Patent ID: 9625717
Date: 2017-04-18
CPC Classifications: G02B

Claim:
1. An optical device wherein the device is a near-to-eye display (NED) or an exit pupil expander (EPE) or part thereof comprising: a transparent substrate, a grating on or within the substrate, the grating comprising a first transparent grating layer which further comprises periodically alternating zones having different refractive indices, wherein the grating has a second transparent grating layer located on the first grating layer and also has periodically alternating zones having different refractive indices so that the zones of the first grating layer having higher refractive index are at least partly aligned with the zones of the second grating layer having lower refractive index and vice versa, wherein said first and second grating layers diffract light to produce a first transmission order having a diffraction efficiency and a first reflection order having a diffraction efficiency, wherein the period, layer thicknesses and refractive indices of the first and second grating layers are adapted to allow diffraction of light by the grating to non-zero reflection orders and to make the diffraction efficiency of the first transmission order lower than the diffraction efficiency of the first reflection order over the wavelength range of 450-650 nm for preventing visible rainbow effect caused by transmitted light; wherein said grating is an out-coupling grating adapted to diffract light directed to the out-coupling grating from the substrate on the surface of which or within which the out-coupling grating is located; and an in-coupling grating adapted to diffract light from the outside of the substrate into the substrate and further towards the out-coupling grating so that light propagates in the light-guiding substrate via total internal reflections.