0402] The absorptive color filter is configured to not attenuate the first emission wavelength, so light emitted at the first emission wavelength is transmitted with a first transmission luminance L 1T from the absorptive color filter.

[0403] The absorptive color filter is configured to attenuate the second emission wavelength by a second luminance L2, so that the second emission wavelength is transmitted with a second transmission luminance L 2T from the absorptive color filter, where L 2T < L2. The absorptive color filter is further configured to attenuate the third emission wavelength by a third luminance L3, so that the third emission wavelength is transmitted with a third transmission luminance L 3T from the absorptive color filter, where L 3T < L3.

[0404] To calibrate the display device, the first duty cycle (the duty cycle for controlling the shortest of the device's primary emission wavelengths) is set to 100%. The second and third duty cycles are given a shorter duration than the first duty cycle, and the durations of the second and third duty cycles are set to calibrated second and third duty cycles, respectively, at which the first emission wavelength λ1, the second emission wavelength λ2, and the third emission wavelength λ3, and the transmitted first transmission luminance L 1T , the second transmission luminance L 2T , and the third transmission luminance L 3T are complementary and sum to a standard illuminant point.

[0405] In use, the preferred three-primary embodiment of the display device of the present invention is driven by driving the variable wavelength LED to emit the first, second, and third primary emission wavelengths in proportions needed to form an output signal having a desired output chromaticity and output luminance. The output chromaticity of the output signal can be changed by altering the relative proportions of the three primary wavelengths emitted by the LED during a display frame. The output luminance of the output signal can be changed by altering the durations of the duty cycles of the drive conditions provided to the variable wavelength LED.

[0406] In a particularly preferred embodiment of the application, the display device comprises a plurality of variable wavelength LEDs forming a matrix of pixels. Each variable wavelength LED is preferably separately drivable, and can be separately calibrated and driven using the method of the application.

[0407] Variable wavelength LED

[0408] In the foregoing and hereinafter references are made to Figure 9 to Figure 15E Variable wavelength LEDs that can be driven in the application are further described.

[0409] In a particularly preferred embodiment of the application, the device epitaxial structure forming all device mesas is a variable wavelength LED epitaxial structure. It is particularly advantageous that a single variable wavelength LED epitaxial structure can be grown over a multiwell region of Ill-nitride material at wafer scale to form a monolithic device wafer comprising an array of variable wavelength LEDs formed from the same material.

[0410] The entire wafer scale device structure can be formed during a single epitaxial growth process, and then the method of the application can be used to process the resulting device wafer into optoelectronic devices, with each individual device mesa acting as a separate variable wavelength LED.

[0411] This provides a large number of possibilities for display devices, since the "color" (peak emission wavelength) emitted by each LED can be controlled by controlling the drive current density provided to a particular LED mesa in use.

[0412] The drive circuit can thus be configured to drive the variable wavelength LEDs in the device in a number of ways, depending on the desired result:

[0413] Each variable wavelength LED in the device can be dynamically driven by the drive circuit, with each variable wavelength LED receiving a drive current density whose magnitude is varied in real time during a display frame, so that the same variable wavelength LED can be controlled to emit a plurality of different wavelengths during a single display frame.

[0414] Each variable wavelength LED can be driven by the drive circuit with a fixed drive current density, so that the variable wavelength LED behaves as a fixed wavelength emitter in use. However, the drive circuit can be configured to provide different magnitudes of drive current density to different variable wavelength LEDs in the device. Although the different variable wavelength LEDs are formed from the same epitaxial process and have the same device structure, the different drive current densities control different device mesas to emit different fixed peak emission wavelengths. Thus, by providing different fixed drive conditions to different device mesas, a multi-color display can be provided from a single device epitaxial process.

[0415] - It is also possible to provide a mix of these two options, where the drive circuit is configured to dynamically control some of the device mesas in the device, and to control others of the mesas as fixed wavelength emitters by providing a fixed drive current to these mesas.

[0416] During fabrication, the size, shape, and location of the variable wavelength LEDs can be controlled by controlling the size, shape, depth, and location of the trenches etched into the device wafer.

[0417] By forming all of the individual device mesas from a single wafer epitaxy process, it is possible to eliminate many of the time-consuming, low-yield, and expensive processing steps required for device integration in the prior art.

[0418] In preferred embodiments of the invention, some or all of the drive circuit and variable wavelength LEDs are preferably configured to receive a variable magnitude supply of drive current from the drive circuit, such that the magnitude of the drive current to each variable wavelength LED is variable. By varying the magnitude of the drive current to a variable wavelength LED, the peak emission wavelength of that LED can be changed as the display device is used. The drive current provided to each variable wavelength LED can be individually controlled, such that the peak emission wavelength of each variable wavelength LED in the display can be individually controlled and changed. Alternatively, the device and drive circuit can be configured such that the same drive conditions are provided to a group of variable wavelength LEDs at the same time, such that all of the variable wavelength LEDs in the group emit light at the same peak emission wavelength when the drive current is on, and the peak emission wavelength of the entire group can be changed by varying the magnitude of the drive current.

