Bonus content from the May 2009 REPTILES magazine article "Catching Wavelengths."
The size of an enclosure is a vital factor in deciding whether to use UVB fluorescent tubes or mercury vapor lamps. UVB fluorescent tubes combined with a heat source are recommended for smaller enclosures. Using a mercury vapor lamp in a cage too small can result in dangerous levels of UVB and heat.
When using a mercury vapor lamp over a basking area, there must be enough space under the lamp for the reptile to bask at a safe distance below the lamp surface, enough space above the lamp for ventilation, and enough space on the lamp’s sides to provide a temperature and ultraviolet gradient. Calculate the size of tank needed for the lamp, and always follow the manufacturer’s recommendations regarding lamp placement.
Here’s an example. A brand’s 100-watt lamp has a minimum recommended distance of 12 inches from lamp surface to reptile. If you have a basking shelf 4 inches above the tank floor and a 4-inch-tall bearded dragon, you therefore need at least 20 inches from the tank floor to the surface of the lamp hanging above.
An 18- to 20-inch-tall tank with a screen top might therefore be suitable if the lamp is hung 1 to 2 inches above the mesh. But check the temperature at the basking spot.
If the enclosure has a solid top, however, a much greater height is needed because the lamp will need to be hung inside the cage. In its fixture, the lamp may be 8 to 10 inches tall. Additional space is needed for ventilation between the top of the lamp fixture and the solid roof of the enclosure, or it may overheat. The vivarium for this lamp therefore needs to be at least 3 feet tall. Vivaria like this also need large, open-mesh ventilation panels on their sides and roof because hot air builds up around lamps in an enclosed space.
Whether the enclosure has a mesh or solid top, it must be long enough to provide a temperature gradient with the lamp over one end and a cool zone at the other. As long as ventilation is adequate, a 100-watt mercury vapor lamp is unlikely to overheat a tank measuring at least 3 feet long (e.g. a 40-gallon breeder). If hung over mesh, it may not even provide enough heat; additional non-UVB basking lamps may be required.
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