A source of vitamin D- must also be provided. Lack of vitamin D3 is a major cause of metabolic bone disease. While vitamin D1 certainly should be provided in the diet, it is still necessary to provide it to reptiles by using UV light, especially since some question remains as to whether some reptiles (iguanas) can absorb dietary vitamin D3. In addition to providing vitamin D, these lights, as well as full-spectrum lights, seem to have a positive impact on the psychological well-being of the pet.
The best recommendation is to provide the UV light using a two-bulb fixture. The lights should be within 15-30 cm of the pet for max imum effectiveness. Ideally, the UV light should emit light in the UV-B range (290-320nm). Combining a blacklight (such as the blacklight blue from General Electric) with a Vita-Lite, Chroma-50, or Colortone-50 in a two-bulb fixture seems to meet the needs of the pet. Other acceptable UV lights include TL-09 and TL-12 from Philips, and the Ultra-Vitalux from Osram. The plant blacklight from Sylvania, the Gro-lux, definitely needs to be combined with a UV light. Because the output of UV lights decreases with age, many experts recommend replacing the lights every 6-12 months. For UV lights to supply adequate vitamin D3, the light must reach the pet unfiltered, so no glass or plastic should be interposed between the pet and the light (a screen or mesh covering on the top of the cage allows the UV light to enter and prevents the reptiles from escaping). Some manufacturers sell cages constructed of material that supposedly does not filter UV light. According to their package inserts, however, UV light in the critical 290-320nm range is filtered, making these cages useless for reptiles. Plant lights (without additional UV light supplementation) and poster blacklights are not recommended, as they don't provide much light in the 290-320 nm range.
Exposing the pet to direct, unfiltered sunlight is always a good idea. Several words of caution should be given, however. First, the UV light of sunlight is filtered by plastic or glass. Second, putting a glass aquarium in direct sunlight can easily cause the habitat to become dangerously hot (similar to leaving a child or pet in a closed car in the summer). Third, taking the pet out of its normal cage is fine, but care must be given to prevent its escape or attack by other pets. An outdoor cage constructed of wood and wire screen is an accept-able method of exposing reptiles to sunlight. Fourth, some reptiles become aggressive immediately following exposure to sunlight.
(from reptile diseases by Shawn P. Messonnier)