Yeah, i've always covered my carboys. This is my first winter with a fermentation chamber, and was just hoping I wouldn't have to cover them in there. I guess I'll just have to find a darker way to heat the thing eventually.
If they do it has to be very little. Glass absorbs UV. I bought an eprom eraser once and the manual went into detail about how hard it is to get glass to pass UV to build things like that. So the the glass on the bulb will block most of what little a light bulb produces. The lower the wattage, the lower the UV. An appliance bulb is made to be 'wasteful' on purpose so it will last longer and the color temperature doesn't matter as much. http://www.newton.dep.anl.gov/askasci/chem00/chem00539.htm
One of the most brilliant men on the planet decided not to wear special glasses to watch the first atomic bomb blast figuring the windshield of a vehicle would cut out the harmful UV radiation: http://www.around.com/genius.html