Ultra-violet radiation, also know as UVB, is the main cause of skin and eye damage in reptiles and mammals alike.
Thinking logically, nocturnal(active at night) animals do have not evolved the same set of defenses to this radiation that diurnal(active at day) animals have. I would NOT want to place my gecko in an environment with this radiation coming from a light 12" above them for 12 hours a day.
As far D3, leopard geckos and other nocturnal reptiles have the ability to absorb D3 from the food they eat much more efficiently than other diurnal reptiles. This is why no additional D3 supplementation is needed, either through vitamins or through UVB exposure. Perhaps it is the UVB exposure that causes the excess of D3 that the poster above keeps bringing up.