[0419] In alternative embodiments, the drive circuit can be configured to provide a fixed magnitude (i.e., non-variable) drive current to some or all of the variable wavelength LEDs in the display device, either on or off. When the fixed drive current is on, the variable wavelength LED will behave as a known LED, and emit at a single peak emission wavelength determined by the drive conditions provided to the LED. Thus, a variable wavelength LED configured to receive a fixed magnitude drive current can be used as a fixed emission wavelength LED in the display device.

[0420] Preferably, the drive circuit is configured to control at least one sub-pixel of each pixel as a dynamically variable wavelength LED, whose peak emission wavelength can be changed within a single display frame.

[0421] The drive circuit can be configured to separately control the drive current provided to each of a plurality of LEDs, such that each of the plurality of LEDs can be individually driven. The drive circuit can be configured to provide a plurality of different drive currents to a plurality of LEDs, such that different peak emission wavelengths can be emitted in response to different drive currents driving individual LEDs.

[0422] Alternatively, the drive circuit can be configured to separately control groups of two or more LEDs such that each LED in a group emits at the same peak emission wavelength. The drive circuit can be configured to provide different drive currents to different groups of LEDs such that individual groups of LEDs can be driven to emit at different peak emission wavelengths in response to the different drive currents.

[0423] Figure 9 A series of five EL images of the same variable wavelength micro-LED InGaN pixel driven at different currents in the continuous wave (CW) mode, showing five different emission colors. In the left image, at a drive current of 50 μΑ, the micro-LED is seen to emit a red color. In the second image from the left, at a drive current of 100 μΑ, the micro-LED is seen to emit an orange-red color. In the third image from the left, at a drive current of 1 mA, the micro-LED is seen to emit an orange color. In the fourth image from the left, at a drive current of 10 mA, the micro-LED is seen to emit a yellow-green color. In the right image, at a drive current of 20 mA, the micro-LED is seen to emit a green color.

[0424] By varying the drive current between 50 μΑ and 20 mA, the same micro-LED is thus able to emit at wavelengths in the range from red to green. The spectral width of this emission wavelength range is about 90 nm (from about 570 nm to about 660 nm). This is far greater than the emission wavelength range that can be achieved with a single LED in the prior art.

[0425] Figure 10A Emission wavelength versus current density plot for a 25 μm x 25 μm InGaN LED pixel array (100 x 100 array, containing 10,000 pixels) driven in pulsed mode with 100 μβ pulses at a 1% duty cycle. Figure 10B Emission wavelength versus current density plot for a 30 μm x 30 μm InGaN LED pixel array (100 x 100 array, containing 10,000 pixels) driven in pulsed mode with 100 μβ pulses at a 1% duty cycle.

[0426] Both of these plots show the controllability of the peak emission wavelength with pulsed power supply. In particular, the wavelength is linearly related to the current density (plotted on a logarithmic scale). This linearity can also be manipulated when driven with a pulsed voltage power supply. Thus, the variable emission wavelength of the LED can be controlled with either a voltage or current drive scheme in either CW or pulsed mode, all of which are standard ways of driving display driver ICs.

[0427] This linear relationship between the driving current density and the resulting emission wavelength is highly advantageous for the purpose of LED display design, since it enables accurate control of the emission wavelength by changing the current density of the power supply.

[0428] Figure 11 Plot of intensity versus wavelength for a variable wavelength InGaN LED driven at different DC currents. The power supply was operated in pulsed mode with 100 μβ pulses at 1% duty cycle.

[0429] Figure 11 Again, the gradual continuous shift in the peak emission wavelength of the LED as the current of the power supply is changed is reflected. At a driving current of 200 mA, the peak emission wavelength is about 575 nm with an intensity of about 10 μβ / nm. However, as the driving current is reduced, the peak emission wavelength gradually moves to longer wavelengths and lower emission intensity. When the driving current reaches 7 mA, the peak emission wavelength is approximately 675 nm with an intensity of about 0.1 μβ / nm.

[0430] Figure 12A to Figure 12G An alternative embodiment of a light emitting region of a variable wavelength LED that can be used as a semiconductor device in embodiments of the present invention is illustrated.

