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Too far and your dragon gets no usable UVB; too close and the UV index runs dangerously high. Enter your bulb type and whether it sits over mesh to get a mounting distance range for the basking spot — then confirm with a UV index meter if you can.
UVB distance chart (to basking spot, no mesh)
| Bulb | Distance to basking spot | Over mesh | Notes |
|---|---|---|---|
| T5 HO 12% (e.g. Arcadia Desert 12%) | 12–18 in (30–45 cm) | Move ~30–50% closer or mount inside | Best all-round choice for 4×2×2 ft enclosures |
| T5 HO 10% (e.g. ReptiSun 10.0 T5 HO) | 10–16 in (25–40 cm) | Move ~30–50% closer or mount inside | Similar; follow manufacturer chart for your reflector |
| T5 HO 6% / 5.0 | 6–12 in (15–30 cm) | Best mounted inside | Only for short enclosures or low basking spots |
| T8 10% (older tech) | 6–10 in (15–25 cm) | Not recommended over mesh | Weak output — upgrade to T5 HO where possible |
| Compact / coil UVB | — | — | Not recommended — uneven output, hard to place safely |
Aim for a UV index of about 3–6 at the basking spot (Ferguson zone 3–4). Fine mesh blocks roughly 30–50% of UVB; wide-hole mesh blocks less. Replace T5 tubes about every 12 months even if they still light up, and re-check distance after any decor or mesh change. If your dragon squints, avoids the basking spot or hides constantly, re-check heat and UVB and ask a reptile vet.
FAQ
Should the UVB tube go over or under the mesh?
Inside the enclosure is best for output. If it must sit on top, use a T5 HO 10–12% with a reflector and mount the basking spot closer to compensate for mesh loss.
How do I know the UVB is really right?
A Solarmeter 6.5 UV index meter is the only sure way. Without one, stick to the chart ranges above, replace bulbs yearly, and watch behavior — constant hiding or lethargy deserves a vet visit.
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