Melissa Kaplan's
Herp Care Collection
Last updated January 1, 2014

Catch the Wave

Reptile Lighting

©1994 Melissa Kaplan, News from the North Bay, November/December 1994


Considering that light is something that surrounds us and is such an integral part of our lives, most of us have only the vaguest notion of just what, exactly, it is. While this lack of knowledge causes us no harm in our daily lives, it is harmful when it comes to the health and well-being of our reptiles and amphibians. It is this ignorance which leads many people to buy the wrong kind of bulbs for their herps because they appear to be the same and, usually, are much cheaper than the bulbs sold in pet stores that are made especially to enable herps to metabolize calcium properly.

People frequently ask if it is acceptable to use plant grow lights, or the so-called full-spectrum (actually, wide-spectrum) and neodymium-type incandescent bulbs. As discussed in Incandescent Light Bulbs: Short on Spectrum, No UVB article in Vol 1(9) of this newsletter, many of these lights are being marketed to reptile owners. This article will explain the differences between the spectrum generated by the various types of bulbs, and which bulbs present the best combination of options for most herp owners. LIGHT

Light is energy; specifically, it is waves of discrete energy particles called photons. Like waves in the ocean, where each wave is distinct, one light wave cannot truly be separated from the other waves or from the ocean itself. Such is the case with light waves and the photons which make up the waves.

When you read about wavelengths, the measurements refer to the distance from the peak of one wave to the peak of the following wave - but don't go getting out your ruler just yet. The measurement most often used is the nanometer (nm), with measurements stated in hundreds of nm's. A nanometer is one billionth of a meter; an angstrom (Å) is one-tenth of that, or one ten-billionth of a meter (10-8).

The wave length determines if we see the light, feel the heat caused by it, or are damaged by it. On the short-wave end of the spectrum are found x-rays and those important ultraviolet (UV) rays (less than 3500 Å in length). In the middle of the light spectrum is that small portion that we see, the visible light, which includes the color wavelengths of violet, indigo, blue, green, yellow, orange, and red. Each of these colors is actually made of waves of differing lengths, ranging from 4 x 103 to 7.5 x 103. Towards the longer end of the wavelength spectrum are heat-producing rays (from 7500 Å to over a meter), including the infrared waves (which are produced by infrared lamps and ceramic heating elements).

Chart of wavelengths from The Physics Classroom.






UV rays do not produce heat, but they produce so much energy that they can knock electrons off atoms. When electrons are knocked off, that atom becomes ionized, carrying a positive charge. It then reacts with other atoms around it, trying to replace its missing electrons. Ultraviolet and X-rays are both ionizing forms of radiation, knocking electrons off DNA. This causes chemical changes in the DNA, causing genetic mutations.

We cannot usually see radiation except as a prism effect. The sunlight hitting the prism (or the glass corner of an aquarium) causes the light waves to split, rendering the individual colors of the middle spectrum visible. We are still unable to see wavelengths longer and shorter than that range; humans see wavelengths between 4000 to 7000 angstrom. Reptiles, and many other animals, see in ultraviolet wavelengths, less than 4000 angstrom.

Visible light spectrum from The Physics Classroom.




If you remember back to the time when you first learned about combining colors, you learned, for example, that yellow and blue made green, and that yellow and red made orange. You also found that if you mixed too many colors together you got a muddy brown or black. When all the different lightwaves are combined, instead of creating black or brown, the result becomes colorless, perceived by us as white. In live theater, the stage is illuminated using red, yellow, and blue spotlights which, when the beams of the three lights are focused together, form white light.

While daylight is generally white, the color is affected by atmospheric changes such as smog, dust, and water vapor. That is why the light at dawn is very different from that at noon, or even late afternoon.


Kelvin and CRI
There are two different Kelvins. Degrees Kelvin is used by photographers as a unit of measurement of the color temperature or the degree of combination of blue, red and yellow. Physicists use Kelvin degrees (K) to measure absolute heat temperature.

The Kelvin value, based on degrees Kelvin, is that which resembles the quality (the wavelengths) of light at high noon. The Color Rendering Index (CRI) rates how a light compares to the natural sunlight at high noon; high noon light is assigned a CRI value of 100. No artificial light source attains a CRI of 100. Expensive bulbs, and some bulbs used in herpetoculture and aquaculture, usually (but not always) have the CRI indicated on the package; the higher the CRI, the more closely it resembles the sunlight at noon.


Lumens and Lux
Another measurement is of the intensity (volume) of radiation produced by the light. Think of a 25-watt white incandescent bulb and a 150-watt white incandescent bulb. The CRI they produce is the same, but the intensity of the light (the amount of radiation) they produce is very different.