[0431] Examples of MQWs:

[0432] 1. Continuous MQW

[0433] 2. V-pits

[0434] 3. Broken QWs, Gapped QWs, Fragmented QWs

[0435] 4. QDs

[0436] 5. Well width fluctuations

[0437] 6. Alloy composition

[0438] 7. Different combinations of MQWs and underlayers

[0439] Such structural characteristics can be identified and examined by standard material characterization techniques, such as cross-sectional transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX or EDS), 3D atom probe (3DAP).

[0440] Figure 12A A continuous MQW light emitting region of an LED is shown, in which three uniform QWs are disposed between four uniform quantum barriers (QB).

[0441] Figure 12B A continuous MQW light emitting region of an LED is shown, in which three uniform QWs are disposed between four uniform quantum barriers (QB). Figure 12AMQW with V-pits diffused through the light emitting region. The V-pits terminate in a threading dislocation and have QWs on their semi-polar facets.

[0442] Figure 12C MQW are shown with QW layers that include discontinuities or gaps in the semiconductor material.

[0443] Figure 12D MQW are shown with quantum dots (QDs) creating non-uniformity in the MQW. The QDs can be disposed on or in the QB or QW layers, for example in the gaps in the QW structure.

[0444] Figure 12E MQW are shown with well width fluctuations, where the thickness of the QW layers is not uniform across the light emitting region. The QWs can have different widths from each other, and also have varying widths within a single QW.

[0445] Figure 12F MQW are shown with fluctuations in the alloy composition in the light emitting region. The composition of the QBs and QWs varies from layer to layer. In particular, the indium In% composition varies within the same QW, i.e. in QW2, the In% varies between 10% to 12% or 10% to 15% or 10% to 25% or 10% to 35%.

[0446] Figure 12G MQW are shown with different combinations of MQW and underlying layers. The In% composition varies throughout the different QWs. For example, the In% in QW1 is 15%, the In% in QW2 is 25%, and the In% in QW3 is 30%. In embodiments of the invention, lower In% QWs are preferably located at the bottom of the MQW due to their strain and thermal effects, while high In% QWs are preferably on top. In one preferred embodiment, for example, QW1 is a blue emitting QW, QW2 is a green emitting QW, and QW3 is a red emitting QW.

[0447] Figure 13A A TEM image of the cross-section of a known non-variable wavelength LED. In this non-variable wavelength LED, the MQW is uniform and smooth in both the upper and lower interfaces (5 MQWs are shown here).

[0448] Figure 13B and Figure 13C A TEM image of a variable wavelength LED including V-pits that can be used in an embodiment of the invention. In this variable wavelength LED, the MSD is non-uniform. This non-uniformity can be induced by various methods, one example being V-pits and semi-polar facets that incorporate more indium and thinner QWs. Another example is also shown in Figure 7In B, the MQW is not uniform in terms of broken QWs, non-continuous QWs, fragmented QWs, QWs with well width, or In composition fluctuations.

[0449] Figure 13C A cross-section of a variable wavelength LED that can be used in a preferred embodiment of the present invention is shown Figure 13B showing a porous region and a light emitting region comprising a plurality of v-pits.

[0450] In this structure, the light emitting region contains a plurality of emission wavelength regions intentionally introduced, such as a plurality of types of QW regions with v-pits extending through the light emitting region.

[0451] The v-pit (V-pit) is actually a hexagonal pit as viewed from above, the v-shape being the shape when viewing the cross-section. The V-pit can be initialized at misfit sites under special epitaxial growth conditions, such as low growth temperature (e.g. < 1000°C, or < 900°C, or < 800°C, or < 700°C) and nitrogen ambient, during growth of the InGaN / GaN superlattice structure under the InGaN, GaN, InGaN / InGaN superlattice or MQW.

[0452] Figure 14A A plot of peak emission wavelength versus drive current density for a known non-variable wavelength LED. By varying the drive current density applied to the LED, the emission wavelength can be varied slightly across an emission wavelength range of about 15 nm.

[0453] Figure 14B A plot of peak emission wavelength versus drive current density for a variable wavelength LED that can be used as the first or second semiconductor device in an embodiment of the present invention. In the variable wavelength LED, varying the current density of the drive power supply results in a much greater change in the peak emission wavelength (WLP) emitted by the LED. In this embodiment, the drive current density is varied from about 0.1 to 100 A / cm 2 causing the peak emission wavelength to change from about 635 nm to about 550 nm (an emission wavelength range of about 85 nm).

[0454] Figure 14C A plot of peak emission wavelength versus drive current density for a variable wavelength LED according to another embodiment of the present invention. In this embodiment, varying the drive current density causes the peak emission wavelength to change from about 720 nm to about 580 nm (an emission wavelength range of about 140 nm).

