Optical system for head-up displays

Claim 1

An optical system for a head-up display having a collimator (5) for producing a collimated image of one or more display objects (1) and including an optical relay unit for producing an intermediate image for each display object in the focal plane (2) of the collimator, the optical relay unit comprising: a first group of relay elements (6,7,8) for each display object; a second group of relay elements (9,10) for receiving light from each of the first groups of relay elements and focusing it in the focal plane of the collimator; and a plurality of optical folding elements for producing a double fold in the optical path between said first and second groups of relay elements, the folding elements comprising one or more first folding elements (12) one for each display object for reflecting light from the display object to a second folding element (11); characterised in that said second folding element is adapted to reflect light incident on it at a defined combination of angle of incidence and wavelength and to transmit display light incident on it at a different combination of angle of incidence and wavelength, and is so disposed that light from at least one display object passes through the second folding element before being reflected by the first folding element to impinge on the second folding element at a different angle of incidence and is hence reflected towards the second group of relay elements.