i dont consider the glass protecting the light, i consider the the glass protecting the plants. the glass is there to disipate heat away from the plants, the whole hood generally is supposed to allow you to hook up ducting and a fan to it so blow/suck the hot air out. without the glass, the heat would hit your plants and that might be no bueno. as far as lumens dimishing, i dont think so... but its possible you could get 20% less lumens. idk, thats a number out of my ass. lmao