That glass cover is for air cooling the hood. I've been told that you should not use the glass if you're not blowing air through the hood. The lamp could get too hot.
I suppose that you can use it with the glass and no forced air as long as it doesn't get too hot in there. Why not put a thermometer inside and see just how hot it gets? Then check with the lamp maker to see if that's too hot to run the lamp.