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US-12627779-B1 - White balance calibration method

US12627779B1US 12627779 B1US12627779 B1US 12627779B1US-12627779-B1

Abstract

A method comprising: applying a respective value of a first plurality of values of a first control parameter to each light source of a projector system, wherein each of the first plurality of values of the first control parameter determines an output luminance of a corresponding light source; measuring a first luminance and first colour space coordinates of a light pattern displayed based on the first plurality of values; converting the first luminance and first colour space coordinates to corresponding tristimulus values; calculating therefrom a respective colour luminance value corresponding to each light source; calculating a linear relationship between the first control parameter and each colour luminance value; determining a second plurality of values of the first control parameter based on the linear relationships and a target colour luminance value for each light source; and controlling the plurality of light sources using the second plurality of values.

Inventors

  • Sebastian de Echaniz

Assignees

  • ENVISICS LTD

Dates

Publication Date
20260512
Application Date
20250912
Priority Date
20250414

Claims (18)

  1. 1 . A method comprising: (i) applying a respective value of a first plurality of values of a first control parameter to each light source of a plurality of light sources of a projector system, each light source of the plurality of light sources configured to output a different wavelength of light, wherein applying each of the first plurality of values of the first control parameter causes each light source of the plurality of light sources to output light having a respective first output luminance; (ii) measuring a first luminance and first colour space coordinates of a light pattern displayed by the projector system, wherein the light pattern is generated by applying the first plurality of values of the first control parameter to the plurality of light sources; (iii) converting the first luminance and first colour space coordinates to corresponding tristimulus values; (iv) calculating, based on the corresponding tristimulus values, a respective colour luminance value corresponding to each light source of the plurality of light sources, wherein the calculation comprises solving a plurality of linear equations, each linear equation of the plurality of linear equations being derived from a sum of the contributions of the plurality of light sources to a respective tristimulus value, to obtain each colour luminance value; (v) calculating a linear relationship between the first control parameter and each colour luminance value based on: (a) the respective colour luminance value; (b) either a previous colour luminance value from a previous iteration or an initial colour luminance value; (c) the respective first value of the first control parameter; and (d) either a previous value of the first control parameter from a previous iteration or an initial value of the first control parameter; (vi) determining a second plurality of values of the first control parameter based on the linear relationships and a target colour luminance value for each light source; and (vii) applying the second plurality of values of the first control parameter to the plurality of light sources, wherein applying each of the second plurality of values of the first control parameter causes each light source of the plurality of light sources to output light having a respective second output luminance.
  2. 2 . The method of claim 1 , wherein the plurality of light sources comprises a first light source configured to output a first wavelength of light having colour space coordinates x R , y R , z R =(1−x R −y R ), a second light source configured to output a second wavelength of light having colour space coordinates x G , y G , z G =(1−x G −y G ), and a third light source configured to output a third wavelength of light having colour space coordinates x B , y B , z B =(1−x B −y B ).
  3. 3 . The method of claim 2 , wherein the first wavelength is in the range of 620 to 750 nm, the second wavelength is in the range of 490 to 570 nm, and the third wavelength is in the range of 450 to 495 nm, and wherein the first wavelength and the second wavelength are separated by at least 150 nm, and the second wavelength and the third wavelength are separated by at least 60 nm.
  4. 4 . The method of claim 2 , wherein the conversion of the first luminance and first colour space coordinates into respective tristimulus values, X, Y and Z, comprises the calculations: X = Lv ⁢ x y , Y = Lv , and Z = Lv ⁢ ( 1 - x - y ) y , wherein Lv is the first luminance and (x, y) are the first colour space coordinates.
  5. 5 . The method of claim 4 , wherein the calculation of the respective colour luminance values comprises solving the three linear equations: X = L ⁢ v R ⁢ x R y R + L ⁢ v G ⁢ x G y G + L ⁢ v B ⁢ x B y B , Y = L ⁢ v R + L ⁢ v G + L ⁢ v B , and Z = L ⁢ v R ⁢ z R y R + L ⁢ v G ⁢ z G y G + L ⁢ v B ⁢ z B y B , wherein x γ , y γ , z γ =(1−x γ −y γ ), are the colour space coordinates for the first wavelength, second wavelength, and third wavelength, such that γ={R, G, B}, in order to obtain a first colour luminance value Lv R corresponding to the first light source, a second colour luminance value Lv G corresponding to the second light source, and a third colour luminance value Lv B corresponding to the second light source.
  6. 6 . The method of claim 5 , wherein the linear relationship between the first control parameter and the respective colour luminance value is determined based on the calculation: m γ = P i γ - P i - 1 γ Lv i γ - Lv i - 1 γ , b γ = P i γ - m γ ⁢ L ⁢ v i γ , wherein P i γ is the respective value of the first control parameter of the first plurality of values corresponding to each respective light source, P i - 1 γ is the previous value of the first control parameter from the previous iteration or the initial value of the first control parameter corresponding to each respective light source, L ⁢ v i γ is the colour luminance value for each respective light source, L ⁢ v i - 1 γ is the colour luminance value from the previous iteration or the initial colour luminance value, m γ is the gradient of the respective linear relationship, and b γ is the offset of the respective linear relationship.
  7. 7 . The method of claim 6 , wherein each of the second plurality of values of the first control parameter, P i + 1 γ , is determined based on the calculation of: P i + 1 γ = m γ ⁢ L ⁢ v t γ + b γ , wherein L ⁢ v t γ is the target colour luminance value for each light source.