The intensity of a 100-watt light six inches from the source is very different from its intensity five feet from the source. At six inches, far more light is hitting a tank than the amount of light from the same bulb would if placed five feet away. The light that actually hits the tank is called illuminance and is measured in lumens per square meter of surface. The ratio of lumens to surface is known as lux. In other words, lumens measure the light as it leaves the source, and lux measures the light that actually reaches the target. So, while the lumens emitted by a 100-watt bulb is going to be the same regardless of the bulb's distance from the target, the lux will vary depending upon how far away the target is from the light.

At this point, I can hear you asking "So what?" Well, the next time someone asks you what wattage heat light they should use, you will be able to help them more effectively by discussing exactly how they plan to use the light.


Ultraviolet Light
Ultraviolet light is broken into short- or middlewave (285-315 NM) and long- or nearwave (315-400 NM) which accounts for 90 percent of the solar ultraviolet spectrum.

As with all light, the amount of UV that actually reaches the ground is dependent upon several factors: latitude, atmospheric conditions, the elevation of the sun, and the amount of cloud cover. The greatest amount of UV reaches the surface at high mountain elevations where the air is thin, in the tropics where the rays are most direct, and in the highly reflective deserts.

Exposure to UV in the middlewave 295-320 NM range (UVB) results in the synthesis of D3 in the skin. While excessive exposure to the shorter UVA wavelengths can be dangerous, causing blindness, skin damage, immune dysfunction, and even death, moderate exposure is beneficial. In diurnal reptiles, it increases activity levels, promotes basking and feeding and, through its influence on the pineal gland, can positively affect reproductive cycles in some species.

Mercury vapor lamps are often mentioned as a source of lighting for herps. Studies with desert iguanas (Dipsosaurus dorsalis), collared lizards (Crotophytus collaris), and rainbow lizards (Agama agama) showed dramatic increases in male threat behavior during the time these sunlamps were on. The aggressive behaviors started within two minutes of the lights being turned on. While the physiological mechanisms at play here may be poorly understood, evidence suggests that sensitivity to UV intensity may play a role. Another possibility for the aggression is suggested by the fact that many reptiles can see in the ultraviolet range, and many bear markings which either reflect or absorb UV. For example, certain anoles have UV-receptive cones in their eyes (in the 365 NM range) and have UV reflectance patterns on their dewlaps and at the corners of their mouth, areas exposed during threat displays. Femoral pore secretions of desert iguanas are visible under black light and probably appear as dark markings against the bright background of the desert sand; tongue flicking then provides additional information about the iguana who left the trail of pore secretions. Many desert flowers absorb UV which may help the desert iguanas and tortoises locate food. This may also be part of the appetite promoting mechanism exhibited when reluctant or inappetant feeders are exposed to UV light for a period of time.


The Best Lighting To Use
In decreasing order of UVB output, the best lights are unfiltered sunlight, FS-fluorescent sun lamp (various manufacturers such as National Biological Corp's FS lamps), black-light (BL) fluorescent tubes (not BLB [black-light blue] psychedelic poster/party/Halloween lights), by various manufacturers including General Electric's F20T12-BL (20 watts) and F40 (40 watts), Westinghouse's 20T-12 BL (20 watts) and F40 BL (40 watts), and full-spectrum fluorescent tubes, such as Vita-Lite by Duro-Test, and Repti Sun and Repti Iguana by Zoo Med.

Despite their occasional appearance in herp books and articles, the Chroma 50 and 75 by General Electric, Designer 50 by Sylvania, Color Tone 50 and Phillips Ultralume do not produce sufficient UVB for herp keeping. (This has been confirmed and reconfirmed through the years by contact with the manufacturer.)

The following lights do not promote D3 synthesis but may be used simply provide light or, in the case of the incandescents listed, heat, for reptiles: wide-spectrum plant and aquarium lights, interior and exterior household lights, Lumiram (Chromalux), Energy Savers Unlimited and Neo-Lite neodymium and other incandescent lights, halogen lamps. BLB blacklights cause eye damage and should not be used.

Halide/halogen lights must be used with extreme caution as they may cause eye damage if left on too long. See the above comments on the use of mercury vapor lamps. Any sunlamp (FS) for which humans should wear protective eye covering should not be used for reptiles - you simply can't get tight-fitting goggles small enough and the animals just won't listen when you tell them to keep their eyes closed!


Spectral Flux
Spectraradiometric tests are done on bulbs to determine their output. It is a method of testing used to measure the UVA and UVB radiation in many of the full-spectrum bulbs.

Zoo Med recently came out with two new full-spectrum fluorescent bulbs, and they are freely giving out the results of the independent testing done on their FS-24 24" tube and two other relatively new full-spectrum tubes on the market. The lights tested were Energy Savers Unlimited (ESU) FR20T12 24" and Lumiram's Lumichrome T8/24. The percentage of UVA and UVB radiation, from 200-250 NM was measured. The findings showed that the Zoo Med light rated higher than the other two.

