Holographic display devices

[0032]We have previously described, in UK patent application number 0512179.3 filed 15 Jun. 2005, incorporated by reference, a holographic projection module comprising a substantially monochromatic light source such as a laser diode; a spatial light modulator (SLM) to (phase) modulate the light to provide a hologram for generating a displayed image; and a demagnifying optical system to increase the divergence of the modulated light to form the displayed image. Absent the demagnifying optics the size (and distance from the SLM) of a displayed image depends on the pixel size of the SLM, smaller pixels diffracting the light more to produce a larger image. Typically an image would need to be viewed at a distance of several metres or more. The demagnifying optics increase the diffraction, thus allowing an image of a useful size to be displayed at a practical distance. Moreover the displayed image is substantially focus-free: that is the image is substantially in focus over a wide range or a