[0455] Figure 15A A plot of peak emission wavelength versus drive current density for another variable wavelength LED according to the present invention. In this embodiment, the drive current density is varied from about 0.1 to 200 A / cm2 This would change the peak emission wavelength from 615 nm to 508 nm (an emission wavelength range of approximately 100 nm). Due to limitations in testing capabilities, the data in this figure only reaches 514.5 nm. Therefore, the current density is estimated at 508 nm. However, the achievable emission wavelength range can be significantly increased in either way.

[0456] Figure 15B to Figure 15D for Figure 13A The image shows a variable-wavelength LED emitting at four different wavelengths across its emission wavelength range. The inserted emission spectrum shows the different peak emission wavelengths at different drive current densities. This shows the same variable-wavelength LED emitting at peak emission wavelengths of orange (615 nm), yellow (556 nm), green (534 nm), and blue (508 nm) in response to different drive current densities.

Claims

1. A display device comprising: a variable wavelength LED, the variable wavelength LED being one of a pixel of the device, or a plurality of sub-pixels of a device pixel; and an absorptive color filter positioned between the variable wavelength LED and an exit of the display device, such that light emitted by the variable wavelength LED passes through the absorptive color filter before emerging from the exit of the display device, wherein the variable wavelength LED or second LED sub-pixel is drivable to emit light at a first emission wavelength in response to a first electrical drive condition, and wherein the variable wavelength LED is drivable to emit light at a second emission wavelength in response to a second electrical drive condition, wherein the second emission wavelength is shorter than the first emission wavelength, and wherein the absorptive color filter is configured to attenuate the luminance of light emitted at the second emission wavelength. The display device is a sub-pixelated display, wherein the variable wavelength LED is a first sub-pixel drivable to emit light at the second emission wavelength, and wherein the display includes a second LED sub-pixel drivable to emit light at the first emission wavelength, the first and second sub-pixels being sub-pixels of the same display pixel, and the first and second sub-pixels being independently drivable from one another.

2. The display device of claim 1, wherein, The second LED sub-pixel is a second variable wavelength LED sub-pixel.

3. The display device of claim 2, wherein, The variable wavelength LED is the pixel of the device, wherein the variable wavelength LED is drivable to emit light at the first emission wavelength in response to the first electrical drive condition provided to the variable wavelength LED, and wherein the variable wavelength LED is drivable to emit light at the second emission wavelength in response to a second electrical drive condition provided to the variable wavelength LED.

4. The display device of claim 1, wherein, The variable wavelength LED is controllable to emit light at two or more selected primary color emission wavelengths, such that, in use, the light transmitted out of the exit of the display device during a display frame forms an observation signal that is a temporal combination of the primary color emission wavelengths emitted by the variable wavelength LED during the display frame.

5. The display device of any one of the preceding claims, wherein, The absorptive color filter is configured such that, in use, light emitted at the first emission wavelength has a first lightness LI before transmission through the absorptive color filter and a first transmitted lightness L 1T after transmission through the absorptive color filter, and light emitted at the second emission wavelength has a second lightness L2 before transmission through the absorptive color filter and a reduced second transmitted lightness L 2T after transmission through the absorptive color filter.

6. The display device of any one of the preceding claims, wherein, The absorptive color filter is configured to reduce or modulate the lightness of light having the second emission wavelength such that, in use, light emitted at the second emission wavelength has a second lightness L2 before transmission through the absorptive color filter and a reduced second transmitted lightness L 2T such that L 2T < L2.

7. The display device of any one of the preceding claims, wherein, The absorptive color filter is configured to not reduce the lightness of light having the first emission wavelength, such that the first transmittance lightness Li 1T has a magnitude that is at least 90%, or at least 92%, or at least 95%, or at least 98% of the magnitude of the first lightness Li.

8. The display device of any of the preceding claims, wherein, The absorptive color filter is configured to absorb light emitted at the second emission wavelength to a greater extent than light emitted at the first emission wavelength, such that L 2T : L 1T the ratio of L2:L1.

9. The display device of any of the preceding claims, wherein, The first and second emission wavelengths are first and second primary color emission wavelengths.

10. The display device of claim 9, wherein, The display device includes a controller programmed to drive the variable wavelength LED to emit the first and second emission wavelengths in response to first and second respective drive conditions, the first and second emission wavelengths being wavelengths within an emission wavelength range of the variable wavelength LED.

11. The display device of any one of the preceding claims, wherein, The controller is programmed to drive the variable wavelength LED to emit the first and second emission wavelengths within discrete sub-frames of the same display frame in a field sequential mode.

12. The display device of claim 11, wherein, The first drive condition is a first current density, and the second drive condition is a second current density that is greater in magnitude than the first current density.

