OPTICAL PATH LENGTH ADJUSTER

An optical path length adjuster ( 53 ) enables electro-optical control of a physical path length between two optical elements, suitable for use in the adjustment of an optical path length within three dimensional display devices that generate a virtual image within a defined imaging volume. The adjuster varies an optical path length between an input optical path and an output optical path and includes: a plurality of first optical ( 61 ) elements and second optical elements arranged in alternating sequence along an optical path, each first optical element ( 62 ) for determining a polarisation state of a light beam passing through that element and each second optical element for selectively transmitting or reflecting a light beam incident on that element depending on the selected polarisation state of the incident light beam, wherein the optical path length traversed by an input beam on the optical path can be varied by is selecting a particular second optical element at which reflection of the input beam is to occur, the reflected input beam emerging along the output optical path.