3.1The spectrum in one page
Everything a lamp emits sits somewhere on this scale. What the sun delivers and what our equipment delivers are very different mixes.
| Band | Wavelength | What it does |
|---|---|---|
| UVB | 280–320 nm | Drives vitamin D3 synthesis. Essential. |
| UVA2 | ~320–400 nm | Essential for some metabolic processes. |
| UVA1 | ~320–400 nm | A color reptiles see and we don’t. Drives behavior, not metabolism. |
| Visible | 400–780 nm | Brightness. Circadian rhythm, mood, appetite, activity. |
| IR-A | 780–1,400 nm | The strongest, deepest infrared. This is basking heat. Aids D3 synthesis and digestion. |
| IR-B | 1,400–3,000 nm | Intermediate. What deep heat projectors produce. |
| IR-C | 3,000–10,000 nm | Weakest, surface only. What CHEs and heat pads produce. |
Two sub-ranges are worth knowing. D3 synthesis happens between 295–315 nm, peaking at 297 nm. A second band at 310–335 nm limits D3 production, which is the body’s own brake and the reason sunlight can’t overdose a dragon.
A wavelength scale from 280 nanometres to 10,000 nanometres divided into six bands: UVB, UVA, visible light, and infrared A, B and C. The vitamin D3 synthesis window sits at 295 to 315 nanometres inside the UVB band, peaking at 297. Each infrared band is annotated with the equipment that produces it.
The sun against our equipment
| Source | UV | Visible | IR-A | IR-B | IR-C |
|---|---|---|---|---|---|
| Sunlight | 8% | 44% | 32% | 14% | 2% |
| Tungsten-halogen | none | 11% | 38% | 39% | 12% |
| Deep heat projector | none | none | 8% | 48% | 44% |
| Ceramic heat emitter | none | none | none | 1% | 99% |
Read that against the band table and the conclusions fall out. Halogen is the closest thing we have to the sun’s basking warmth, because it’s the only one rich in IR-A. A ceramic heat emitter is 99% IR-C, which warms air and skin surface and delivers essentially none of the deep infrared a basking dragon needs. That’s why a CHE is a night-time air temperature tool and never a primary heat source.
3.2Why indoor light has to be so much stronger than it looks
There’s a photograph worth describing here, because it changes how people think about their setup better than any number.
The same vivarium, an a 39W 12% T5 HO tube and a 100W halogen flood, photographed twice. In the first the camera exposes for the enclosure and it looks bright and well lit. In the second the camera settings are left as they were outdoors, and the same enclosure is nearly black.
Both are around 4,000 lux. Your eyes adapt and the camera doesn’t, and the reptile brain is calibrated to the outdoors.
| Light source | Brightness |
|---|---|
| Natural sunlight, full sun | up to 120,000 lux |
| Natural sunlight, in shade | ~60,000 lux |
| Typical basking bulb | ~4,000 lux |
| T5 and heat lamp alone | ~2,500 lux |
| Target for the enclosure | 30,000–60,000 lux |
The shade figure matters as much as the full-sun one: even out of direct sun, the wild is roughly fifteen times brighter than a typical vivarium, which is why the whole enclosure (not just the basking spot) wants to be bright. You can’t get there with a heat lamp. You get there by adding LEDs, covered in 3.5.
3.3UVB
Without adequate UV a dragon can’t carry out certain metabolic processes, and it develops metabolic bone disease. Bone density can be rebuilt with treatment, but any deformity that has already set is permanent, so every week it goes unnoticed costs the animal something it doesn’t get back. Chapter 5 covers the calcium mechanism behind it.
Several meters read the same scale and any of them will do the job. Without one, your equipment choices narrow, because you’re relying on published distances being right for your exact bulb and fixture; and as the table below shows, that varies more than people expect.
