Reptiles synthesize vitamin D3 in their skin when UVB (290–315 nm) hits it — the same photochemistry humans run. Without it, dietary calcium cannot be absorbed, and the result is metabolic bone disease: soft shells, rubbery jaws, deformed limbs, death. UVB is not a lamp upgrade — it is the calcium delivery system.
Ferguson zones: the classification
Professor Gary Ferguson's zones classify species by their natural UV exposure behavior:
| Zone | Behavior | Examples | Typical bulb |
|---|---|---|---|
| 1 — shade | Crepuscular, forest floor | Crested gecko, some pythons | 2–5% T8/T5, or optional |
| 2 — partial sun | Occasional basker | Leopard gecko, ball python | 5–7% T5 at distance |
| 3 — open basker | Regular full-sun | Bearded dragon, water dragon | 10–12% T5HO |
| 4 — mid-day open | Maximum exposure | Uromastyx, some tortoises | 12–14% T5HO close |
The distance math
UVB output decays with the square of distance — the lamp's strength means nothing without the distance to the basking spot. A 12% T5HO delivers zone-3 levels at ~30 cm; the same lamp at 60 cm delivers a fraction. Measure with a solar meter if serious; use manufacturer distance charts if sensible; replace lamps every 12 months regardless of appearance (UV output dies before the light does).
The mesh-roof tax
- Screen tops block 30–50% of UVB — mount lamps inside or reduce distance accordingly.
- Glass and acrylic block UVB entirely — never place the lamp outside glass.
- Gradients matter: the animal needs a basking zone WITH UVB and shade zones without — self-regulation is the point.
- Supplements are backup, not the plan: D3 dusting helps, but the lamp is the biology.
Check your species' zone in the database, mount the right lamp at the right distance, and replace it yearly. MBD is the most preventable tragedy in the hobby — and it is prevented by one bulb, correctly hung.