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[APD] Re: replacement for metal halide lamp

Can anyone tell me the difference between a metal halide lamp and normal
halogen lamp (those 35W small bulbs)?
Metal halide lamps are expensive and generate a lot of heat. I wonder if
halogen can be a close substitute. They also give out a lot of light
(albeit the spectral density will be different), and consume less space.

Tony Lau

Halogens are very different and far inferior to Metal Halide (MH) lights in terms of efficiency. Halogen bulbs produce far more heat per unit of light output than do MH lights. Halogen lights are incandescent (they use a filament) while MH lights are arc lights (which are far more efficient). Also, typical halogen lamps are in the 3200K to 3600K range where most MH lights are 4200K or 5600K (or the specialty aquarium lamps that go much higher). The resulting "crisper" white is usually preferred in aquariums over the more yellow light give off by halogen lamps.

A properly designed halogen light fixture will be similar in size to a properly designed MH fixture of the same wattage. You can't make a fair comparison of the little 20 watt 12 volt halogen lights to the 400 watt MH fixtures, for example. A better comparison is a 250-300 watt halogen and a 175 watt MH fixture which can be similar in size (with the MH fixture a bit bigger due to the larger bulb), but you will get a *lot* more light out of the MH fixture and a *lot* more heat out of the halogen fixture.

I have used 10 watt 12 volt halogens in aquarium hoods as accent lights (mostly to get the water-ripple patterns on the substrate), but the primary light in those tanks are PCF lights. It was a lot of work to install the halogen lights, they tended to melt little spots in the plastic hoods, and the PCF light tended to overpower the ripple effects anyway. If you want an intense, point-source light MH is really the only way to go. For general lighting, PCF is easier and a bit cheaper too unless you go the DIY route with the MH fixtures. I would not consider halogen lights as a sole source of light for a tank due to the low color temperature, the very poor efficiency, and the high heat-to-size ratio of the lamps themselves.


***************************** Waveform Technology UNIX Systems Administrator

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