Why 4–5 and not more
Different species bask at different points along the sun’s daily arc, from the weak first light a gecko emerges into up to the fierce midday sun a uromastyx tolerates. On that scale a bearded dragon is a late-morning basker, the strong, open sun of scrubland, but not the desert-noon extreme. That is where the UVI target of 4–5 comes from: it’s the strength of the sun the animal evolved under at the time of day it chooses to bask. Aiming much higher isn’t “more of a good thing,” it’s a different climate than the one the dragon is built for.
What produces usable UVB, and what doesn’t
UVB for a dragon comes from a linear T5 HO fluorescent tube. The format matters for a physical reason: UVB has to reach the animal across the whole width of its body, and a long tube produces a broad band of it rather than a point.
| Property | Why it matters |
|---|---|
| Linear T5 HO tube | Produces UV along its length, so the whole animal is covered. Compact coils and screw-in bulbs concentrate output into a small area, which is why they cover a fraction of the dragon. |
| Tube strength | Quoted as 12% or 10.0 for the stronger option, 6% or 5.0 for the weaker. Two labeling conventions describing the same thing, 12% and 10.0 are equivalent, as are 6% and 5.0. |
| Tube length | The middle of the tube is the strongest point and has to sit over the basking spot, with tube to spare either side. In a 4 ft enclosure that generally means a 22–24″ tube. |
| Reflector | Roughly doubles the UV that reaches the animal instead of being lost upward. This is why mounting distance varies so much between fixtures. |
| Age | UV output falls steadily while the tube still looks perfectly bright. Visible light is no indication of UV. See 13.4. |
Mounting distances
Measured from the tube to the animal’s back, with the middle of the tube over the basking spot. Three things decide the distance, and leaving any of them out gives you a number that is wrong by inches: the fixture, the tube strength, and what the light passes through on the way down.
The fixture matters more than people expect. An Arcadia ProT5 or Vivarium Electronics unit puts a deep reflector behind the tube and throws far more of the output downward than a Zoo Med hood does, so it has to sit further away to land on the same reading. Same tube, same enclosure, four inches of difference.
| Screen | Fixture | 12% / 10.0 tube | 6% / 5.0 tube |
|---|---|---|---|
| None, tube inside | Arcadia ProT5 12% / 14% · Vivarium Electronics | 16–18 in 40–46 cm | 10–12 in 25–30 cm |
| Zoo Med ReptiSun T5 HO hood | 12–14 in 30–35 cm | 8–9 in 20–23 cm | |
| About 35% block | Arcadia ProT5 / Vivarium Electronics | 12–14 in 30–35 cm | 8–10 in 20–25 cm |
| Zoo Med T5 HO hood | 10–12 in 25–30 cm | not suitable | |
| About 45% block | Arcadia ProT5 / Vivarium Electronics | 10–14 in 25–35 cm | not suitable |
| Zoo Med T5 HO hood | 8–10 in 20–25 cm | not suitable |
Two elevations, both for an Arcadia ProT5 fixture with a 12 percent tube. On the left the tube is mounted inside the enclosure, 16 to 20 inches above the dragon's back. On the right it sits on a screen top blocking about 35 percent of the UV, so the distance closes to 12 to 14 inches.
Or let a tool do this for you
The table above is the arithmetic. If you would rather not do it by hand, the free Reptile Lighting Tool at lightmyreptile.com does the whole calculation live: enter your enclosure size, pick your actual fixture and tube from a tested catalogue, set your mesh and dome offset, and it reads back the UVI, W/m² and lux at the animal’s back. It uses the same targets and the same measured data this chapter does.
It is the fastest way to sanity-check a setup before you spend money on it, and the only practical way to see how moving a lamp two inches changes every reading at once. Built by Nathan Burton, whose work we know and trust. See 3.9.
What the mesh and the fixture take away
Anything between the tube and the animal costs you output, and it costs the same fraction of UVB, heat and visible light alike. A standard screen top blocks roughly 35%, which is why the weaker tube usually isn’t worth using over one. Some brands block more, Zen Habitats screens are nearer 45%; so if you know your enclosure’s figure, use it. A wire lamp guard around the bulb takes its own share, around 35% again; guards aren’t needed for bearded dragons, though they matter for climbing species.
