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A C Highfield

Above: Metabolic bone disease is revealed by the spinal region depression visible on this Geochelone sulcata (African Spurred tortoise). MBD is typically caused by lack of exposure to UV-B as well as by dietary factors.

There is considerable confusion among many regarding the 'correct' type of lighting to provide for tortoises and turtles which are over wintered, or which are kept in terrarium habitats indoors for much of the year. Much of the blame for this confusion can be laid at the doors of manufactures and retailers who often issue inadequate and often seriously misleading information on the topic. In this article, we will attempt to set the record straight.

First, a few basic facts:

Most reptiles require high quality and appropriate lighting to meet a number of different metabolic needs. The only exceptions are certain nocturnal and dense rainforest species. It is not necessarily the case that all of these needs can be accommodated by one single form of lighting. A combination of different lighting systems may be required in some cases.

Under natural conditions, in the wild, many reptiles synthesise their own vitamin D3 from the UV component of sunlight. Vitamin D3 is essential for the effective metabolism of dietary calcium in reptiles. Certain wavelengths in the UV spectrum (290 - 320 nm) react with sterols in the skin to produce pre-vitamin D3. This is in turn converted into vitamin D3 itself, using a process which also depends upon heat. . Carnivorous and omnivorous reptiles get a high proportion of their vitamin D3 requirement from their food, however, plants do not contain D3, cholecalciferol, instead they contain D2, ergocalciferol, which is far less efficient in calcium metabolism than D3. Herbivorous reptiles kept indoors are, therefore, far more dependent upon the quantity and quality of artificial lighting than carnivorous specimens.

If inadequate vitamin D3 is available, the animal will rapidly develop the condition known as MBD or Metabolic Bone Disease. In this condition, bone density suffers and various other serious metabolic problems occur. Symptoms include swellings, lethargy, general weakness and tremors. The shell may also become soft and pliable. MBD remains the number one killer of captive lizards, tortoises and turtles (snakes are less affected as being highly carnivorous they easily obtain their D3 requirement via their prey). To prevent MBD, adequate levels of calcium must be present in the diet, and adequate (but not excessive) quantities of D3 must be provided by means of dietary supplementation or by exposure to adequate levels of UVB lighting. Rapidly growing specimens such as hatchlings are most at risk, although adults too will be affected if maintained in a state of deficiency for long enough. Egg laying females are also at great risk, due to the extra demands egg production places upon their calcium metabolism.

Above: This South African Homopus femoralis (Padloper) had been raised in a vivarium tank with inadequate UV-B lighting. The 'lumpy' shell and spinal depression is self-evident.

In the tropics the light intensity of the sun at ground level is in the region of 1,000,000 Lux, while even in the rainforest canopy it can exceed 7,000 Lux. By comparison, a 40W incandescent lamp typically produces less than 50 Lux when measured at a distance of only 1 meter! This demonstrates very clearly just how very "dim" even the best illuminated terrarium is compared to nature.

Artificial lamps used in vivaria and terraria fall into two main categories:

  • Incandescent lamps
  • Spotlights/Reflector Lamps

These are standard incandescent lights with a silvered reflector backing to direct the beam. They are more useful as a source of heat than light. They can make excellent basking lamps, however.

Infra-Red Reflector Lamps

Usually only available in high powers, e.g., 150-250 watts. They do not offer a great advantage over standard spot lamps and their red glow is particularly unnatural. Not generally recommended for reptile use.

Reptile Basking Lights (neodymium lamps)

Above: Don't be fooled! This lamp emits NO UV-B and will do nothing to prevent MBD. An ordinary domestic bulb is just as good, and a lot cheaper. This type of lamp should not be confused with genuine 'full spectrum' lamps.

These lamps are sometimes misleadingly, in my opinion, described as "full spectrum". In fact they are ordinary incandescent lamps with a built-in blue filter which serves only to change the color balance of the illumination. They emit no useful UV and must not be relied upon as a sole source of lighting in reptile enclosures.

None of the above lamps emits useful quantities of UV-B or UV-A. They are therefore of no use whatever in promoting vitamin D3 synthesis.

