UVB distance and Ferguson zones

The Ferguson zone table, with the numbers

Two columns, two different things. The "zone UVI range" describes the average exposure an animal experiences across its active period. The "basking zone maximum" is the peak figure at the basking site, and it is the one you set your lamp against. Those figures come from Northampton Reptile Center, with the species assignments corroborated by Zen Habitats.

Species targets used on this site

Mount distances, all in one place

What the mesh assumption means in practice

Mesh blocks a substantial fraction of UVB output - the published corrections on this page assume somewhere between 30% and 45% depending on the species page they come from. Your lid may be finer or coarser than the one those figures were measured through, which is the single largest uncontrolled variable in a typical setup.

Three practical consequences. First, if you mount the fixture inside the enclosure, the mesh correction disappears and you need the no-mesh distance instead - 16 to 17 inches rather than 11 for an Arcadia Desert 12%. Second, if you do not own a meter, the published advice for bearded dragons is to add 1 to 3 inches to the figure to account for the animal's own height. Third, a meter removes the guesswork entirely, which is why it is the only item on this site we describe as making every other figure verifiable.

Building the gradient, not just the peak

The basking figure is the maximum, not the whole specification. For a bearded dragon the optimal UVI gradient runs from 0 at the furthest point from the lamp up to 4.0-4.5 at the basking site. That gradient is what lets the animal choose its own exposure, which is the entire mechanism Ferguson zones describe. An enclosure where UVI is uniform everywhere - usually because it is too small - removes the choice.