You do not have to guess at your own screen. Measured figures exist for most of the mesh sold with vivariums, and the spread is wide enough to matter: the difference between a 33% screen and a 60% one is several inches of mounting height.
| Blocks | Screen |
|---|---|
| 12% | ½″ chicken wire welded mesh |
| 12–17% | Welded wire hardware cloth, depending on wire thickness |
| 23% | Custom Reptile Habitats; Tetrafauna; egg-crate panel |
| 25% | ½″ galvanised mesh |
| 30–32% | Imagitarium; XL ReptiBreeze; Zilla Fresh Air |
| 33–35% | Thrive habitat screen cover; Dubia.com wide mesh (2024); Zoo Med; Exo Terra; Focus Habitats; Ecoflex Mojave; Zen Habitats 3.0 |
| 37% | Vision Cage |
| 43–45% | Dubia.com (2022); Swell; Atasuki; Carolina Custom Cages; Reptile One type 2; ReptiZoo; Zen Cage; Symton BSF (Nov 2021 version) |
| 50–55% | National Geographic; Apogee Reptarium; T-Rex; Reptile One type 1 |
| 60% | Symton BSF (first version); Thrive standard mesh as sold with the vivarium |
| 70% | Perforated metal panel in an aluminium-framed vivarium |
A wire lamp guard around the bulb takes its own share, around 35% again; guards aren’t needed for bearded dragons, though they matter for climbing species.
Whatever your screen, the distance table above already accounts for it. Set the tube and platform heights to land in the right band, then confirm with a meter if you have one. A reading at the animal’s back beats any published figure, including these.
3.4Heat: power density, not surface temperature
This is the biggest shift in modern bearded dragon husbandry and it contradicts advice that’s still everywhere.
Basking bulbs (heat lamps, incandescent lamps, halogens) produce mostly IR-A, the wavelength that penetrates deepest, drives core body temperature, aids digestion and contributes to D3 synthesis.
IR-A comes from a hot filament. Incandescent and halogen lamps produce it because something inside them is glowing at a few thousand degrees. LEDs don’t; they are efficient precisely because they don’t waste energy as heat, which makes them useless as a basking source however bright they are.
Beam shape matters too. A flood spreads warmth across the whole animal; a narrow spot heats a patch of its back while the rest of it stays cold.
Wattage is not a specification you can look up. The same lamp at two different distances delivers completely different irradiance, so what works depends on your enclosure height and where the fixture sits. This is why the target below is stated in W/m² at the animal’s back rather than in watts at the bulb.
One bulb or two
A single basking bulb works, but in a 4-ft enclosure it tends to heat a bearded dragon unevenly, warm over the shoulders, cooler at the hips and tail. Two bulbs set side by side, a hand’s width or so apart, give a broader, more even basking zone from nose to tail. Where their beams overlap the infrared adds together, so two lower-wattage bulbs can reach the same power density as one hot one while covering more of the animal. If you go this route, measure the overlap zone; that’s where the dragon will sit, and where the combined figure has to land in range.
One more bit of geometry: a basking bulb almost never sits flush on the screen. How far the dome’s face floats above the mesh, the dome offset, changes the irradiance reaching the animal. A couple of centimeters is normal and fine; just remember it’s part of the distance, and re-measure if you move the fixture.
Why the temperature gun lies
Every material heats differently under identical infrared. Two spots receiving the same IR-A, verified with a power density meter, read completely differently on a gun.
| Surface | Gun reading | Infrared received |
|---|---|---|
| White half of test disc | 90.8°F | identical |
| Black half of same disc | 107.0°F | identical |
| Cork bark, correct infrared | 120–140°F | feels cool to touch |
| Light pavers, correct infrared | 90–100°F | suitable |
A test disc, half pale and half black, under one lamp. A temperature gun reads 90.8 Fahrenheit on the pale half and 107 on the black half, although a power meter confirms both halves receive the same infrared.