13. The display device of any of the preceding claims, wherein, The first emission wavelength is a red emission wavelength that is higher than 595 nm or higher than 630 nm, preferably wherein the first emission wavelength is between 595 nm and 700 nm, or between 610 nm and 680 nm, preferably between 625 nm and 670 nm.

14. The display device of any of the preceding claims, wherein, ​ 15. The display device of any one of the preceding claims, wherein, The second emission wavelength is less than 595 nm.

16. The display device of any one of the preceding claims, wherein, The second emission wavelength is a green emission wavelength between 500 nm and 590 nm or between 510 nm and 580 nm, preferably between 525 nm and 570 nm, or wherein the second emission wavelength is a blue emission wavelength below 495 nm, preferably wherein the second emission wavelength is between 400 nm and 490 nm, or between 410 nm and 480 nm, preferably between 430 nm and 470 nm.

17. The display device of any one of the preceding claims, wherein, The first emission wavelength and the second emission wavelength are separated by at least 40 nm, or by at least 50 nm, within the emission wavelength range of the variable wavelength LED.

18. The display device of any of the preceding claims, wherein, The display comprises a plurality of absorptive color filters, each absorptive color filter configured to attenuate the luminance of light emitted at a respective emission wavelength.

19. The display device of any of the preceding claims, wherein, The variable wavelength LED is drivable to emit light at a third emission wavelength in response to a third electrical drive condition provided to the variable wavelength LED, the third emission wavelength being shorter than the first emission wavelength, and the first emission wavelength, the second emission wavelength, and the third emission wavelength being within the emission wavelength range of the variable wavelength LED.

20. The display device of claim 19, wherein, The absorptive color filter is configured to attenuate the luminance of light emitted at the third emission wavelength as well as the luminance of light emitted at the second emission wavelength.

21. The display device of claim 19 or 20, wherein, The absorptive color filter is a second emission wavelength absorptive filter configured to modify the luminance of the second emission wavelength, and wherein the device comprises an additional third emission wavelength absorptive filter configured to modify the luminance of the third emission wavelength.

22. The display device of claim 19, 20 or 21, wherein, The absorptive color filter is configured to reduce or attenuate the lightness of light having the third emission wavelength such that, in use, light emitted at the third emission wavelength has a third lightness L3 before transmission through the absorptive color filter and a reduced third transmitted lightness L 3T .

23. The display device of any of claims 19-22, wherein, The first wavelength, the second wavelength, and the third wavelength are first, second, and third primary color emission wavelengths.

24. The display device of any of claims 19-23, wherein, The absorptive color filter is configured to reduce the luminance of light emitted at the second emission wavelength and to reduce the luminance of light emitted at the third emission wavelength, and wherein the absorptive color filter is configured to not reduce the luminance of light emitted at the first emission wavelength.

25. The display device of any of claims 19-24, wherein, The absorptive color filter is configured to reduce the luminance of the light emitted by the variable wavelength LED at the second emission wavelength and at the third emission wavelength, and wherein the absorptive color filter is configured to reduce the luminance of the light emitted at the third emission wavelength at a different proportion than it reduces the luminance of light emitted at the second emission wavelength.

26. The display device of any of claims 19-25, wherein, The third emission wavelength is less than 595 nm, preferably less than 495 nm.

27. The display device of any of claims 19-26, wherein, The second emission wavelength is a green emission wavelength between 500 nm and 590 nm or between 510 nm and 580 nm, preferably between 525 nm and 570 nm, and wherein the third emission wavelength is a blue emission wavelength below 495 nm, preferably wherein the third emission wavelength is between 400 nm and 490 nm, or between 410 nm and 480 nm, preferably between 430 nm and 470 nm.

28. The display device of any of claims 19-27, wherein, The first, second and third emission wavelengths are at least 40 nm apart from each other, or at least 50 nm apart from each other, within the emission wavelength range of the variable wavelength LED, such that the highest and lowest of the first, second and third emission wavelengths are at least 80 nm apart, or at least 100 nm apart.

29. The display device of any of claims 19-28, wherein, The absorptive color filter is configured to attenuate a second luminance L2 from a second emission wavelength emitted by the variable wavelength LED to a second transmissive luminance L 2T , and to attenuate a third luminance L3 from a third emission wavelength emitted by the variable wavelength LED to a third transmissive luminance L 3T such that the wavelengths and the transmissive luminances of the first, second, and third emission wavelengths are complementary and sum to a standard illuminant point.

30. The display device of any of the preceding claims, wherein, The absorptive color filter is configured to attenuate a second lightness L2 of the second emission wavelength from the variable wavelength LED emission free the first lightness L1 to a transmitted second lightness L 2T such that the wavelengths and the transmitted lightness of the first emission wavelength and the second emission wavelength are complementary and sum to a standard illuminant point.

