Mercury Vapor Bulbs (MVB)
The Pros and Cons
1) The biggest pro for MVB's are they are convenient in the sense they are a "all in one" bulb, they offer light, heat and UVB.
2) MVB's have a safety feature that will turn the bulb off for a cooling down period (roughly 15 minutes) and then turns itself back on, this prevents the bulb from burning itself out when over heating.
1) MVB's are expensive, actually very expensive and depending on the brand name, outrageously expensive.
2) MVB's only offer UVB to the location the light is aimed at.
3) As you adjust the MVB up and down to try getting the proper temperature at the basking spot surface, you are also messing with the UVB levels, the farther away the less UVB.
4) MVB's are large bulbs (except the Powersun 100 and some MegaRay) thus needing special fixtures like a plain porecelain or ceramic household fixtures (looks like light fixtures in a unfinished basement) or the specialized metal wire cage fixture so the bulb can ventilate properly since they to get quite hot.
5) Quality by brand names..... this is one instance where "you get what you pay for" does not apply, in example, a Zoo Med Powersun 100 is more expensive than a SolarGlo 125 or 160 but yet puts out pathetic amounts of UVB. Many brand names offer fancy packaging and exagerated claims of performance. Best bet before buying a MVB, do your research and maybe talk to people who have actually tested these lights with SolarMeters. At this time, the only two brands of MVB I would consider is the MegaRay and the SolarGlo.
6) Unlike incandescent or halogen lights, MVB cannot be used with a dimmer, so the only way to adjust the temp at the basking spot if too hot... is to raise the MVB until the desired temps are reached.
I use to use a MVB, I've since went back to a regular basking bulb (Halogen) and a tube for UVB, I like the consistency of the tube UVB but also the convenience of a basking light that can be dimmed with a dimmer. When I was using a MVB, I found it difficult to keep the basking temps at the right levels as the house got cooler and warmer (winter and summer months), and by adjusting the height of the MVB, I was rasing or lowering the levels of UVB due to distance (just one inch of distance can affect meter reading by quite a bit).