Waveguide grating device

An optical waveguide comprising:

• at least two TIR surfaces and containing a grating of a first prescription configured such that an input TIR light with a first angular range along a first propagation direction undergoes at least two diffractions within said grating and undergoes a change in propagation direction from said first propagation direction to a second propagation direction, wherein each ray from said first angular range and its corresponding diffracted rays lie on a diffraction cone of said grating, wherein each diffraction provides a unique diffraction efficiency versus angle characteristic along said second propagation direction, wherein one of said diffractive efficiency versus angle characteristics corresponds to rays that do not meet the condition for TIR at said TIR surfaces.