31. The display device of claim 29 or 30, wherein, The standard illuminant point is a white point, preferably wherein the standard illuminant point is the CIE standard illuminant point D65.

32. The display device of any of the preceding claims, wherein, The absorptive color filter comprises a glass, plastic or semiconductor film substrate.

33. The display device of any of the preceding claims, wherein, The absorptive color filter is a long pass filter.

34. The display device of any one of claims 1-32, wherein, The absorptive color filter is a band pass filter.

35. The display device of any one of claims 1-32, wherein, The absorptive color filter is a low pass filter.

36. The display device of any of the preceding claims, wherein, The variable wavelength LED has an emission wavelength range with a width between 100 nm and 200 nm, or between 125 nm and 185 nm, or between 150 nm and 175 nm, wherein the variable wavelength LED is continuously controllable within the emission wavelength range by varying the electrical driving conditions provided to the variable wavelength LED.

37. The display device of any of the preceding claims, wherein, The emission wavelength range of the variable wavelength LED extends from a lower limit, which is preferably below 500 nm, or below 450 nm, to an upper limit, which is preferably above 595 nm, or above 630 nm.

38. The display device of claim 37, wherein, The lower limit of the emission wavelength range is between 440 nm and 495 nm, or between 440 nm and 485 nm, preferably between 450 nm and 470 nm.

39. The display device of claim 37 or 38, wherein, The upper limit of the emission wavelength range is between 595 nm and 700 nm, or between 610 nm and 680 nm, preferably between 625 nm and 670 nm.

40. The display device of any one of the preceding claims, wherein the variable wavelength LED has a FWHM of 50 nm or less, or 40 nm or less, or 30 nm or less, or 20 nm or less.

41. The display device of any one of the preceding claims, comprising an array of variable wavelength LEDs, each variable wavelength LED being a pixel of the device, or one of a plurality of sub-pixels of a device pixel.

42. The display device of any of the preceding claims, wherein, The absorptive color filter is positioned between the variable wavelength LED and an outlet of the display device, such that light emitted by the variable wavelength LED passes through the absorptive color filter before emerging from the outlet of the display device.

43. A method of calibrating a display device, the display device comprising: a variable wavelength LED, the variable wavelength LED being a pixel of the device, or one of a plurality of sub-pixels of a device pixel; and an absorptive color filter positioned between the variable wavelength LED and an outlet of the display device, such that light emitted by the variable wavelength LED passes through the absorptive color filter before emerging from the outlet of the display device, The method comprises the steps of: driving the variable-wavelength LED or the second LED sub-pixel with a first driving condition to emit a first emission wavelength λ1 having a first luminance L1 for a first working period, wherein the first emission wavelength is transmitted with a first transmission luminance L 1T from the absorptive color filter; driving the variable wavelength LED with a second driving condition for a second working period to emit a second emission wavelength λ2 having a second luminance L2, the second emission wavelength being shorter than the first emission wavelength; driving the variable wavelength LED with a second driving condition for a second working period to emit a second emission wavelength λ2 having a second luminance L2, the second emission wavelength being shorter than the first emission wavelength; wherein the absorptive color filter is configured to attenuate the second lightness L2 of the second emission wavelength such that the second emission wavelength is transmitted at a second transmitted lightness L 2T from the absorptive color filter, where L 2T < L2; The second duty cycle is set to a shorter duration than the first duty cycle, and the duration of the second duty cycle is controlled such that the first emission wavelength λ1 and the second emission wavelength λ2 and the first transmission luminance L 1T and the second transmission luminance L 2T are complementary and sum to a standard illuminant point.

44. The method of claim 43, wherein, The display device is a sub-pixelated display, wherein the variable wavelength LED is a first variable wavelength sub-pixel, and wherein the method comprises the steps of: driving a second LED sub-pixel to emit light at the first emission wavelength; and driving the first variable wavelength sub-pixel to emit light at the second emission wavelength.

45. The method of claim 43, wherein, The variable wavelength LED is a pixel of the device, and wherein the method comprises the steps of: driving the variable wavelength LED with a first driving condition for a first duty cycle to emit a first emission wavelength λ1 of the device; and driving the variable wavelength LED with a second driving condition for a second duty cycle to emit a second emission wavelength λ2.

46. The method of claim 43, 44, or 45, wherein, The duration of the second duty cycle is controlled to find a calibrated second duty cycle at which the first emission wavelength λ1 and the second emission wavelength λ2 and the transmitted first lightness L 1T and the transmitted second lightness L 2T are complementary and sum to the standard illuminant point.

