UVB is the light you cannot see that lets most reptiles build strong bones. The right lamp depends on how much sun your species gets in the wild, how far the lamp sits from the animal, and whether there is mesh in the way. This guide explains each of those in plain terms, so you can pick a tube, hang it at the right height and know when to replace it.
The short version
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Strong UVB for sun baskers Arcadia ProT5 UVB Kit, Desert 12%, 24W Amazon - Strong UVB for sun baskers Arcadia ProT5 UVB Kit, Desert 12%, 24W Amazon UK
- Low UVB for shade dwellers Arcadia ShadeDweller ProT5 UVB Kit (7%, 8W) Amazon
- Low UVB for shade dwellers Arcadia ShadeDweller ProT5 UVB Kit (7%, 8W) Amazon UK
- Gentle UVB for snakes Arcadia ShadeDweller Max ProT5 UVB Kit (2.5%, 14 W) Amazon
- Gentle UVB for snakes Arcadia ShadeDweller Max ProT5 UVB Kit (2.5%, 14 W) Amazon UK
- UV index meter Solarmeter Model 6.5R Reptile UV Index Meter Amazon
- UV index meter Solarmeter Model 6.5R Reptile UV Index Meter Amazon UK
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Basking heat to pair with UVB Zoo Med Repti Tuff Splashproof Halogen Lamp, 75W (2 pack) Amazon - Basking heat to pair with UVB Arcadia Halogen Basking Spot, 50W Amazon UK
- Thermometer gun for the basking spot Etekcity Lasergrip Infrared Thermometer Amazon
- Thermometer gun for the basking spot Etekcity Lasergrip Infrared Thermometer Amazon UK
What UVB actually does
Sunlight contains two kinds of ultraviolet light that matter to reptiles:
- UVB (290 to 320 nm): VCA explains that reptiles need it to make vitamin D3 in their skin, and that vitamin D3 is what lets the gut absorb calcium.
- UVA (320 to 400 nm): it does not help make vitamin D3, but VCA notes it matters for behavior.
When UVB, heat or diet falls short, calcium goes missing from the bones. The MSD Veterinary Manual names lack of UVB, too little heat and a poor diet as causes of nutritional secondary hyperparathyroidism, the most common form of metabolic bone disease (MBD). VCA calls MBD a common condition of pet reptiles that can be fatal if it is not caught early.
Care sheets disagree on whether a few species, such as leopard geckos and snakes, strictly need UVB. The RSPCA and most of the other sources on our species pages now recommend at least low-level UVB for them too, with shade nearby so the animal can choose how much it gets.
The lamp types
| Lamp | What it is | Our view |
|---|---|---|
| T5 HO tube | A thin, high output fluorescent tube in a fitting with a reflector | The first choice for most setups. Strong, even output along its length. |
| T8 tube | A wider, older fluorescent tube | Works, but gives less UVB at the same distance and fades faster. |
| Mercury vapor | A screw-in bulb that gives heat, light and UVB together | For large enclosures only. It cannot go on a thermostat. |
| Compact or coil | A small screw-in fluorescent | We do not recommend these. They light a small patch instead of a long gradient. |
Why linear tubes win. Arcadia’s method is to run the UV lamp over about half to two thirds of the enclosure, so the animal can move from a bright basking zone into full shade. A long tube on the warm side makes that gradient. A coil bulb makes a small pool of UV, so the animal has to sit in one exact spot to get it.
Mercury vapor bulbs do have a place in big enclosures, but they need care. The MSD Veterinary Manual lists them as one kind of basking light. Because they are both the heat and the UV source, you cannot dim them, so the basking temperature is set only by how high you hang the bulb.
Strong T5 HO kit
Arcadia ProT5 UVB Kit, Desert 12%, 24W
A complete high-output T5 UVB kit: the 12% Desert lamp, the controller, a removable curved reflector and a link cable. The 24W size is about 22 inches long, which is roughly half the length of a 4 ft enclosure.
- Lamp
- T5 HO, D3+ 12% UVB (Desert)
- Wattage
- 24W
- Length
- About 570 mm (22.4 in) fitting
- In the box
- Controller, lamp, removable reflector, power and link cables
- Mounting
- Inside the enclosure or over suitable mesh
Best for
The core UVB light for a 4 x 2 x 2 ft bearded dragon setup, in either market.
