An optical element comprises a body having opposed planar major surfaces with a plurality of open-mouthed inclusions (grooves) formed on its light output surface. Each groove has a closed apex with an included apex angle. The body is formed from a material having an index of refraction of at least 1.3 and an induced absorbance rate ΔAbs/Dose less than or equal to about 0.4. The apex angle is selected such that light incident upon the body is conveyed the light output surface by the mechanism of total internal reflection from the boundary walls of the grooves and/or without retro-reflection toward the incident surface.