The basking surface temperature tells us practically nothing useful about the quality of the basking area. Every material heats differently whereas the dragon will receive the same infrared radiation hitting their back. Disregard anyone telling you that your basking surface needs to be a specific temperature; this has since become an outdated method that is ineffective. What matters is the wavelength and power density reaching your dragon’s body while basking.
Aaron Carter · Admin team, Reptile Lighting
Surface temperature and power density are not the same. One does not replace the other. They live on the same street, but not in the same house.
Thomas Griffiths · Admin, Reptile Lighting
There is a way to make the gun useful. Leave a digital thermometer face up on the basking spot for an hour, then compare its settled reading to the gun. In the test above, a gun reading of 90°F on the pale surface corresponded to roughly 108°F of delivered heat. Once you know your own offset the gun becomes a decent relative check.
The number that matters
Measured at the level of the dragon’s back. Power density doesn’t correlate with surface temperature or ambient temperature. It’s governed by distance from the lamp.
If you don’t own a power meter
Dr. Frances Baines of Reptile Lighting gives a hand test that works surprisingly well.
- 1.Hold your hand over the basking platform at the height of your dragon’s back and feel the warmth on the back of your hand, from above. It should be a gentle, evenly spread warmth you could sit under for several minutes without discomfort.
- 2.Then rest the back of your hand directly on the platform. It should be comfortable to hold there for a full minute.
If the lamp passes both, irradiance is very likely in the 250–350 W/m² IR-A range, because that’s what sunlight feels like on human skin.
There’s a slower backstop worth running once, when you first set up or change a lamp: leave everything on at full power for 3 hours, then read the basking surface with a temperature gun. This isn’t the primary measurement (surface temperature still isn’t what you’re tuning for) but it catches a spot that heats up dangerously over time, which a thirty-second hand test can miss.
Reading the dragon
Bearded dragons are biphasic baskers. They bask a few hours a day in two main phases, first thing when the lights come on, then again after eating, sometimes once more before lights out. The rest of the day is for exploring, digging and hunting.
| What you see | What it means |
|---|---|
| Basking all day | Not enough infrared. Or the enclosure is too small and there’s nothing else to do. |
| Avoiding the basking area | Too much infrared. |
| Two or three sessions, exploring between | Correct. |
Mounting over mesh
Mesh blocks infrared as well as UV, so a bulb chosen for an open enclosure will under-deliver through a screen. Correct for it like this.
- 1.Measure from the mesh to the animal’s back when basking. With a deep dome, measure from the bulb itself. Say 12 inches.
- 2.Find your mesh blockage, a standard screen is about 35%, so 0.65 passes through.
- 3.Divide the target range by what passes through: 300 ÷ 0.65 = 462 and 350 ÷ 0.65 = 538.
- 4.Find a bulb on an irradiance chart delivering 460–540 W/m² at 12 inches.
These are estimates and you may still need to adjust distance or wattage, but they get you close. If you have no UV index meter, set your basking distance from the UVB requirement first, then solve for the heat lamp.
3.5LED and visible light
Not life or death, but the most visible quality-of-life improvement you can make. Keepers consistently report changes in color and activity within weeks.
Adding suitable LED provides stimulation to a dragon’s pineal gland, releases hormones such as serotonin, melatonin, dopamine; resulting in a happier healthier, more active dragon. It also increases energy, appetite, and brightens skin pigments so it will actually make most lizards physically brighter and have more vivid colors.
Thomas Griffiths · husbandry educator, Moderator at Reptile Lighting
Why most LEDs look wrong to a dragon
Bearded dragons are tetrachromatic. Four cone types against our three, and stimulating that fourth cone needs UVA1.
Most of the bright panels people use are horticultural grow lights. They produce plenty of brightness, but they were designed around what plants photosynthesise rather than what a reptile sees, so to a dragon they read yellowish rather than white.
A small number of reptile LEDs specify UVA1 output. Those are the ones that look genuinely white and closer to midday sun, because they reach that fourth cone. If a product doesn’t mention UVA1 anywhere, assume it doesn’t produce it.
