Sunlight does not only provide a source of heat for many reptiles. It also provides an intense source of ultraviolet radiation in the 290 to 320nm range, commonly called UVB. Exposure to UVB, even for a short amount of time, allows reptiles, amphibians, and many other vertebrates to produce Vitamin D, which is vital for proper calcium absorption in the intestines.
Not all reptiles and amphibians require the same amount of UVB to be healthy. Exposure to sunlight will vary greatly depending on species, their natural environment, and their habits in the wild. Many species of reptiles and amphibians are nocturnal or live in burrows or on the floor of the rainforest, such as crested geckos, leopard geckos, or poison dart frogs, and are not exposed to significant levels of UVB in the wild. For these species, low levels of UVB, such as 2.0 bulbs, (or none at all!) are sufficient. Some species, such as many chameleons, live in forests, where much of the sunlight is absorbed by foliage. For these species, a moderate level of UVB, such as a 5.0 bulb, is appropriate. At the opposite end of the spectrum, there are reptiles that thrive in full sun environments, such as deserts. Common pet lizards such as bearded dragons and uromastyx thrive under intense UVB expose from 10.0 or Mercury Vapor Bulbs. Josh's Frogs carries a wide variety of UVB Bulbs.