I have two Metal Halide systems - one came with a glass cover over the bulb (the 600W system) the other did not (a 1000W system). I have used these fixtures for the winter season for two years now.

When changing the bulbs today, I questioned whether using the glass cover is good, bad, or neither. For example:

1) Is there really a safety issue that the glass (tempered) is addressing? If there is, why didn't the 1000W system come with one, too?

2) Am I reducing the light output of the 600W bulb when it is behind glass, i.e., does some get filtered? Is it significant.

3) Wouldn't the bulb perform better and last longer without the glass - since the heat around the bulb can better disperse without it?

I am now operating both fixtures without glass covers, and it does appear brighter now under the 600W fixture.

Any advice is greatly appreciated.

Tom

It is unfortunate that so many retail web sites describe these glass covers variously as for water protection (they are not designed to be water tight and certainly provide no protection of the wiring from splashes or humidity), UV protection (very limited except during failures since the tempered glass will absorb very little UV that wasn't absorbed by the glass cover of the bulb itself), increasing ventilation, reducing ventilation, or various other bizarre explanations.

Metal halide lamps should strictly only be operated without a tempered glass cover with O-rated bulbs which are designed with an integral cover and end-of-life cutout protection. Non-tempered glass or plastic covers provide limited protection and would certainly need replacement following any failure. At your own risk, you may wish to operate a lamp without a cover if you, or anything else you value, is unlikely to be present when any failure occurs. Remember, exposed high voltage and pieces of molten glass could cause a fire even if you weren't present to be hurt. The risks are reduced but not eliminated by replacing bulbs on a timetable so that they will only rarely fail.