I had the same question and did this experiment last night. I have two 250W MH lights and turned them on with a glass top on my tank for 15 minutes. I then measured the temperature (using a Raytech Laser Non-Contact Thermometer) of the glass and the plastic hinge using both a fan blowing ambient air into the canopy and sucking air out of the canopy. As it turns out, without the fan, the temperature of the glass/hundges was 174 degrees (the surface of the bulb is 250 degrees), when I turned on the fan and sucked hot air out, the temperature of the hinge/glass was 140 degrees and when I blew the ambient room temperature air into the canopy, the temperature of the hingle/glass was 104 degrees. I'm setting up my tank without the glass top to give me another 3 inches to the water, but I will be blowing the air into the canopy.