An optical display device (600, 700) is disclosed. A diffractive optical element (618, 718) is mounted on a substrate (611, 711). The diffractive optical element comprises a blazed grating (619, 719) and a remnant layer (617, 717) which is formed of the same polymer-based material as the blazed grating. The remnant layer (617, 717) is positioned adjacent the substrate (611, 711) and a refractive index mismatch is provided between the two. The depth of the remnant layer is selected so that light diffracted by the grating and light reflected by the interface re-combine and interfere constructively for selected angles of incidence in order to undergo thin-film interference.