How far should the UVB be from the basking spot?

Start with the UV zone

A 2016 study by Baines and colleagues sorted reptiles and amphibians into four Ferguson zones by how much sun they take in the wild. Each zone has a UV index (UVI) the animal should reach at its basking spot:

• Zone 1, shade dwellers (leopard geckos, crested geckos, ball pythons, most frogs): a UVI of about 0.5 to 1.
• Zone 2, partial sun (hognose snakes, White's tree frogs): about 1 to 2.
• Zone 3, sun and shade (blue tongue skinks, chameleons, tegus, Russian tortoises): about 2 to 3.5.
• Zone 4, full sun (bearded dragons, uromastyx): about 3 to 6.

Then the tube

A linear T5 HO tube in a reflector gives an even gradient across the enclosure, the way sunlight does. Coil and compact bulbs throw a small, uneven patch and are best left out. The strength printed on the tube (5 to 7% for shade dwellers, 10 to 14% for sun baskers) sets how far it can sit from the back.

Then the mesh

A mesh top blocks roughly a quarter to a half of the UV, depending on the mesh. That is why a 12% tube for a bearded dragon sits about 11 to 13 in above its back through mesh, and farther with nothing in between.

Move the platform, not the lamp

Once the tube is mounted, you set the distance by raising or lowering the basking platform. Read the UVI at the basking spot with a UV index meter if you have one, and replace the tube every 12 months: it keeps glowing long after the UV fades.