Care guide
UVB, Ferguson zones, and the UV index
A zone is a way to match a lamp to how the species basks. It is not a promise that any one bulb is correct at any distance.
What the zone means
Frances Baines and colleagues described Ferguson zones from how reptiles use sun in the field, and turned those observations into a UV-tool for captivity (Journal of Zoo and Aquarium Research, 2016). Zone 1 is shade or crepuscular. Zone 2 is partial sun. Zone 3 is open basking. Zone 4 is midday sun for a few desert species.
Reading a care sheet
Repticentra records a zone and a rough UV index at the animal, not a brand of lamp. A T5 tube that is fine at 30 cm can be too strong at 15 cm, or useless behind glass or fine mesh. Measure, or follow a distance chart for that exact lamp, and replace UVB tubes on the maker's schedule. Output falls before the lamp looks dark.
Who needs it
Diurnal lizards, tortoises, and basking turtles use UVB to support vitamin D3 and calcium metabolism. Many snakes and crepuscular geckos are kept in zone 1, where a low-output lamp is useful and a desert lamp is too much. Axolotls do not get a basking UVB lamp.
What UVB does not fix
UVB does not correct a diet with no calcium, a temperature that is too cold to digest, or a lamp with no shade. The animal needs a gradient: a brighter basking end and a retreat where it can sit out of the light.
Source
Baines and others, "How much UV-B does my reptile need? The UV-Tool, a guide to the selection of UV lighting for reptiles and amphibians in captivity", Journal of Zoo and Aquarium Research 4(1), 2016. https://www.jzar.org/jzar/article/view/150