The CRI, color temperature and lumens were also analyzed. The Zoo Med outranked both of the bulbs in the output of UVA and UVB, but slightly below the Lumiram in CRI and color temperature and lumens.

While the Lumiram is a brighter, more intense bulb (due to the higher CRI), since UV bulbs should be no farther than 18-24 inches from the basking area to minimize the dispersion of the UV waves, the difference in CRI, K, and lumens would appear to balance out.

Tests have been done on Vita-Lite (24" 20 watt), comparing it to the Sylvania F20T12/350 BL (24" 20 watt) and the Westinghouse FS-20 (24" 20 watt) sunlamp (now no longer available, but similar lamps are made by National Biological Corp.) This is somewhat akin to comparing apples to oranges, as these are three different types of lights.

Ball and several others recommend using the sunlamp for the shorter periods time, stating they are preferable to the less effective UVB-producing fluorescent lights. However, whether a sunlamp is on for only 30 minutes a day, or for 10 minutes an hour for several hours, the problem is that the animal may not be under the light at the necessary distance during the time it is on for its use to be effective. It also is creating abnormal periods of light intensity during the daytime which can ultimately cause stress and stress-related health problems. Keeping the less effective light UVB-producing fluorescents known to provide sufficient UVB during a standard day photoperiod will result in the animal getting sufficient doses of UVB to enable proper vitamin D3 synthesis.


Whenever possible, provide your reptile with natural, unfiltered sunlight. Glass and plastic filter out the beneficial UV and so any light reaching the interior of a tank through such a substance lacks what it takes to promote optimal behavior and D3 synthesis. Even aluminum screen, a common feature of many enclosures, filters out 30 percent of the beneficial UV. There is special glass, UVT Plexiglas, manufactured for the greenhouse industry, which does not filter out the beneficial UV.

When putting any animal in the sun, whether outside or inside, you must assure that the animal has a cool, shady place to go to if it gets to warm for it. Glass and plastic enclosures, especially aquariums placed in front of closed windows, are heat traps, no different than an closed up automobile which on a mildly warm day becomes a lethal oven. Temperatures inside solid-walled enclosures build up extremely fast and can become hotter than the air on the outside of the tank, especially in tanks with little ventilation.

If you live in a part of the country where you have lots of sunny days warm enough to expose your reptile to unfiltered sun, you can do with a UVB-producing fluorescent tube on days when they cannot be in the sun. In parts of the country which get fewer sunny days warm enough for direct exposure, or if you live in the northern parts of country and have tropical species, you may want to supplement the UVB-producing fluorescent with a BL tube to increase the amount of UVB your desert and montane species are exposed to. As new tubes with higher UVB output hit the market (such as the 5.0 and 10.0 additions to Zoo-Med's line of reptile & iguana lights).

At night, when supplemental heating is required, a non-light emitting ceramic heating element may be used, or blue light bulbs, such as the nocturnal reptile lights (not the BLB black lights). These lights produce fewer lumens than red and green lights, and are thus less likely to disturb a sleeping reptile.

Studies indicate that dietary D3 alone may be inadequate and may affect behavior. In a study at the National Zoo (Bernard et al 1991), groups of iguanas were fed a diet rich in calcium and D3, while other groups were injected with D3. Both groups fared poorly compared with the control group of iguanas which were exposed to daily doses of UV light. And, contrary to what is often reported, iguanas exposed to the UV "appeared to become less irritable after exposure to the UV light." Quite often the "aggressive" behavior reported in iguanas exposed to unfiltered sunlight is actually the normal behavior of an iguana which has been kept in an enclosure which has been kept at below-optimum temperatures and lighting for too long which results in slow movements, lethargic, mildly apathetic behavior and reduced appetite.



Alberts, Allison. 1994. Ultraviolet light and lizards: more than meets the eye. The Vivarium 5(4):24.

Ball, James C. Vitamin D3 synthesis: A comparison of the UV B output of commercial lights and sunlight. Northern Nevada Herpetological Society, V(3):7.

Barten, Stephen L., DVM. The medical care of iguanas and other common pet lizards. Exotic Pet Medicine I 23(6):1213-1249

Bernard, JS, OT Oftendal, PS Barboza, ME Allen, SB Citino, DE Ullry, and RJ Montali. 1991. The response of vitamin D deficient green iguanas (Iguana iguana) to artificial ultraviolet light. Proc Am Vet 1991:147-150.

Gehrmann, William H. No UV from tungsten filament incandescent light. ARAV 2(2):5. 1992.

Goldstein, Robert. 1990. Spectrum lighting. Reptile & Amphibian Magazine, Mar/Apr 1990.

Richards, Ann. 1994. Understanding your reptile's lighting needs will help you keep it healthy. Reptiles 1(1):24.

Use of full spectrum lighting at the Arizona Sonora Desert Museum. Iguana Times 3(1):11. March 1994.

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