An optical element is provided which comprises a plurality of fluorophores disposed in a medium. The fluorophores have a quantum yield greater than 50% and an absorption spectrum with a maximum intensity at wavelengths less than 400 nm, and emit a spectrum of light having a maximum intensity at wavelengths within the range of 400 nm to 1200 nm. The optical element is at least partially transparent over the visible region of the spectrum. The optical element is especially useful as a window or other optical component of a greenhouse structure.
