Hologram calculation

A method of determining a multi-wavelength hologram, said multi-wavelength hologram being configured to project a first image and a second image when it is displayed on a pixelated display device and illuminated by light of a first wavelength to project the first image and by light of a second, shorter wavelength to project the second image, wherein the first image is different to the second image;

• the method comprising:
• i) obtaining a first hologram, comprising a first set of hologram pixel values corresponding to the first image;
• ii) obtaining a second hologram, comprising a second set of hologram pixel values, corresponding to the second image;
• iii) determining a first operating range of voltage drive levels, wherein each pixel of the display device is configurable to provide a light modulation value in a full range of light modulation values at the first wavelength, when driven within the first operating range;
• iv) determining a maximum number of discrete light modulation levels for the display device and distributing those discrete light modulation levels over a voltage range that equals or exceeds said first operating range of voltage drive levels;
• v) using the distributed discrete light modulation levels to separately represent each of the first hologram and the second hologram and outputting a corresponding first set of pixel drive levels for the first hologram and a second set of pixel drive levels for the second hologram;
• vi) for each pixel of the multi-wavelength hologram, selecting a first drive level from the first set of pixel drive levels, to represent the corresponding pixel of the first hologram, and selecting a second drive level from the second set of pixel drive levels, to represent the corresponding pixel of the second hologram, and outputting a multi-wavelength drive level for that pixel, based on the selected first and second drive levels;
• vii) using the multi-wavelength drive level output for each pixel to form the multi-wavelength hologram.

A voltage selection unit for driving a pixelated display device to display a multi-wavelength diffractive structure, said multi-wavelength diffractive structure being configured to represent each of a first diffractive structure and a second, different diffractive structure, the voltage selection unit being configured to:

• a) determine a first plurality of discrete voltage levels at which the display device may be driven, wherein each level of said first plurality of discrete voltage levels corresponds to a respective discrete light modulation value for the first diffractive structure, in the full range of light modulation values thereof;
• b) determine a correspondence between each level of said first plurality of discrete voltage levels and a respective discrete light modulation value for the second diffractive structure, in a range exceeding the full range of light modulation values thereof;
• c) determine a first set of pixel drive values for representing the first diffractive structure on the display device and a second set of pixel drive values for representing the second diffractive structure on the display device, using the first plurality of discrete voltage levels;
• d) for each pixel of the display device, select an optimised pixel drive value that represents each of the pixel drive value for the first diffractive structure and the pixel drive value for the second diffractive structure.