Glass between light and plants

Is a piece of glass between my hps lamp and my plants better or worse for light production?

probaly

Slightly worse - Likely its tempered glass, and not "optically perfect". But the benefits of being able to get the plants closer to the light source and cool the light fixture is well worth it. Plus if the bulb shatters you have something (besides your plants) to catch the burning shrapnel :)

If tempered it should be OK, I have a glass on my air cooled, but they say that you loos a little lumen output because the glass reflects some light back into the hood, but I aint complaining, it is better than roasting my girls. This is why some folks use cool tubes.
Adieu

if the glass was supplied with the lamp then its better,its there mainly for use with metal halide lamps and is purely to dissipate the heat...