UV index

UV index is a way of quantifying how much UVB is actually reaching a reptile at a given point in the enclosure, rather than relying on a bulb's advertised strength or its age. Different species evolved under different natural light conditions, so a desert species that basks in open sun typically needs a higher UV index at its basking spot than a shade-dwelling forest species.

Checking UV index means holding a UV index meter or solarmeter at the exact spot the animal basks, not near the bulb or at the enclosure floor, since output falls off quickly with distance and is affected by screen mesh, glass, or plastic covers between the bulb and the animal. Readings should be checked periodically as bulbs age, since UVB output can fall well before a bulb stops emitting visible light.

A common mistake is checking UV index once at setup and never again, missing the gradual decline that most bulbs go through over months of use. Another is measuring at the wrong height, since a reading taken at the enclosure floor will not reflect what an animal experiences on an elevated basking platform closer to the bulb. Matching the meter reading to published index targets for the specific species being kept is more reliable than any general rule of thumb.