Was the lamp with the glass intended for use with an aquarium ?, they usually have glass to protect the bulb from water. Glass shouldnt cut down on the light but it will reduce uv.

Tom,
The glass covers are totally optional. The main reason for them that I see is if you want to air-cool your reflector. Other than that, it may help keep some of the heat away from the tops of the plants or keep bugs etc. out of the reflector.
I dont see much need for them though unless air-cooling.

I'm afraid I can't let this stand without reply. The covers are not optional, they are a safety feature to prevent intense UV or hot (potentially molten) broken glass landing on any unfortunate passers-by if the bulb reptures which they sometimes do at the end of their lives.

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.