Reptile UVB and Ferguson Zones Explained: How Much UVB Your Reptile Needs

The short answer: give your reptile a UV gradient that tops out, at the spot closest to the lamp, at the UV Index (UVI) its Ferguson zone calls for, and falls to zero in the shade. For a bearded dragon (zone 3 to 4) that means a basking spot of roughly UVI 3 to 6. For a leopard gecko or crested gecko (zone 1) it means a gentle maximum of about UVI 0.6 to 1.4. The zones come from field measurements of wild lizards and snakes by Ferguson and colleagues (Zoo Biology, 2010), and the way to apply them in an enclosure comes from the UV-Tool published by Baines and colleagues (Journal of Zoo and Aquarium Research, 2016). For the zone, basking temperature and day and night temperatures of 59 species, see our reptile temperature and UVB chart.

This guide explains what the zones are, which zone the common pet species fall into, how to turn a zone into a lamp and a distance using the makers' own output charts, and what mesh, glass and lamp age do to the numbers. Ask a reptile veterinarian about your own animal's health and lighting.

What the Ferguson zones are

Ferguson et al. measured the UV exposure that 15 species of lizards and snakes chose for themselves in the wild, in the southern United States and Jamaica, and sorted them into four groups by behaviour. The median UVI ran from 0.35 for zone 1 to 3.1 for zone 4. Baines et al. summarised the four zones in their Table 1, reproduced here.

The Ferguson zones (Baines et al. 2016, Table 1, summarised from Ferguson et al. 2010)

Zone	Behaviour	Zone range (average UVI)	Maximum UVI recorded
1	Crepuscular or shade dweller, thermal conformer	0 to 0.7	0.6 to 1.4
2	Partial sun or occasional basker, thermoregulator	0.7 to 1.0	1.1 to 3.0
3	Open or partial sun basker, thermoregulator	1.0 to 2.6	2.9 to 7.4
4	Mid-day sun basker, thermoregulator	2.6 to 3.5	4.5 to 9.5

The "zone range" is the average of all readings taken where the animals were found. The "maximum" is the highest UVI any animal of that group was seen in. Baines et al. suggest giving the captive animal a gradient from zero, in full shade, up to the maximum for its zone at the closest point it can reach to the lamp.

Which zone is my reptile in?

The UV-Tool appendix assigns 254 species to zones from their known basking behaviour. These are the common pets this site covers.

Zone assignments from the UV-Tool appendix (Baines et al. 2016)

Species	Ferguson zone	Method that fits
Leopard gecko (Eublepharis macularius)	1	Shade method
Crested gecko (listed as Rhacodactylus ciliatus)	1	Shade method
Corn snake (Pantherophis guttatus)	1 to 2	Shade method
Ball python (listed as royal python, Python regius)	2	Shade method
Eastern blue-tongued skink (Tiliqua scincoides)	2 to 3	Shade or sunbeam method
Inland bearded dragon (Pogona vitticeps)	3 to 4	Sunbeam method

Albino and hypomelanistic animals are the exception to the table. Baines et al. note that melanin absorbs UV, so animals lacking it are likely to need much lower exposure than their species' zone suggests. Ask a reptile veterinarian before lighting one.

The shade method and the sunbeam method

Shade method (zones 1 and 2). A low level of UV across a broad area, highest near the lamp and fading to zero. Baines et al. describe T8 fluorescent tubes as suited to this in small enclosures where the maximum needed is no more than UVI 0.7 to 1.0, and high-output T5 tubes for larger enclosures, mounted further away to reach the same low UVI at animal level. Most snakes, amphibians and crepuscular lizards fall here.

Sunbeam method (zones 3 and 4, and some zone 2 baskers). A higher UVI restricted to the basking spot, like a patch of sunlight, with a gradient to zero in the shade. The target is the UV of an early to mid-morning basking session: in open tropical and subtropical sun between 8:30 and 9:30 am the UVI is typically 3.0 to 5.0, per the authors' measurements. Even zone 4 species seen basking at UVI 9.5 or above spend most of their basking time at about UVI 3.0 to 5.0, so the authors place the best range for them there, and treat UVI 7.0 to 8.0 at reptile level as the absolute maximum under artificial UV.

Two more rules from the paper apply to both methods. The UV patch has to be at least as wide as the animal's whole body, and the heat and UV should overlap so the animal is not forced to choose between warmth and UV. That is why makers such as Arcadia advise grouping heat and light over one basking zone.