  8. 8 . The method of claim 1 , wherein steps (i) to (vii) form an initial iteration in which calculating the linear relationship between the first control parameter and each colour luminance value is based on: (a) the respective colour luminance value; (b) an initial colour luminance value; (c) the respective first value of the first control parameter; and (d) an initial value of the first control parameter; wherein the method steps (i) to (vii) are repeated for one or more further iterations in which calculating the linear relationship between the first control parameter and each colour luminance value is based on: (a) the respective colour luminance value; (b) a previous colour luminance value from a previous iteration; (c) the respective first value of the first control parameter; and (d) a previous value of the first control parameter from a previous iteration.
  9. 9 . The method of claim 8 , wherein the one or more iterations are performed while a difference between the first luminance and a target luminance is below a first threshold and a difference between the first colour space coordinates and target colour space coordinates is below a second threshold.
  10. 10 . The method of claim 8 , wherein the method steps of claim 1 are repeated at a first temperature of the projector system.
  11. 11 . The method of claim 10 , wherein the first temperature and the second plurality of values of the first control parameter is saved in a lookup table implemented on a computing device in communication with the projector system when the difference between the first luminance and a target luminance is below a first threshold and a difference between the first colour space coordinates and target colour space coordinates is below a second threshold.
  12. 12 . The method of claim 1 , wherein the first control parameter comprises one of the group comprising: a pulse width modulation parameter for each of the plurality of light sources, a digital-analogue converter parameter for each of the plurality of light sources, a photodiode signal parameter for each of the plurality of light sources, and a scaling factor applied to a uniformity map for controlling the amount of light projected by each of the light sources.
  13. 13 . The method of claim 12 , wherein the selection from the group is determined based on the measured value of the first luminance.
  14. 14 . The method of claim 13 , wherein a first one of the group is selected for a first range of measured first luminance values, and a second one of the group is selected for a second range of measured first luminance values.
  15. 15 . The method of claim 12 , further comprising applying a first plurality of values of a second control parameter each corresponding to a respective one of the plurality of light sources, wherein the second control parameter is a different one of the group than the first control parameter, wherein the second control parameter is used to control an overall luminance of the projector system.
  16. 16 . A projector system comprising: a plurality of light sources, each configured to output a different wavelength of light; a display device configured to display a hologram, such that a light pattern is displayed at an image plane when the display device is illuminated by the plurality of light sources; a measurement device configured to measure a first luminance and first colour space coordinates of the light pattern displayed at the image plane; and a computing device in communication with the plurality of light sources and the measurement device, wherein the computing device is configured to: apply a respective value of a first plurality of values of a first control parameter to each of the plurality of light sources, wherein applying each value of the first plurality of values of the first control parameter causes each light source of the plurality of light sources to output light having a respective first output luminance; receive the first luminance and first colour space coordinates from the measurement device; convert the first luminance and first colour space coordinates to corresponding tristimulus values; calculate, based on the corresponding tristimulus values, a respective colour luminance value corresponding to each light source of the plurality of light sources, wherein the calculation comprises solving a plurality of linear equations, each linear equation of the plurality of linear equations being derived from a sum of the contributions of the plurality of light sources to a respective tristimulus value, to obtain each colour luminance value, calculate a linear relationship between the first control parameter and each colour luminance value based on: (a) the respective colour luminance value; (b) either a previous colour luminance value from a previous iteration or an initial colour luminance value; (c) the respective first value of the first control parameter; and (d) either a previous value of the first control parameter from a previous iteration or an initial value of the first control parameter; determine a second plurality of values of the first control parameter based on the linear relationships and a target luminance value for each light source; and apply the second plurality of values of the first control parameter to the plurality of light sources, wherein applying each of the second plurality of values of the first control parameter causes each light source of the plurality of light sources to output light having a respective second output luminance.
  17. 17 . A method of projection comprising: applying a respective value of a first plurality of values of a first control parameter to each light source of a plurality of light sources of a projector system, each light source of the plurality of light sources configured to output a different wavelength of light, wherein applying each of the first plurality of values of the first control parameter causes each light source of the plurality of light sources to output light having a respective first output luminance; projecting a first light pattern by applying the first plurality of values of the first control parameter to the plurality of light sources and illuminating a hologram corresponding to the light pattern with the plurality of light sources; measuring a first luminance and first colour space coordinates of the light pattern; converting the first luminance and first colour space coordinates to corresponding tristimulus values; calculating, based on the corresponding tristimulus values, a respective colour luminance