Black Lights

Some types of black light are used for special effects in discotheques. This type of lamp is of no use to reptiles. Other types are used for various industrial purposes, for example, in sterilizers. This type of lamp can be positively dangerous to living creatures, including humans, as they emit UV-C which can cause skin cancer and eye-damage. The only "black light" which is of any use to reptiles for D3 synthesis are those specified as "BL" types. These emit both UV-A and UV-B radiation at a relatively high level. They have been used successfully with many species of reptile, but high output UV-B full spectrum types are generally preferable. Black lights tend to emit an unpleasant violet glow, and provide no "daylight" frequency light whatsoever. They should therefore only be used in combination with a true full spectrum tube. As FS tubes and mercury vapour lamps with sufficiently high UV-A and UV-B outputs are readily available these days, black lights are now largely redundant and we do not recommend this approach.  An alternative "black light" is now on the market aimed at simulating moonlight for nocturnal reptiles.. This should in no way be confused with the true UV black lights described above. These are in any event not suitable for use with tortoises.

Full Spectrum Fluorescent Lights

No lamp is truly "full spectrum", but some get closer to this ideal than others. Today, there are a wide range of high quality fluorescent tubes available which are specifically designed for use with reptiles. Some of these, are, however, better than others and the intensity and quality of light emitted does vary considerably from brand to brand and from model to model. Note that fluorescent tubes do not provide any heat output and that a separate, incandescent, basking lamp is always required in addition. There are three main properties which are of special interest to reptile keepers, and I would rate them in the following order of importance:

  • UVB output - critical to vitamin D3 synthesis and the calcium metabolism;
  • Color temperature - nothing to do with heat, but rather the color from 'warm' red to 'cold' blue expressed in degrees Kelvin. Daylight at noon is typically estimated at 5,500 K. At the tropics, or in a desert, the color temperature can reach 6,500 K.
  • UVA output - many reptiles are believed to be able to see into the UVA range (320-400 nm), and this is likely to have a profound effect upon behaviour, and specifically, how they visualise food items.
  • Color Rendering Index: Color rendering is the degree to which a light source shows the true colors of the objects it illuminates. This is measured on a color rendering index, rated from 0-100. A normal fluorescent lamp, for example, rates 54 on the CRI scale. High quality fluorescent lamps designed for reptile use will rate 80-90 on the same scale. Color rendering is very important as many reptiles rely upon color signals for reproductive and feeding purposes.

The combination of sufficient UVA content and a 'natural' >5,500įK color temperature is most probably the reason why so many keepers report a marked improvement in activity patterns and feeding when high quality full spectrum lighting systems are utilized in enclosures. In addition to the quality of the lamp, its proximity to the animal, its output intensity and duration of use are also critical.

The illumination intensity of tubes is primarily dependent upon their size. A 24" tube producers less than half the light output of a 48" tube. Do not expect to be able to provide adequate levels of lighting in a large vivaria using a single small tube. One keeper found that in order to provide a satisfactory level of lighting for a 10' X 5' indoor enclosure eighteen 48" 40W full spectrum fittings were required. Our own 6' X 2' terraria for juvenile Mediterranean tortoises have at least two 48" fittings. One useful tip: fittings purchased from specialist suppliers can be very expensive. Full spectrum tubes work equally well in standard household or industrial fluorescent fittings which can be obtained from electrical and hardware suppliers at considerable discounts.
When installing full spectrum or UVB producing tubes, it is absolutely critical that nothing is placed between the envelope of the tube and the recipient animal. UVB is greatly attenuated by glass, plastic and even fine mesh (the tube envelope itself is a special type of glass which does permit UV transmission). The amount of UVB received also diminishes extremely quickly with distance. It is generally recommended that such tubes be no further than 18" (46 cm) away from the subject. At greater distances than this, the amount of UVB actually received will be minimal. For reptiles with very high UVB requirement, such as desert species, tubes should be placed as close as 10-12" (25-30 cm) above the basking site - although the new 'UV-Heat' type lamps are better suited overall to this application (see below). It may be necessary to install fittings on a sub-framework located within the terrarium in order to achieve this. Tubes also have a very limited life. Most will require changing every at least 12 months in order to guarantee continued UVB output. There is evidence that some tubes fail to produce advertised levels of UV-B after only 6 months of use. Although there may be no visible deterioration in the performance of the tube, there is ample evidence that the invisible UV content decays rapidly as the tube ages. It is a good idea to place a small adhesive label near each fitting with the date the tube was last changed clearly marked.