Worth knowing
Distance matters more than the box. Mesh between lamp and dragon cuts UVB, so the basking spot sits closer when the lamp is over mesh. Replace the tube on the schedule Arcadia gives (ReptiFiles says every 12 months for T5).
Ferguson zones: how much UVB your species needs
In 2010, Ferguson and colleagues measured the UV that wild reptiles actually experience and sorted species into four groups. Baines et al. (2016) built on that work in the UV-Tool, which assigns 254 species to these zones. Exo Terra’s summary of both papers gives these figures:
| Zone | Behavior | Average UVI | Highest UVI recorded | Examples |
|---|---|---|---|---|
| 1 | Shade dweller or active at dusk, rarely basks | 0 to 0.7 | 0.6 to 1.4 | Crested gecko, leopard gecko, Burmese python, milk snake |
| 2 | Partial sun, basks now and then | 0.7 to 1.0 | 1.1 to 3.0 | Green anole, boa constrictor, ornate box turtle |
| 3 | Open or partial sun basker | 1.0 to 2.6 | 2.9 to 7.4 | Frilled lizard, Yemen (veiled) chameleon, leopard tortoise |
| 4 | Midday sun basker | 2.6 to 3.5 | 4.5 to 9.5 | Chuckwalla, uromastyx, rhinoceros iguana |
The “highest recorded” column is the figure to aim for at the basking spot. The average column is what the animal gets across its day as it moves in and out of the light. That is why every setup needs shade as well as a bright spot.
Care sheets add species detail. For example, the RSPCA gives royal (ball) pythons a gradient from about UVI 1.0 at the basking spot down to zero in the shade, and a lower maximum of 0.7 for albinos. Pale morphs of other species usually get a lower target too.
Low output kit for zone 1
Arcadia ShadeDweller ProT5 UVB Kit (7%, 8W)
A complete low-output T5 UVB kit: an 8 watt ShadeDweller lamp (7% UVB, 17% UVA) with the fitting, mounting hardware, power cable and a link cable. Arcadia makes it for crepuscular and shade-dwelling species, and leopard geckos are one of its listed uses.
- Lamp
- 8W T5, 7% UVB and 17% UVA
- Kit includes
- Fitting, switch, fixings, power cable, link cable
- Mounting
- Inside a vivarium or on a mesh top
- Lamp life
- 12 months
Best for
Low-level UVB over the warm end, leaving the rest of the enclosure shaded.
Worth knowing
UVB output fades before the visible light does. Replace the lamp every 12 months, and place it with a mesh top in mind.
Gentle UVB for snakes
Arcadia ShadeDweller Max ProT5 UVB Kit (2.5%, 14 W)
A low-output T5 UVB and full-spectrum lamp in a fitting with a built-in reflector, made for shade-dwelling and partly basking snakes. Low-output tubes are what the RSPCA and ReptiFiles describe for royal pythons.
- UVB
- 2.5%
- Power
- 14 W
- Length
- About 22 to 24 in (55 cm)
- Type
- T5 with reflector
Best for
Giving a ball python gentle UVB and a bright day across about half of a 4 ft enclosure.
Worth knowing
Mount it on the warm side so the snake can move into shade. Check the distance against a UV meter or the maker's chart, and replace the tube on schedule.
UV index and why a meter helps
The UV index (UVI) is a number for how strong the sunburn and vitamin D producing light is at one spot. It is the same scale used in weather forecasts. Percentages on lamp boxes (6%, 12%, 14%) are not UVI; they describe the lamp’s spectrum, not what reaches your reptile.
A UV index meter turns guesswork into a reading. ReptiFiles says the Solarmeter 6.5 (or its 6.5R version) is the only handheld tool it trusts for UVI, and Arcadia’s own distance tables are measured with one. Hold it where your reptile’s back sits on the basking spot, then move it into the shade to check the gradient falls away.
A meter is an investment. If you skip it, follow the lamp maker’s distance chart closely and replace tubes on schedule.
UV index meter
Solarmeter Model 6.5R Reptile UV Index Meter
A handheld meter that reads the UV index (UVI) your lamp gives at any spot in the enclosure. Its sensor is weighted to the vitamin D range, and a Ferguson zone chart is printed on the front. ReptiFiles says it is the tool to use for measuring UVI.
- Reads
- UV index, 0.1 UVI resolution
- Accuracy
- Plus or minus 10%, NIST traceable (per seller listing)
- Power
- 9V battery
- Extra
- Ferguson zone chart on the meter
Best for
Setting the basking distance by reading instead of guessing, and knowing when a UVB tube has faded and needs replacing.
