The difference between UVA and UVB is nanometer (measure of visible light) range of each. UVA is 320-400 nm and UVB is 290-320 nm. UVA slightly changes the way reptiles view their environment. These differences help in feeding and breeding. The way we see things may appear very different to them under UVA light. UVB is necessary for reptiles to produce vitamin D3 which allows them to metabolize calcium. Sounds familiar right? vitamin D for strong bones. Most lizards and all turtles and tortoises need UVB to prevent metabolic bone disease. There are different levels of UVB; the most common are 5.0 and 10.0. The level needed will be based on the type of reptile you have.