A backlight comprises in order a lightguide plate with a sidelit light source and bonded thereto a light extraction unit containing in order from the lightguide plate, a first polymeric layer, an adhesive layer, a second polymeric layer bearing features at least partly embedded in the adhesive layer wherein the materials of the first polymeric layer, the adhesive layer, and the second polymeric layer differ by less than 0.05 in refractive index, the interstitial regions between the non-embedded portions of the features containing a material having a refractive index at least 0.1 lower than that of the material comprising the features.