A wavelength-selective optical diffuser comprising a substrate of a first material having opposite first and second surfaces, wherein said first material is transparent to a first wavelength λS and a second wavelength λL, with λL>4λS; at least a first surface of said substrate having a surface relief such that a beam of light having the first wavelength λS is diffused, with a rms phase delay S>π/4, when traversing said substrate; and a beam of light having the second wavelength λL is minimally diffused, with a rms phase delay S