Reptile UVB and Basking Guide: Ferguson Zones, Bulbs and UV Meters

This page is for anyone choosing or checking UVB and basking lights for a pet reptile. It gives each common species’ UV zone and basking target, then covers bulb types, how fast they fade, how to measure them, and what blocks UVB.

UVB zone and basking temperature by species

The UV-Tool, compiled by the BIAZA reptile working group, groups species into four Ferguson zones by how much sun they seek. The zone UVI is the average background level; the maximum is the highest a lamp should give at the animal’s closest point (Baines et al., JZAR):

• Zone 1 (shade dweller, crepuscular): UVI 0–0.7, maximum 0.6–1.4
• Zone 2 (occasional basker): UVI 0.7–1.0, maximum 1.1–3.0
• Zone 3 (open or partial sun basker): UVI 1.0–2.6, maximum 2.9–7.4
• Zone 4 (midday sun basker): UVI 2.6–3.5, maximum 4.5–9.5

Species	Ferguson zone	Basking surface temperature	Daytime air	Merck lighting class
Bearded dragon	3–4; basking zone UVI 3.0–5.0 (RSPCA; Baines et al.)	104–113°F (40–45°C) (Baines et al.); RSPCA gives 100–108°F (38–42°C)	77–86°F (25–30°C) (Baines et al.)	UVB essential (Merck)
Leopard gecko	1 (Baines et al.); 2–5% UVB tube (RSPCA)	90°F (32°C) (Baines et al.); RSPCA gives 82–86°F (28–30°C)	77–84°F (25–29°C) (Baines et al.)	No special lighting, UVB still recommended (Merck)
Crested gecko	1 (Baines et al.)	82°F (28°C) (Baines et al.)	77–82°F (25–28°C) (Baines et al.)	Not listed
Ball python	2 (Baines et al.)	95–104°F (35–40°C) (Baines et al.)	82–86°F (28–30°C) (Baines et al.)	No special lighting (Merck)
Corn snake	1–2 (Baines et al.)	88°F (31°C) (Baines et al.)	75–79°F (24–26°C) (Baines et al.)	No special lighting (Merck)
Red-eared slider	3–4 (Baines et al.)	95°F (35°C) (Baines et al.)	77–86°F (25–30°C) (Baines et al.)	UVB essential (Merck)
Green iguana	Not in the UV-Tool appendix	About 9°F (5°C) above the air gradient (Merck)	84–91°F (29–33°C) (Merck)	UVB essential (Merck)
Eastern blue-tongued skink	2–3 (Baines et al.)	95–113°F (35–45°C) (Baines et al.)	82–90°F (28–32°C) (Baines et al.)	Not listed

Where sources disagree. Merck classes corn snakes, ball pythons and leopard geckos as needing “no special lighting”, though it still recommends broad-spectrum light for likely health benefits. The UV-Tool assigns each of them a zone and gives a low-level UVB gradient, noting that nocturnal species can make vitamin D3 in their skin when exposed to daylight. Its authors also stress that “all guidelines to date are still very experimental”. Basking figures differ by a few degrees between the UV-Tool and the RSPCA; measure and stay inside the overlap where you can. Species pages: bearded dragon, leopard gecko, ball python, blue-tongue skink.

Bulb types: shade method versus sunbeam method

The UV-Tool describes two ways to light an enclosure (Baines et al., JZAR):

• Shade method (zones 1–2: most snakes, crepuscular geckos). Low background UVB over a large area, falling to zero in shelters. T8 fluorescent tubes suit small vivaria; T5 high-output tubes can sit further away in larger enclosures to give the same low UVI.
• Sunbeam method (zones 3–4: bearded dragons, sliders). A brighter, sun-like patch over the basking spot. T5-HO tubes with reflectors, mercury vapour or metal halide lamps. Mercury vapour and metal halide lamps also give off heat, and some have very narrow beams. One tested metal halide gave the right UVI for a zone 2 animal over a spot under 10 in (25 cm) across, while a T5 tube with reflector covered more than 51 in (130 cm). The UV patch must be at least as wide as the whole animal.

Fluorescent lamps giving under UVI 0.5 at 6 in (15 cm) aren’t adequate as the only UVB source, even for zone 1. Some “full spectrum” tubes emit negligible UVB except at very close range (Baines et al., JZAR). The RSPCA specifies a 10–12% high-output tube up to half the vivarium length for bearded dragons and a 2–5% tube for leopard geckos.

Measuring output and when to replace

UVB fades while visible light doesn’t, so a lamp that looks fine may be useless. In the UV-Tool’s tests, mean output fell 12.6% after a 105-hour burn-in and 39.9% after 4,000 hours (about a year at 10–12 hours a day). One product lost 64% in 1,000 hours, about three months (Baines et al., JZAR). Fixed replacement intervals therefore don’t fit every brand:

1. Use a UV index meter (the UV-Tool authors used a Solarmeter 6.5) at the animal’s closest point to the lamp, and again in the shade.
2. Record the reading when the lamp is new and recheck it regularly (RSPCA).
3. Replace the lamp when readings fall below the zone’s range or at the manufacturer’s interval, whichever comes first (RSPCA).
4. No meter? The UV-Tool’s distance charts for each lamp are the fallback. They don’t allow for reflectors, mesh guards or branches, which all change the result.

Glass, mesh and placement

• Glass blocks UVB. Ordinary 4 mm window glass let through only 0.4% of UVB at 300 nm, against 80.9% for 3 mm UV-transmitting acrylic (Baines et al., JZAR). Never put glass or plain plastic between the lamp and the animal (RSPCA; Baines et al.).
• Mesh reduces output. Guards are needed against burns and shattering (RSPCA), but choose wide mesh to let more UV through, and measure underneath it (Baines et al., JZAR).
• Mount lamps overhead, so the brow and eyelids shade the eyes (Baines et al., JZAR).
• Distance: keep a bearded dragon’s back at least 10–12 in (25–30 cm) from its basking lamp (RSPCA).

Lighting mistakes and what they cause

• No shade. Even zone 4 reptiles need a gradient falling to zero in a shelter (Baines et al., JZAR).
• Too much UVB. The UV-Tool treats UVI 7.0–8.0 as an absolute maximum at reptile level even for zone 4 species; most basking happens at UVI 3.0–5.0. Overexposure risks UV burns and eye damage.
• Using a “hot rock” for heat. Merck notes these frequently burn larger reptiles.
• Replacing by calendar alone. A weak product can be ineffective after three months (Baines et al., JZAR).

Avoid: UVB through a glass lid, unguarded bulbs, and basking spots narrower than the animal. For tank cleaning, see cleaning and maintenance.