47. The method of any one of claims 43 to 46, wherein, The standard illuminant point is a white point, preferably wherein the standard illuminant point is a CIE standard illuminant point D65.

48. The method of any one of claims 43 to 47, wherein, The first duty cycle is a 100% duty cycle.

49. The method of any one of claims 43 to 48, wherein, The method includes the steps of: selecting a set of two or more discrete primary color emission wavelengths λ P within an emission wavelength range of the variable wavelength LED , the set of primary color emission wavelengths corresponding to a set of two or more respective drive conditions; wherein the first emission wavelength λ1 is a longest of the selected primary color emission wavelengths, and wherein the second emission wavelength λ2 is another of the selected primary color emission wavelengths.

50. The method of any one of claims 43 to 49, wherein, The step of controlling the duration of the second duty cycle controls the second transmission luminance L of the light emitted when the second driving condition is applied 2T .

51. The method of any one of claims 43 to 50, comprising the step of: The variable wavelength LED is driven with a third driving condition for a third duty cycle to emit a third emission wavelength λ3 of the device having a third luminance L3, the third emission wavelength being shorter than the first emission wavelength, wherein the absorptive color filter is configured to attenuate the third lightness L3, such that the third emission wavelength is at a third transmission lightness L 3T transmitted from the absorptive color filter, where L 3T < L3, And wherein the method comprises the steps of setting the third duty cycle to a shorter duration than the first duty cycle; and controlling the duration of the third duty cycle such that the first emission wavelength, the second emission wavelength and the third emission wavelength and the first transmission luminance L 1T , the second transmission luminance L 2T and the third transmission luminance L 3T are complementary and sum up to a standard illuminant point.

52. The method of claim 51, wherein, The second and third emission wavelengths are shorter than the first emission wavelength, and the second and third duty cycles are shorter than the first duty cycle.

53. The method of any one of claims 43 to 52, comprising the step of: using the first luminance L1 as a reference luminance and using the first duty cycle as a reference duty cycle; and setting the second duty cycle and optionally the third duty cycle to a shorter duration than the first duty cycle such that the second transmissive luminance and optionally the third transmissive luminance are shorter than the first transmissive luminance L 1T satisfy a standard illuminant point mixture ratio at which the emission wavelengths of the first emission wavelength, the second emission wavelength, and optionally the third emission wavelength, and the transmissive luminance are complementary and sum to the standard illuminant point.

54. The method of any one of claims 43 to 53, wherein, The variable wavelength LED emits red light in response to the first driving condition, green light in response to the second driving condition, and blue light in response to the third driving condition.

55. The method of any of claims 43 to 54, comprising the step of selecting a first primary color emission wavelength, a second primary color emission wavelength, and a third primary color emission wavelength for driving the variable wavelength LED.

56. The method of claim 55, wherein, The method comprises the following steps: selecting a set of three discrete primary color emission wavelengths λ P in the emission wavelength range of the variable wavelength LED, the set of primary color emission wavelengths corresponding to a set of three respective driving conditions; wherein the first emission wavelength λ1 is the longest of the selected primary color emission wavelengths, and wherein the second emission wavelength λ2 and the third emission wavelength λ3 are the other selected primary color emission wavelengths.

57. The method of claim 55 or 56, comprising the step of: Looking up or calculating standard illuminant point mix ratios, wherein light emitted at the selected primary color emission wavelengths and their respective luminances are combined so that the combined output signal reaches the standard illuminant point.

58. The method of any of claims 55 to 57, comprising the steps of: The required transmission luminances L at other primary color emission wavelengths are calculated using the standard illuminant mixture ratios and the first luminance L1 at the first primary color emission wavelength T which will be complementary to the first emission wavelength and its first luminance L1 and which sum up to the standard illuminant point; and calculating a duty cycle for each of the primary color emission wavelengths that will result in the required transmitted luminance of the variable wavelength LED at each primary color emission wavelength.

59. The method of any one of claims 55 to 58, wherein, The first, second, and third primary color emission wavelengths are selected to be a red (R) primary color emission wavelength, a green (G) primary color emission wavelength, and a blue (B) primary color emission wavelength, respectively, and wherein the three corresponding driving conditions drive the variable wavelength LED to emit red, green, and blue, respectively.

60. The method of claim 59, wherein, The first driving condition drives the variable wavelength LED to emit a red primary color emission wavelength, and wherein the first duty cycle has a duration of 100% of the duration of a display sub-frame, such that the first luminance L1 is the luminance emitted at the red primary color emission wavelength at 100% duty cycle.

61. The method of claim 59 or 60, wherein, The method comprises the following steps: reducing duty cycles of green driving conditions and blue driving conditions until the transmission brightness of the green primary color emission wavelength and the blue primary color emission wavelength meets the first transmission brightness L 1T satisfies the required color mixing ratio.