value corresponding to each light source of the plurality of light sources, wherein the calculation comprises solving a plurality of linear equations, each linear equation of the plurality of linear equations being derived from a sum of the contributions of the plurality of light sources to a respective tristimulus value, to obtain each colour luminance value; calculating a linear relationship between the first control parameter and each colour luminance value based on: (a) the respective colour luminance value; (b) either a previous colour luminance value from a previous iteration or an initial colour luminance value; (c) the respective first value of the first control parameter; and (d) either a previous value of the first control parameter from a previous iteration or an initial value of the first control parameter; determining a second plurality of values of the first control parameter based on the linear relationships and a target colour luminance value for each light source; and projecting a second light pattern by applying the second plurality of values of the first control parameter to the plurality of light sources, wherein applying each of the second plurality of values of the first control parameter causes each light source of the plurality of light sources to output light having a respective second output luminance.
  18. 18 . A non-transitory computer readable medium comprising instructions which, when executed by one or more processors, cause the one or more processors to: (i) apply a respective value of a first plurality of values of a first control parameter to each light source of a plurality of light sources of a projector system, each light source of the plurality of light sources configured to output a different wavelength of light, wherein each of the first plurality of values of the first control parameter determines an output luminance of a corresponding light source of the plurality of light sources; (ii) measure a first luminance and first colour space coordinates of a light pattern displayed by the projector system, wherein the light pattern is generated by applying the first plurality of values of the first control parameter to the plurality of light sources; (iii) convert the first luminance and first colour space coordinates to corresponding tristimulus values; (iv) calculate, based on the corresponding tristimulus values, a respective colour luminance value corresponding to each light source of the plurality of light sources, wherein the calculation comprises solving a plurality of linear equations, each linear equation of the plurality of linear equations being derived from a sum of the contributions of the plurality of light sources to a respective tristimulus value, to obtain each colour luminance value; (v) calculate a linear relationship between the first control parameter and each colour luminance value based on: the respective colour luminance value, a previous colour luminance value from a previous iteration or an initial colour luminance value, the respective first value of the first control parameter, and a previous value of the first control parameter from a previous iteration or an initial value of the first control parameter; (vi) determine a second plurality of values of the first control parameter based on the linear relationships and a target colour luminance value for each light source; and (vii) control the plurality of light sources using the second plurality of values of the first control parameter.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority of United Kingdom Patent Application no. 2505558.3, filed Apr. 14, 2025, which is hereby incorporated herein by reference in its entirety. FIELD The present disclosure relates to a method such as a method of determining or calibrating the white balance or white point of a display device or display system. More specifically, the present disclosure relates to a white balance calibration method, and a projection system for calibrating a white balance. Some embodiments relate to a method of measuring the luminance and colour of a light pattern and estimating new control parameter values to reach a target luminance and white balance based on a linear approximation method. Some embodiments relate to a projector system for implementing the method. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel/Fourier transform holograms or simply Fresnel/Fourier holograms. A Fourier hologram may be considered a Fourier domain/plane representation of the object or a frequency domain/plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and/or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, “HUD”, and head-mounted displays, “HMD”, including near-eye devices, for example. A holographic projector may form a full-colour holographic reconstruction. This may be achieved by displaying three single-colour holograms, and illuminating each hologram using a corresponding single-colour light source to form three single-colour holographic reconstructions at substantially the same time. An approach known as spatially-separated colours, “SSC”, involves simultaneously forming the three single-colour holographic reconstructions, whilst another approach, known as frame sequential colour, “FSC”, involves rapidly forming three single-colour holographic reconstructions in succession, so as to be within the integration time of the human eye. Thus, using either approach, a human viewer perceives a full-colour (i.e. polychromatic) image. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. In a colour display system, the range of displayable colours may be obtained from a mixture of colours each provided by a different light source (e.g. a plurality of laser diodes). By controlling the relative intensity of each colour emitted from the respective light source, a wide range of different colours can be displayed. Since the relative intensities of the light sources affect the output colour, as the skilled person would be aware, such a display may require calibration such that the colours output by the system match the desired colours of an input or target image. One such type of calibration is known to the skilled person as a white balance calibration, in which the system is calibrated to display the desired “colour” of white when, for example, the plurality of light sources are driven at the same grey level. A typical framework for calibrating the white balance of a display system may involve displaying a known pattern at a known mixture of grey levels. For example, the light sources of the display may all be driven at the maximum grey level (either across the whole display a