It is worth noting that a newer type of Compact UV tube is now available. There have been some safety concerns with certain brands and models of these lamps as it appears the coatings used have produced UV-B at wavelengths and levels that have caused tissue damage. It is vital that you check that any such lamp used is safe and that you follow installation and use instructions carefully.

Above: A compact UV-B tube with reflector. These must be used with caution as thee have been safety concerns wih some brands.

Full spectrum UVB tubes produced for reptile use are often classified according to their percentage UVB output. Tubes are available offering from 2% UVB to 8% UVB. The most popular tubes offer 3% or 5% UVB. In the vast majority of cases the 3% tubes are perfectly adequate, provided they are correctly sited, changed regularly, and a sufficient number of hours exposure permitted. For a 3-5% tube, 10-12 hours daily has proved a satisfactory level of exposure for most species. Concerns have been expressed about the safety of tubes with outputs greater than 5% - in particular, there may be a possibility of eye damage occurring with some tubes in some situations.

UV-B Heat Lamps (Self-Ballasted Mercury Vapor)

Superficially, these appear similar to a regular incandescent reflector lamp, but unlike a regular incandescent spot lamp, they also emit very significant levels of essential UV-B. The color of the light they emit is also much whiter, and brighter than a normal spot basking lamp. Not only that, but they also emit a very useful amount of heat. The levels of UV-B and UV-A produced by these lamps is extremely impressive. The lifespan of these lamps is also excellent, with very useful levels of UV-B being produced even after 3,000 hours of use (by this time, tubes are virtually dead in terms of UV-B production).. As the levels of UV-B and radiant heat produced are extremely high, you must install them carefully and follow the makerís instructions to the letter. A heat resistant lamp holder is essential, for example. Two sizes are generally available, 100W and 160W in spot or flood. For general use, we recommend the flood models and taking care to ensure the basking zone produced is sufficient to cover the body of the entire animal.

All basking lamps that produce heat, however, may also be problematic in terms of excess drying and uneven heating patterns. Recent research by the Tortoise Trust highlights these issues.


Despite advances in lamp design, there is still no complete substitute for natural sunlight. Wherever possible, all diurnal species should be given access to unfiltered sunlight. Use outdoor pens as much as possible. Even the very best artificial light source literally pales into insignificance compared to the sun. Bearing in mind concerns about premature UVB  flourescent lamp failure, it is the view of the Tortoise Trust that tubes alone should not be relied upon exclusively to provide vitamin D3. It is our opinion that it is far safer to continue to provide a high quality combined calcium-D3 supplement orally, even where full spectrum tubes are employed. Full spectrum UVB tubes do offer many other advantages apart from stimulating D3 production, however, and we therefore recommend that they should be installed in all indoor enclosures except where 'UV-Heat' type lamps are available. Where 'UV-Heat' lamps are used, however, it is our view that a supplement containing oral D3 should be reduced to once or twice per week, and a plain calcium carbonate (phosphorus-free) supplement used instead on a daily basis.

Some final advice:

  • More is not necessarily better. Provided sufficient UV-B is available to prevent MBD giving even more is not an advantage and may even prove injurious.
  • In the wild, tortoises do not bask indefinitely and they are often not active at mid-day. They spend a lot of time in scrub, under bushes, or in burrows. Do not therefore assume that just because very high UV-B levels can be measured in their natural habitat that they automatically choose to expose themselves to it.
  • Different species have different requirements. Tortoises from forest habitats, for example, have different needs from savannah habitats.
  • Before investing in lighting equipment, research the species you are keeping adequately, look at both its natural habitat AND behaviour, and then compare lighting options in detail.



Excellent detailed discussion of every aspect of UV and reptiles, including tests, behaviour studies, and biology.


New research from the Tortoise Trust exposing the health risks caused by artificial heating sources.


New practical designs that can reduce reliance on artificial heat and light and can greatly reduce energy use and costs.

©A. C. Highfield 1998-2002 (Updated May 2015)