62. The method of any one of claims 43 to 61, wherein, The variable wavelength LED is a device pixel, and wherein the method is a method of calibrating the display for field sequential driving of variable wavelength LED pixels to emit a plurality of primary color emission wavelengths in sequential sub-frames of a display frame.

63. The method of any one of claims 43 to 62, wherein, The variable wavelength LED is a pixel of the display device, and wherein the method is a method of calibrating a field sequential display device comprising variable wavelength LED pixels.

64. The method of any one of claims 43 to 61, wherein, The display device comprises a sub-pixelated display, wherein the pixels of the display each comprise two or more sub-pixels, the variable wavelength LED is a first sub-pixel of a plurality of sub-pixels of a device pixel.

65. The method of any one of claims 43 to 64, wherein, The method comprises the step of storing the calibrated duty cycle for the set of primary color emission wavelengths.

66. The method of any one of claims 43 to 65, wherein, The method comprises the step of programming a display controller with the calibrated duty cycle for the set of primary color emission wavelengths.

67. The method of any one of claims 43 to 66, wherein, The method comprises the step of calibrating the transmitted luminance for a complementary primary color emission wavelength by measuring the transmitted luminance transmitted by the absorptive color filter and emerging from the exit of the display device when driving the variable wavelength LED pixel across a range of emission wavelengths of the variable wavelength LED pixel.

68. A method of driving a display device, the display device comprising: a variable wavelength LED, the variable wavelength LED being one of a pixel of the device, or a plurality of sub-pixels of a device pixel; and an absorptive color filter positioned between the variable wavelength LED and an exit of the display device such that light emitted by the variable wavelength LED passes through the absorptive color filter before emerging from the exit of the display device, The method comprises the steps of: driven with a first drive condition to emit a first emission wavelength λ1 having a first luminance L1 for a first duty cycle, wherein the first emission wavelength is transmitted with a first transmission luminance L 1T from the absorptive color filter; driving the variable wavelength LED with a second driving condition for a second duty cycle to emit a second emission wavelength λ2 having a second luminance L2, the second emission wavelength being shorter than the first emission wavelength; wherein the absorptive color filter is configured to attenuate the second lightness L2 of the second emission wavelength such that the second emission wavelength is transmitted at a second transmitted lightness L 2T from the absorptive color filter, where L 2T < L2; and The durations of the first and second duty cycles are controlled such that the output signal emitted from the outlet of the display device has an output chrominance and an output luminance L O .

69. The method of claim 68, wherein, The display device is a sub-pixelated display, wherein the variable wavelength LED is a first variable wavelength sub-pixel, and wherein the method comprises the steps of driving a second LED sub-pixel to emit light at the first emission wavelength; and driving the first variable wavelength sub-pixel to emit light at the second emission wavelength.

70. The method of claim 68, wherein, The variable wavelength LED is a pixel of the device, and wherein the method comprises the steps of driving the variable wavelength LED with a first driving condition for a first duty cycle to emit a first emission wavelength λ1 of the device; and driving the variable wavelength LED with a second driving condition for a second duty cycle to emit a second emission wavelength λ2.

71. The method of any one of claims 68 to 70, comprising the step of: driving the variable wavelength LED with a third driving condition for a third duty cycle to emit a third emission wavelength λ3 of the device having a third luminance L3, the third emission wavelength being shorter than the first emission wavelength, wherein the absorptive color filter is configured to attenuate the third lightness L3, such that the third emission wavelength is at a third transmission lightness L 3T transmitted from the absorptive color filter, where L 3T < L3, And wherein the method comprises the step of controlling the duration of the first duty cycle, the second duty cycle and the third duty cycle such that the output signal emitted from the outlet of the display device has an output chrominance and an output luminance L O .

72. The method of claim 71, comprising the step of: The duration of the third duty cycle is controlled such that the first emission wavelength, the second emission wavelength and the third emission wavelength and the first transmittance luminosity L 1T , the second transmittance luminosity L 2T and the third transmittance luminosity L 3T are complementary and sum up to a standard illuminant point.

73. The method of claim 71 or 72, wherein, The second emission wavelength and the third emission wavelength are shorter than the first emission wavelength.

74. The method of any one of claims 68 to 73, wherein, The respective duty cycles of the driving conditions are calibrated duty cycles obtained from the method of any of claims 43-67.

75. The method of any one of claims 68 to 74, wherein, The method is a field sequential driving method for driving a display device comprising a variable wavelength LED pixel, the method comprising the steps of: driving the variable wavelength LED pixel to emit light at three or more primary color emission wavelengths in response to respective driving conditions in sequential sub-frames of the display frame.

Images