What the light needs to deliver, either way:
- , Full spectrum, high CRI, daylight color temperature, around 6,500 K.
- , Bright enough to matter, enough output to lift the basking area to 30,000–60,000 lux, which is what the lux meter is for.
- , UVA1, if the fourth cone is to be stimulated at all. It is the difference between light a dragon reads as daylight and light it reads as yellow.
Metal halide is the other route to true white light and gives strong UVA. Many metal halides also emit UVB, which means treating them as part of your UV provision rather than as a separate light, check them with a UV meter, and re-check as the UVB decays.
3.6Placement
The geometry matters as much as the equipment.
A side elevation of an enclosure. The UVB tube, the basking lamp and the LED all point at the same basking surface, and their beams overlap where the animal sits. A cool end sits at the far side with the thermometer.
- , The middle of the UVB tube goes over the basking spot. The center is the strongest point, it’s where every distance figure is measured from, and it’s where the dragon needs to sit.
- , The basking bulb sits right next to the middle of the UVB, shining on the same spot.
- , The dragon must be able to sit under both at once. If it has to choose between heat and UV, the setup is wrong.
- , LEDs overlap the basking surface. You can run them on the cool side too, but the basking surface has to be the brightest place in the enclosure. If you have a UVA1 LED, that is the one that goes over the basking spot.
3.7Temperatures, day and night
Night
All lights on during the day, all lights off at night. Only add heat if the enclosure drops below 55°F / 13°C, and then use a mounted ceramic heat emitter or a deep heat projector over mesh.
If it stays above that, use nothing. Dragons need the night-time cool-down to give their metabolism a break and grow properly. Pick a CHE wattage sized to your enclosure and don’t over-do it, the cool-down is the point.
Never use colored bulbs, including red and infrared night bulbs.
Measuring properly
Three rules people break constantly:
- , Don’t use pet-store dial gauges for temperature or humidity.
- , Don’t use a temperature gun to measure air temperature.
- , Don’t use a digital thermometer to measure the basking surface.
The digital thermometer goes on the cool side, away from the basking bulb, close to the floor. It reads air temperature and humidity. A hygrometer belongs on the shade side too; there’s no need for one at the basking end.
Humidity
Aim for an average of 30–60% relative humidity, measured on the cool side. The basking end will sit far lower, sometimes down around 10%, and that gradient is correct rather than a fault.
The idea that a bearded dragon enclosure should be as dry as possible is one of the more damaging things still circulating. Central Australia isn’t bone dry. Humidity in their native range runs 55–65% at dawn and dusk, drops to 20–30% at midday, and climbs to 75–80% overnight, with burrows and bush bases higher still.[14] Most centrally heated homes are drier than that.
Keeping an enclosure at 15% produces stuck shed and constipation in the same animal, and Chapters 8 to 10 are largely about the consequences.
3.8Thermostats
Thermostats are of limited use for a basking animal and are widely misunderstood. They can really only stop your cool end getting dangerously hot. You still have to pick the right wattage lamp yourself, because you don’t want a thermostat constantly dimming the bulb.
Put the probe on the cool end at roughly the height the animal spends its time, and set the maximum to 83°F / 28°C. If your climate control fails or a heat wave arrives, that keeps the cool side survivable.
3.9A tool that does the maths for you
Everything in this chapter (UVI, power density, lux, mesh loss, dome offset, mounting distance) can be worked out by hand, and the sections above show how. There is also a free tool that does all of it live, and for most keepers it is the better starting point.
The Reptile Lighting Tool at lightmyreptile.com lets you set your enclosure size, pick your actual bulbs from a tested catalogue, set the mesh and dome offset, and read back the UVI, W/m² and lux at the animal’s back, using the same targets this chapter does. It’s built by the same lighting community our numbers come from (it places a bearded dragon at the “late-morning basker” level described in 3.3), and it’s an excellent way to sanity-check a setup before you buy, or to see how moving a lamp changes the readings.