[APD] Re: replacement for metal halide lamp

[Bill Wichers <billw at waveform_net>]

> 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.

        -Bill


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

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