Not sure whether this goes in the lamp or the fixture category but involves three lamps and one fixture so here it is.

I am considering building a fixture having been inspired by this one . I would make in single instead of double as in the picture. The material used would be a metal food can (the big kind, usually bought by restaurants), where it would probably have an open top and a concentric ring louvre on the bottom. I will make the louvre out of smaller cans. On top of the louvre, there will be a circular piece of frosted glass for protection and diffusion. For the lamp, I am thinking either a 300 watt incandescent or a metal halide, either 175 watt or 100 watt, not sure. The GE 175 watt MH lamps run at ~200ºC at the base and ~400ºC on the outer glass, according to their website. The 300 watt incandescent and the 100 watt MH didn't specify operating temperature. Just to be safe, the MH would be an open-fixture rated one.

The question : since this fixture is made of thin metal, what lamp type would generate the most heat ? I don't want to have something overheat, possibly burning some accumulated dust or the like. It will probably end up being suspended very close (but not touching) a wood ceiling.

This fitting will run on 120V 60Hz.

Thanks for any help you may provide.

You should prevent the build up of dirt in the fixture anyway. The dirt will find its way to where it blocks part of th light / make unsightly spots on the glass etc. And pose a fire hazard where ever it'll be if it can get close to the lamp (dry it up to the point where it can combust at the temperature of the fixture), or the electrics (ignite as result of arcing etc)

For that you need either open bottom fixture (so it won't accumulate dirt) or sealed fixture (so it won't let in dirt at all - even mosqitos which will attempt to get in no matter what etc)

In an open bottom fixture (without the glass bottom), consider opening the top as well - in this case it'll cool itself pretty well by convection and you won't need anything special. You can make the louver be a bit deeper (thicker) so preventing direct view of the arc tube even without the glass refractor of the fixture, or use a frosted MH

In a sealed fixture, it can get very hot so all of the materials used must withstand the heat



The incandescent radiates a lot of heat as IR - which mean that this heat is becoming a high temperature not at the lamp, but at the surfaces where the light reach such as parts of the fixture enclosure

The MH, allthough generating about the same amount of heat for the same input power, generate it mostly as high temperature on the lamp itself

So since the MH is better for the fixture even at the same power, but also of lower power, i'd definitely go with the MH

If you make the fixture open bottom, use a open rated MH that is protected from exploding at EOL



The lower the fixture is suspended below the ceiling, the more the hot air rising will diffuse and make less concentrated heat pattern on the ceiling, You can add a dish on the cable (between the ceiling and fixture) to help diffuse the hot air - even few of them as decoration

Even if not too hot to be safe, the heat can slowly dry out the wood or make it darken (as result of dark particales rising with convection and settling on the wood) so it is important to diffuse the stream of hot air to keep the ceiling in good condition

Take into account that the ballast can make few tens W of heat too for a big MH - put it in a metallic box

The bulb and base temperature ratings are not the actual operating temperatures, but the maximum the lamp could withstand. When designing the fixture, you have to check, if the temperatures on different places of the lamp do not exceed the manufacturer rating.
Similar limits apply for all lamps, include incandescents, only those figures is harder to find. But generally you may assume the limits would not differ from the HID ratings of similar wattage.

For your setup I would use 70W pulse start MH, preferably the CMH (It emit it's heat mainly as IR).

Thank you for your suggestions. An open-bottom (without the glass, only the louvre) sounds like a good option.

If I understand correctly, a 70 watt CMH, for example, would generate about the same amount of heat as a 70 watt incandescent. The ballast temperature is not a factor here as the ballast will be remote-mounted in a metal enclosure.

No, I think he meant that the maximum temp ratings of the CMH would be about the same as those of the incandescent of similar wattage.  The CMH couldn't produce the same amount of heat as the same wattage incandescent without having the same efficacy.

So a CMH (or MH) being more efficient than an incandescent of the same wattage, would produce less heat than the incandescent ?

The total amount of heat would be quite close actually

Incandescent is 2% efficien (so 98% energy to heat). CMH lets say 16% efficient (so 84% energy to heat). In the light output it makes a big difference. In the heat output its 84 vs 98 - not so big difference at all

The MH "heats less than the incandescent" since in an actual application what you care about is light output and not input power - so for the same light output you'd install a MH that will take 8 times lower power to begin with - even if in the end they'd make the same heat/power

But the heat of MH and incandescent is different : MH makes more heat at the lamp itself (the lamp is hot), while incandescent emits most of it as IR - this IR heat has a temperature effect not at the lamp but ast the surfaces where the light hit. So for example you have a plastic refractor below a MH or incandescent of the same power. The heat of MH is on the lamp and drawn up and away by convection, while the heat of incandescent is radiated from it and hit the plastic refractor, rising its temperature significantly

OK. Thanks, it makes more sense now.