Enclosure Concepts

UVB Lighting For Your Reptiles

So What's UVB and Why Do I Need It?

Something that seems to be one of the biggest hurdles for newer keepers or keepers that are trying to up their husbandry is the use of UVB. There’s a ton of types of bulbs, tons of info, percentages, distances, etc. It’s a lot to take in. We’ll do a bit of a deep dive here and talk about each of these things. The first step is knowing what UVB is and what it does. Then we’ll hop into the different products that are out there and available. Then we’ll take that knowledge and apply it to use. 

Anyone can tell you that sunlight and reptiles just go together. The sun is a source of light and heat. But it’s actually a bit more than that. The sun outputs UVA, UVB, UVC, visible light, and heat. Heat will be covered in a different article but heat is made up of IR-A, IR-B, and IR-C. Light is measured in nanometers. UVA, for example, is from 315nm to 400nm. UVB is in the range of 290nm to 315nm. UVC is in the range of around 180nm to 290nm. Once you get over 400nm you’re in the range of visible light. The wavelength of light over 400nm is actually what color it is. For example blue light is between 380nm to 500nm. Here’s a picture from Dr. Frances M. Baines that shows the whole spectrum.

So now you’ve got a little science lesson. You’re probably saying, “Cool, how does this matter to my reptiles?”. So obviously we need visible light, we need a good day/night cycle and we like looking at our reptiles. Why do I need UVB? UVB is what starts the whole process in which your reptile utilizes calcium. I’ll explain this process in more of an overview. This article would be 10 miles longer if we did a true deep dive. 

The process in which a reptile utilizes calcium requires UVB and heat. In the past we didn’t have a reliable way of providing the UVB portion of that equation. These days we do. So lets use a bearded dragon as an example. The bearded dragon is out basking in the sunlight, while this is happening there’s many processes at play. The beardie is obviously raising it’s body temperature in order to stay at a proper temperature for survival and to digest it’s food. But the UVB from the sunlight is also converting cholesterol in the skin into what’s called Pre-D3. When the reptile is at it’s optimal temperature this is converted in Vitamin D3. A reptile can never produce too much D3 in it’s natural environment. Higher levels of UVB and UVA handle this by breaking Pre-D3 and D3 down into harmless byproducts. This Vitamin D3 then binds to the calcium from the beardie’s food source and carries it where it needs to go and aids in the absorption. Since we can manufacture Vitamin D3 we’ve taken to adding it to our calcium powders and dusting our feeders with it. This works, and has worked for a long time. But there’s a downside. Too little and we’ve essentially done nothing, too much and it can be toxic to the reptile. Instead of letting the natural processes of the reptile handle this, we’re artificially handling it and humans are not perfect. Here’s a graphical representation for the visual folks.

uvb pathway for reptiles

Now we’ve got a good handle on what UVB is and how it works in regards to our reptiles. Let’s talk about how it’s measured. In the natural world you’ll see UVB measured with the UV Index system. This is a linear system that goes from 0-10. 0 being no UV radiation. So zero is basically night time. Meanwhile 10 is a mid-day summer day with a clear sky. Knowing this info is great if you’re planning a natural enclosure and need to know what levels of UV your reptile is normally exposed to in it’s natural habitat. For Herpetology uses there’s something even more in-depth created by Dr. Gary Ferguson. He created the Ferguson Zones. In the sources section I’ll link to a tool created by Dr. Frances Baines call UV-Tool. The Ferguson Zones are split into 4 zones numbered 1-4. Zone 1 is for crepuscular species and shade dwellers. Zone 2 is partial sun and species that occasionally bask. Zone 3 is reptiles that are out in the open or species that occasionally bask. Zone 4 is for species that are out basking in the midday sun. Each zone relates to specific index range. For example Zone 1 is a UV index of 0.1 to 0.7. This is good information to know about your reptile and I strongly suggest checking out his UV Tool. Here’s a graphic showing some common species in their Ferguson Zones. 

System seems great right? So in a perfect world we’d go to the store and grab a bulb for species in Zone 1 and call it a day, right? Well unfortunately no. Product manufacturers sell their bulbs in percentages. Knowing what percentage you need for what Ferguson zone is where many people struggle. The other struggle is the distance at which the light needs to be placed. A 10% bulb at 30 inches away is going to cater to Zone 1 where as a 5% at 8 inches is going to provide the same amount. The farther away the reptile is from the bulb the less UVB they’ll effectively receive. Here’s a chart from Zoo Med that gives you a visual representation of this. 


At this point we know what UVB is, how it works, and how it’s measured. You should have a pretty decent understanding of what your reptile needs. Choosing the right product to accomplish that is your next big hurdle. There’s tons of different types of bulbs, lengths, and formats. It’s a lot to take in and pretty daunting. We’ll cover the different common form factors here and the use-cases for each. 

