Holographic projection

A holographic projection system comprising:

• a spatial light modulator (SLM) arranged to display a diffractive pattern comprising a hologram;
• a light source arranged to illuminate the diffractive pattern in order to form a holographic reconstruction of the structured light pattern;
• a light detector arranged to detect light from the holographic reconstruction and to output a respective detected light signal;
• an image processor arranged to receive an image for projection, determine a pixel usage of the image, and to form an image frame comprising a first image region and second image region, wherein the first image region corresponds to the image and the second image region corresponds to non-image content representative of a system control parameter;
• the image processor being further arranged to determine a first gain factor based on the pixel usage, apply the first gain factor to the non-image content in order to change a grey level of the non-image content of the image frame, and to calculate a hologram of the image frame, wherein the first gain factor is proportional to the pixel usage;
• the system further comprising:
• a controller arranged to receive the respective detected light signal from the light detector and to obtain a measure of a parameter representative of the luminance of the holographic reconstruction of the non-image content, from said detected light signal;
• the controller being further arranged to:
• scale the measured parameter in accordance with an inverse of the first gain factor, to obtain a parameter value that is representative of true luminance of the holographic reconstruction of the non-image content;
• and
• control the optical power of the light source, illuminating the hologram, until the true luminance of the holographic reconstruction of the non-image content indicates that a target luminance is met, for the holographic reconstruction of the image content of the image frame.

A method of holographic projection, comprising:

• (i) receiving an image for projection;
• (ii) determining a number of pixels in the image;
• (iii) determining a grey level for each of the pixels in the image;
• (iv) calculating a predicted pixel usage, for a holographic reconstruction of a hologram that is to be created to represent the received image, using the determined number of pixels and their respective grey levels;
• (v) using the predicted pixel usage to determine a value of a parameter representative of optical power of a light source, arranged to illuminate the hologram that is to be created, wherein that value would be required in order to meet a target luminance, for the holographic reconstruction.