Optical brighteners require ultraviolet radiation for their effects. Especially in strong sun and clear, blue skies outdoors or under suitable artificial lighting (with high UV content), the white then appears whiter. 

Fluorescence is the cause of the brightening effect of optical brighteners. These are substances that absorb from the invisible ultraviolet (at 290-400 nm) and, according to intramolecular interlude, re-emit most of the absorbed energy in visible light.