Worth knowing
It is a costly tool for one animal. Readings change fast with distance, so always measure at the height of your reptile's back on the basking spot.
Distance and mesh
UVI drops fast with distance. Arcadia’s measurements for its ProT5 kits, taken with a Solarmeter 6.5 directly under the lamp, show how fast:
| Distance | 6% T5 | 12% T5 | 14% T5 |
|---|---|---|---|
| 12 in (30 cm) | 4.5 | 8.5 | 10.5 |
| 15 in (38 cm) | 3.6 | 6.5 | 7.0 |
| 20 in (50 cm) | 2.5 | 3.8 | 4.6 |
| 24 in (60 cm) | 1.2 | 2.4 | 3.2 |
Arcadia sets a minimum of 10 inches (25 cm) from lamp to reptile for these kits. VCA gives a general range of about 9 to 15 inches for UV lights, and the MSD Veterinary Manual says within 18 inches (45 cm) of the basking or resting area. The right answer for your setup is the distance that gives your species’ target UVI, which is why the maker’s table and a meter beat any single number.
Mesh blocks UVB. Arcadia says a mesh lid blocks about 30% of the UV, and more with fine stainless mesh. A lamp over mesh needs to be stronger, or the basking spot needs to sit closer to it.
Glass and plastic block it completely. VCA says there must be no glass or plastic between the lamp and the animal. A UVB lamp shining through a glass lid or a window does almost nothing.
When to replace the lamp
UVB output fades long before the visible light does, so a lamp that still looks bright may be giving very little UVB.
- T5 tubes: every 12 months (ReptiFiles).
- T8 tubes: every 6 months (ReptiFiles).
- Mercury vapor: every 6 to 12 months depending on the maker (ReptiFiles).
- VCA: every 9 to 12 months or as the manufacturer directs.
These overlap. Follow your lamp maker’s schedule, write the date on the fitting, and if you own a meter, check the reading every few months.
Day length
Lights belong on a plug-in timer so the day is the same length every day.
- The RSPCA uses a simple 12 hours on, 12 hours off for royal pythons, on a plug-in timer.
- The MSD Veterinary Manual says tropical and subtropical species get about 10 hours of light in winter and 14 in summer, and temperate species about 8 in winter and 16 in summer. It adds that pets not being bred can stay on the same cycle all year.
At night, all lights go off. Colored “night” bulbs still give off light; if the room gets too cold, add a heater that gives no light. See our heating and thermostats guide.
UVB and supplements work together
UVB and calcium powder are a pair. Insect eaters with good UVB are usually dusted with calcium without D3, while those kept without UVB usually need calcium with D3. The feeder insects and supplements guide covers the schedules.
Quick checks before you switch on
- Is anything between lamp and animal? Glass and plastic must go. Mesh is fine if you allowed for it.
- Is the tube on the warm side? Basking heat and UVB belong together, the way the sun gives both at once.
- Is there real shade? The cool end should get little or no UVB, so the animal can step out of it.
- Can the animal climb too close? Keep branches and rocks low enough that the reptile’s back stays at least the maker’s minimum distance from the lamp.
- Is the date written down? Note when the tube went in, so you know when to change it.
By species
- Leopard gecko (zone 1): low output T5 over the warm end, with plenty of shade. See leopard gecko lighting and heating.
- Bearded dragon (zones 3 to 4): a strong 10 to 14% T5 HO tube about half the enclosure length. See bearded dragon lighting and heating.
- Ball python: a low output tube over the warm side, UVI about 1.0 at the basking spot falling to zero (RSPCA). See ball python lighting and heating.
- Crested gecko (zone 1): a low output 5 to 7% T5 tube with plenty of shade. See crested gecko lighting and heating.
- Corn snake: low-level UVB over the warm side, as the RSPCA and RVC advise. See corn snake lighting and heating.
- Veiled chameleon: a 5 to 6% T5 HO tube, aiming for a UVI of about 2.5 to 3 at the basking branch. See veiled chameleon lighting and heating.
See a reptile vet if your animal has a soft jaw, swollen limbs, trembling or trouble walking. This page is general guidance, not veterinary advice.
UVB is invisible, so you cannot judge it by eye. Pick the zone, hang the tube at the maker’s distance, put shade under the rest of the enclosure, and change the tube on its birthday.