Turning a zone into a lamp and a distance

The UVI an animal receives depends on the lamp, its reflector, the distance and anything in between. The only reliable way to set it is with the maker's published output chart and, ideally, a UV Index meter (the studies and makers quoted here use the Solarmeter 6.5). Two examples from makers' charts:

Arcadia ProT5 kit (lamp in reflector), UVI directly under the lamp, per Arcadia

Distance	Forest 6%	Desert 12%	Dragon 14%
12 in (300 mm)	4.50	8.50	10.50
15 in (380 mm)	3.60	6.50	7.00
20 in (500 mm)	2.50	3.80	4.60
24 in (600 mm)	1.20	2.40	3.20

Zoo Med ReptiSun T5 HO 24-inch hood with 10.0 lamp: distance from bulb for each zone, per Zoo Med's chart

Zone (Zoo Med's UVI band)	Distance from bulb
Above UVI 7: no animal access	Under 10 in
Zone 4 (2.6 to 3.5+)	10 to 19 in
Zone 3 (1.0 to 2.6)	19 to 31 in
Zone 2 (0.7 to 1.0)	31 to 36 in
Zone 1 (0.4 to 0.7)	36 to 38 in

Read these as starting points. For a bearded dragon, Arcadia sets its Dragon 14% lamp at about 15 in (35 to 40 cm) from the animal's back for an upper basking index of UVI 6. For a leopard gecko, Arcadia's ShadeDwellerPro 7% kit gives UVI 0.80 at 10 in and 0.65 at 12 in, which sits inside the zone 1 maximum. Baines et al. stress that charts are no substitute for measuring in your own enclosure, because reflectors, guards, branches and plants all change the result. The best UVB lights for reptiles page applies these charts to specific kits.

Mesh, glass and plastic

What sits between the lamp and the animal

Material	Effect on UVB	Source
Stainless steel mesh	Can reduce UVI by 30% or more	Arcadia (ShadeDwellerPro page)
Terrarium screen cover	Can filter out up to 50% of UVB	Zoo Med (T8 ReptiSun 10.0 page)
Ordinary 4 mm window glass	0.4% transmission at 300 nm	Baines et al. 2016
4 mm high-transmission low-iron glass	16.9% transmission at 300 nm	Baines et al. 2016
3 mm UV-transmitting acrylic	80.9% transmission at 300 nm	Baines et al. 2016

The practical rule: never put ordinary glass or plastic between the lamp and the animal, and if the lamp sits on top of a mesh lid, expect a lower reading than the chart and move the lamp closer or choose a stronger lamp. Baines et al. recommend wide mesh where a guard is needed. Inside a PVC or wooden enclosure the lamp can be mounted under the ceiling with no mesh in the way, which is one reason the T5 vs T8 comparison matters more for mesh-top glass tanks.

UVB lamps lose output with age

UV output falls with use, mainly because the glass of the lamp solarises under UV bombardment. In the UV-Tool tests, seven Arcadia and Zoo Med fluorescent tubes lost an average of 12.6% of their UVI (range 6 to 23%) in the first 105 hours, and 39.9% on average (range 30 to 48%) after 4,000 hours, about a year at 10 to 12 hours a day. A lamp from another maker lost 64% after only 1,000 hours. Arcadia rates its T5 lamps for 12 months, and Zoo Med runs a 12-month UVB replacement reminder. A UV Index meter is the only way to know where a particular lamp stands.

Do nocturnal species need UVB?

The UV-Tool authors argue there is no reason to withhold it, provided the animal has a retreat and access to zone 1 levels. They cite work showing leopard geckos exposed to low-level UVB had 25-hydroxyvitamin D3 levels 3.2 times those of geckos given dietary supplements alone (Wangen et al. 2013), and corn snakes making vitamin D3 under low-level fluorescent UVB (Acierno et al. 2008). The species setups for leopard geckos, ball pythons and bearded dragons put these zones into a full enclosure plan.

Checklist

• Find your species' zone in the table above or in the UV-Tool appendix.
• Pick the shade method (zones 1 and 2) or the sunbeam method (zones 3 and 4).
• Choose a lamp whose maker publishes UVI at distance, and set the closest point the animal can reach to the zone maximum or below.
• Keep the hottest and highest-UV spot in the same place, wide enough for the whole animal, with shade to retreat to.
• Allow for mesh: expect 30 to 50% less UV through a screen top, per Arcadia and Zoo Med.
• Never let the animal reach a spot above UVI 7 to 8, the UV-Tool's absolute ceiling for zone 4.
• Replace fluorescent UVB lamps on the maker's schedule (12 months for Arcadia T5), or sooner if a meter shows the reading has dropped.