Kind of a trick question, but I think you're asking what range the bulb emits of UVB since UVB is emitted at a certain range on the wavelength scale. Its not about what part of the spectrum its emitting, its the amount its producing as an animal needs a minimum amount and that is defined on how powerful the bulb is and the distance it would be to the animal. A 10.0 bulb is great when you're using a very tall tank where the animal would be 18-36 inches away from the bulb. A 5.0 is 12-24 inches (for the T5s) Since the bulb also emits UVA and visible, the spectrum is pretty large, to answer your original question.