Lizard UVB Lighting: Lamp Type, Distance, Mesh and Replacement

Quick answer

Dusting food with calcium does not prevent metabolic bone disease. Calcium is absorbed from the gut only in the presence of active vitamin D3, and most pet lizards make that vitamin in their own skin using ultraviolet B light. Without usable UVB, the calcium goes straight through the animal.

That makes the lamp, its distance from the basking site, and what sits between the two the most consequential equipment decision in lizard keeping. It is also the one most often got wrong, because a UVB lamp that has stopped producing useful output still looks exactly the same.

• Write the installation date on the lamp fitting. UVB output falls long before the light goes out.
• Distance matters as much as the lamp. Output drops sharply as the animal sits further away.
• Standard fine mesh blocks a large proportion of UVB. Glass and most plastics block essentially all of it.
• Put the UVB and the heat in the same place, so the animal gets both while basking.
• The lizard must be able to get out of the UV. A shaded retreat is part of the lighting design, not an optional extra.

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A lizard sunbathing behind a window gets no vitamin D3 at all.

Ordinary window glass blocks the ultraviolet B wavelengths that drive vitamin D3 synthesis. So do most plastics and acrylics. A lizard basking in a sunny window is getting warmth and visible light and nothing else, and owners who rely on it produce animals with advanced metabolic bone disease while believing they are providing natural sunlight.
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Decide in 60 seconds

How the whole system works

Vitamin D3 in a lizard usually starts in the skin. Ultraviolet B light converts a cholesterol precursor into previtamin D3, which converts to vitamin D3. That is then modified in the liver and again in the kidneys into the active hormone.

The active form is what tells the intestine to absorb calcium. Without it, dietary calcium is largely unavailable no matter how much is dusted onto the food.

When blood calcium falls, the parathyroid glands respond by pulling calcium out of the skeleton to keep the blood level stable. That is the process that produces metabolic bone disease: soft, weakened, deformed bone in an animal that has been eating calcium the whole time.

Two implications follow. First, calcium powder is not a substitute for lighting. Second, an animal can look perfectly well for months while its skeleton is being dismantled, because blood calcium is maintained until late.

Ultraviolet A matters too, at longer wavelengths. Many lizards see into the ultraviolet A range, and it influences colour perception, activity, feeding and reproductive behaviour. It comes from the same lamps and it is a reason a lizard under good lighting is often more active and better coloured.

Which lizards need how much

Species differ enormously, because they occupy different light environments in the wild.

Open-country diurnal baskers. Bearded dragons, uromastyx, chuckwallas, many agamids and iguanids. These animals sit in full desert or savannah sun and need high output.

Partial-sun and forest-edge diurnal species. Many day geckos, some skinks, water dragons and anoles. Moderate output, with plenty of shade to move into.

Shade dwellers. Species that live under cover and take filtered light need much less, and too much is a real hazard for them.

Crepuscular and nocturnal species. Leopard geckos, crested geckos and similar. These were long assumed to need no UVB at all. Current thinking supports providing low-level UVB as a benefit rather than a requirement, with plenty of shade and hides so the animal chooses its own exposure.

Reptile lighting is commonly specified using a published zone system that groups species by how much sun they seek in the wild, and lamp manufacturers give recommended mounting distances by zone. Find your species' zone, then follow the manufacturer's distance figures for the specific lamp you buy. Do not transfer distances between lamp models or brands.

If you are not sure, a reptile-experienced vet or a specialist supplier can tell you which zone your species belongs in.

Lamp types, and what each one is good for

Linear fluorescent tubes.

The most reliable general solution. They give broad, even coverage along the length of the enclosure, which lets the animal move in and out of the light rather than being forced to sit under a single spot. High-output tubes reach further than standard ones.

A reflector matters. Without one, a large proportion of the output goes into the ceiling of the enclosure. With one, both the intensity below the tube and the effective coverage improve considerably.

The tube should span a good part of the enclosure length, not a small section at one end.

Compact and coil fluorescent lamps.

Screw-in lamps with a small, folded tube. They produce a narrow, intense beam over a small area, which means the animal is either in it fully or not at all. Historically some of these lamps were associated with eye inflammation in reptiles. They are not a good primary source for anything but the smallest enclosures.

Mercury vapour lamps.

Produce heat, visible light, ultraviolet A and ultraviolet B from a single unit, which is efficient and mimics the sun's combination of heat and UV in one place.

The important caveat is that they cannot be run through a dimming thermostat. Reducing the power either destroys the lamp or stops the ultraviolet output. That means temperature has to be controlled by changing the lamp height or wattage, or by using a separate heat source and lamp guard arrangement, rather than by dimming.

Output close to the lamp can be very high, so mounting distance is critical.

Metal halide lamps.

Similar combination of heat, high visible output and ultraviolet, with the same requirement that they are not dimmed. Bright and efficient for large or tall enclosures.

Newer LED-based ultraviolet products.

An evolving area. Judge these on the manufacturer's published output and distance data for the specific product, and be cautious about anything that does not provide it.

The three things that destroy a good lamp's output

Distance.

Ultraviolet intensity falls sharply as distance increases. A lamp that is correct at one height is inadequate at twice that height. The measurement that matters is from the lamp to the point where the lizard's back sits while basking, not to the floor of the enclosure.

If the numbers do not work, the usual fix is to raise the basking site rather than to buy a stronger lamp.

Mesh.

Fine metal mesh screens absorb a large proportion of the ultraviolet passing through them, and the finer the mesh the greater the loss. Coarser mesh with larger apertures loses less.