Compact Fluorescent Bulbs (CFLs)

If you go to a large chain store and look for UVB bulbs this will most likely be the first thing you find. It uses a standard light socket, is relatively cheap, and you probably already have a dome. Reptile Haven will never endorse this as a good solution. These bulbs are a prime example of a company trying to make a quick buck. However if you’re in a pinch something is better than nothing in some cases. There’s a multitude of problems with these bulbs however. Due to the nature of the bulb and the fixture it’ll be placed in, you will always only have a narrow beam of light. Essentially it’s an absolute waste if your reptile isn’t directly underneath it. This defeats the purpose of emulating sunlight. The other problem is the amount of UVB produced by this style of bulb varies wildly from bulb to bulb. They’ll be intense and over rating in the beginning and the dramatically drop over time. 

T5HO(High Output) Fluorescent Bulbs

This form factor along with T8 style bulbs is the go-to recommendation from Reptile Haven. Brands and Length vary with Arcadia being recognized as the best brand currently. T5HO bulbs are best used when you have a large amount of vertical height and you have to provide a large amount of UVB. An example would be for a bearded dragon in a 4x2x2 enclosure. 

T8 Fluorescent Bulbs

T8 Bulbs are very similar to T5HO bulbs other than the fact they’re a bit thicker and aren’t as powerful. If you’re working in a shorter tank or with tropical species a T8 is sometimes better than a T5HO.

Mercury Vapor/Mercury Halides

Mercury Vapor and Mercury Halide bulbs are commonly confused with a standard halogen bulb. Standard Halogen bulbs do not produce UVB. These bulbs produce large amounts of UVB and heat. These are ideal for desert species and large enclosures. Keep in mind this bulb has the same drawback of CFLs in that the beam is very narrow, so supplemental UVB for the rest of the enclosure will be needed.

Common Mistakes and Misconceptions

So now you should have a pretty good understanding of UVB and what type of bulb you need. Due to the nature of this hobby there’s always tons of push back on newer technologies. Due to this many misconceptions come about, usually from common mistakes. We’ll cover a few them here. There’s many more, but if it’s not listed here feel free to pop into the discord server and ask. 

Mistake or Misconception

Resolution or Truth

  • I don’t need UVB because my animal is nocturnal or crepuscular.
  • Not true. While they aren’t as exposed as a Zone 4 reptile they are exposed to low levels during the day. Some species practice cryptic basking where just a tail or piece is exposed during the day time. Some crepuscular species, like tokays, are known to openly bask even during daytime.
  • UVB will burn their skin or cause eye issues.
  • This one is partially true, however it takes making a mistake for it to be true. If you use a bulb meant for species meant to be in Zone 4 on a species meant to be in Zone 1 then you will see skin damage and possible eye issues. This is why identifying what your reptile needs is important. If you pick the correct strength bulb and have it at the correct distance you will not see these issues.
  • UVB on top of Glass/Tight Mesh
  • UVB cannot go through glass. Many newer keepers make this mistake. Likewise keep in mind that tight mesh will block around 30% of the UVB. 
  • Length of the T5 or T8 Bulb
  • When deciding on a size your coverage should be the entire enclosure or at a minimum 3/4 of the enclosure. So if you have a 4ft long enclosure you should shoot for a 4ft bulb or at the very least a 3ft bulb.
  • More strength is better. 
  • Hopefully after reading this far you know this to be false. A higher strength will be detrimental to the health of the reptile if the output is higher than the ferguson zone of that species. 
  • I’ve bought a bulb, I’m good until it goes out.
  • UVB bulbs degrade over time. They’ll still produce visible light but the amount of UVB produces will decay over time. A good rule of thumb is to write the date on the bulb when you put it in and replace every 6 months. Some bulbs are rated for a year. But we recommend to replace at 6 months to make sure. 
  • It’s really expensive
  • While there is some initial cost, a lot of that is in the purchase of the fixture for the bulb. In reality once you have the fixture you’re only looking at $40-50 a year for your UVB needs. 
  • Only rare/high quality/expensive reptiles need UVB
  • This one is so far from the truth it’s borderline ridiculous. Some of the cheapest species available have the highest needs for UVB exposure. 


UVB is an essential thing for keepers to understand. Without a good understanding of what purpose it serves and why you need it you’ll never be able to make a good informed decision. Research your animals ferguson zone, find a bulb that works for the height and length of your enclosure, then make sure to replace it every 6 months. When typed out like that it doesn’t seem as much of a hurdle anymore right? Once I cannot recommend UV-Tool enough. It’s an invaluable resource for all keepers. It’s linked below in the Sources Section.


UV-Tool by Dr. Frances Baines –Link

UV Wavelength Image – Dr. Frances Baines

How Much UVB Does My Reptile Need – Dr. Frances Baines

Fire — The Sun: Its Use & Replication Within Reptile Keeping by John Courteney-Smith

Evaluating the Physiologic Effects of Short Duration Ultraviolet B Radiation Exposure in Leopard Geckos (Eublepharis maculariusby Amelia Gould

An In-Depth Look At UV Light And Its Proper Use With Reptiles by Dr. Frances Baines

Nutrition and Husbandry Conditions of Palearctic Tortoises(Testuodo spp.) in Captivity by Thomas Bauer

Lighting Chapter from Reptile Medicine and Surgery in Clinical Practice by Dr. Frances Baines

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