UVB Lighting
UVB lighting is not a decorative detail of a terrarium — it's part of how a reptile breathes calcium into its bones. Here's how to think about it without relying on the marketing "10.0" or "5.0" printed on the box.
Key facts
- UVB drives vitamin D3 synthesis and calcium metabolism — it matters most for diurnal, sun-basking species such as bearded dragons and similar lizards.
- Zoo Med states directly that its ReptiSun T8 5.0 bulb provides optimum UVB for about 12 months, and should be replaced then — even though it still lights up.
- There is no universal bulb-to-animal distance: how much real UVB reaches the basking spot depends on the specific bulb and distance, not just its printed rating.
Why reptiles need UVB
UVB drives vitamin D3 synthesis in the skin, which in turn is needed for calcium metabolism. The requirement varies sharply by species: it is highest in diurnal, actively sun-basking species such as bearded dragons and many day geckos, and considerably lower or close to absent in some nocturnal and fossorial species.
Ferguson zones — the real tool, not "10.0 vs 5.0"
Rather than reading a bulb's marketing number, serious sources use the Ferguson-zone system, which sorts reptile species into four UV Index zones — from shade-dwellers to open-desert baskers — based on field measurements of natural sunlight in their habitat. The system was published in the peer-reviewed paper "How much UV-B does my reptile need? The UV-Tool" (Baines et al., Journal of Zoo and Aquarium Research 4(1), 2016), associated with herpetologist Frances Baines of UVGuide.co.uk.
The practical takeaway: for a given species, look up its Ferguson zone rather than the number on the bulb box, and choose bulb and distance to match it.
When to replace the bulb
Zoo Med states directly for its ReptiSun T8 5.0: "this lamp will provide optimum levels of UVB for 12 months so it should be replaced once a year." The bulb keeps lighting up after that, but its actual UVB output has already declined by then. Other bulb technologies (mercury-vapor bulbs, for instance) are commonly cited elsewhere at roughly 6 months — so the bulb type and manufacturer matter, and it's worth checking the guidance for your specific bulb rather than treating 12 months as universal across every bulb type.
Distance and mesh screens
The real UV Index at the basking spot depends on distance from the bulb as much as on the bulb's strength — which is exactly why the Ferguson-zone system works from a measured UV Index at a given distance rather than an abstract "bulb power" figure. A mesh screen or guard between the bulb and the terrarium noticeably reduces the UVB that gets through; we could not find a single reliable figure for how much it reduces it in general, so account for it when placing the fixture rather than plugging in a specific percentage.
Photoperiod
Zoo Med states for the same ReptiSun T8 bulb that it "should be on for 10-12 hours a day to maintain your animal's day and night cycle." That is a general manufacturer default, not a figure tuned to a specific species or season — individual species and seasonal changes can call for a different schedule.