Two options. Mount the lamp above the mesh and compensate by reducing the distance according to the manufacturer's mesh guidance. Or mount the lamp inside the enclosure with a guard so the animal cannot touch it, which removes the mesh loss entirely but changes the safe distance considerably.

Age.

This is the failure that catches most keepers. Ultraviolet output declines steadily from the day a lamp is switched on, and it falls to useless levels while the lamp continues to produce perfectly normal visible light. There is no way to see it.

Manufacturers publish a replacement interval for each lamp. Follow it, and write the installation date directly onto the fitting or the lamp end with a marker.

The only way to actually know is to measure. A handheld ultraviolet index meter designed for reptile keeping is the single best purchase a serious keeper makes, because it answers the distance question, the mesh question and the age question at once.

Put the heat and the UV in the same place

In the wild, a basking lizard gets warmth and ultraviolet from the same source at the same moment, and it regulates both by moving in and out of the sun.

If the heat lamp is at one end of the enclosure and the UVB tube at the other, the animal will go where it is warm. It will thermoregulate correctly and receive very little ultraviolet.

Position the ultraviolet source over the basking site so that the animal receives both together. Then provide a shaded, cooler area at the other end that is out of the ultraviolet as well as out of the heat.

That shaded area is not a luxury. Overexposure is a real problem, and the correct arrangement is a gradient the lizard can choose from rather than a fixed dose it cannot escape.

Clear, open, symmetrical eyes. Squinting, held-shut or swollen eyes under a lamp point at excess or unsuitable ultraviolet.

What failure looks like, in both directions

Too little ultraviolet, over months.

Early: reduced appetite, lethargy, muscle tremors or fine twitching, reluctance to climb, difficulty lifting the body clear of the ground when walking, and in juveniles, slow or uneven growth.

Established: swollen or bowed limbs, a jaw that feels soft or rubbery, a lower jaw that no longer meets the upper, a kinked or curved spine, difficulty eating, constipation.

Late: fractures from normal activity, tetany and seizures, hind limb paralysis, and death.

Gravid females and rapidly growing juveniles are at the greatest risk because their calcium demand is highest.

Too much or the wrong wavelengths.

Squinting, holding the eyes closed, swelling of the eyelids, avoiding the basking site, rubbing the face, and in severe cases skin redness. Eye signs typically appear within a day or two of a new lamp being installed too close, or of an unsuitable lamp type being used.

If a lizard develops eye signs after a lighting change, switch the lamp off and get veterinary advice rather than waiting to see whether it settles.

Diet, and how it interacts

Calcium supplements come with and without added vitamin D3, and the choice depends on the lighting.

An animal with good ultraviolet provision generally makes its own D3 and needs calcium without D3 for routine dusting. Adding oral D3 on top of good lighting risks oversupply.

An animal with no ultraviolet provision depends entirely on dietary D3, and getting that balance right is difficult. Too little and the animal develops metabolic bone disease anyway; too much causes hypervitaminosis D, in which calcium is deposited in soft tissues including the kidneys and blood vessels. That is a serious and sometimes irreversible condition.

The safe position for almost all diurnal species is to provide correct ultraviolet lighting and use dietary supplementation as directed by a reptile vet, rather than trying to substitute one for the other.

The calcium to phosphorus ratio of the diet matters as well. Many feeder insects and many vegetables commonly fed are naturally high in phosphorus, which works against calcium absorption. That is a separate topic worth discussing with a vet for your specific species.

Natural sunlight

Real sun is the best ultraviolet source there is, and it comes with conditions.

It must be direct. Glass, acrylic and most plastics block ultraviolet B almost entirely, so windows and clear-lidded enclosures indoors provide nothing.

It must be temperature-appropriate. An outdoor enclosure in direct sun becomes lethally hot very quickly, and a lizard that cannot move into deep shade will die. Shade must be available at every hour.

It must be secure. Escape and predation are both real, and a bird of prey or a cat does not need to get into an enclosure to cause a fatal panic.

Latitude and season change the available ultraviolet enormously. Winter sun at high latitude provides very little even on a bright day.

Used properly, occasional outdoor time is excellent. It is a supplement to indoor lighting rather than a replacement for it.

What never to do

Do not rely on a window, a conservatory or a clear plastic lid for ultraviolet.

Do not run a mercury vapour or metal halide lamp through a dimming thermostat.

Do not leave a lamp in place past its replacement interval because it still lights up.

Do not use a compact coil lamp as the only ultraviolet source for a basking species.

Do not mount a lamp inside an enclosure without a guard. Burns from direct contact are severe.

Do not build an enclosure with no shaded retreat.

Do not assume calcium dusting compensates for missing lighting.

Do not add oral vitamin D3 on top of good lighting without veterinary advice.

Do not transfer mounting distances between different lamp models. Use the figures for the exact lamp you own.

Do not ignore squinting after a lighting change.

What the vet will want to know

When to get help

Book a reptile-experienced vet within the week for tremors, twitching, reluctance to move, difficulty lifting the body when walking, a soft or asymmetric jaw, bowed limbs, or slow growth in a juvenile.

Go urgently for a lizard with a fracture, a seizure, hind limb weakness, or an inability to right itself.

Turn the lamp off and get advice the same week for any lizard that starts squinting or holding its eyes closed after a lighting change.

Bright daylight through a window looks like sunbathing and provides no ultraviolet B at all. The lamp inside the enclosure is what does the work.

Take photographs of the lamp label and the enclosure. In lizard medicine, the diagnosis of metabolic bone disease is usually made from the animal and the cause is usually found on the lamp.