I have a desktop lamp with a halogen bulb. I accidentally got a piece of tape stuck to the glass UV filter on the bottom a few minutes ago, and it promptly melted. I turned off the lamp and used a razor blade to remove the adhesive, but it occurred to me that I may have damaged the UV filter while doing so. The lamp has a warning printed in large letters about not operating it with a damaged or missing UV filter.
Is it possible I damaged it? It’s a thick glass disk, so physically it looks okay, but I don’t know what part of it actually does the UV filtering. Is it the glass itself? Is there a coating on the outside? If the latter, is it possible that I scraped off some of the coating with my razor?
Hmm, some googling indicates that ordinary glass doesn’t transmit UV radiation. So, the UV filter on my lamp is nothing more than a solid hunk of ordinary glass, and I shouldn’t be worried about damage to it unless it cracks.
The large glass front of a halogen lamp should be quartz as glass might not stand the heat. Quartz is very good at letting UV through.
Good quality lamps have doped quartz wich stops UV. Cheap imported lamps may not. However normal halogen lamps probably do not get hot enough to emit much if any UV anyway (high power projection lamps probably do). Glass is very good at stopping UVB but lets UVA through which is much less damaging.
[nitpick]
The quartz you refere to is actually quartz glass, i.e. glass made from just silica, of which quartz is the crystalline form. Normal glass has sodium added to it which makes it melt at lower temperatures and is much less viscous when molten, making it easier and cheaper to produce.
[/nitpick]
While you probably didn’t ruin any UV filtering that may or may not be occuring, one thing you proably DID do is get oil from your hands all over the bulb. I often have to replace halogen bulbs at my job, and one of the big things they make sure of, is that you NEVER touch the glass with your bare hands, and if you do by accident, then wipe it off with a couple alcohol pads. They claim that when the bulb gets super hot, the oil from your hands on the glass surface will heat up, and cause the bulb to shatter. I’m not sure of how, or if that’s even possible, but I imagine they wouldn’t put so many warnings on the package if it hadn’t happend at least once.
Granted, your bulb is a much lower wattage than mine (mine require a large heatsink and cooling fan to not blow themselvse up as it is) so odds are, you don’t have to take such precautions, but it couldn’t hurt to clean off the bulb with a cotton ball with a bit of rubbing alcohol on it. Just make sure it dries before putting it back in the socket.
It’s not the oils but the salts in sweat that causes problems. As has already been mentioned, halogen bulbs use quartz glass envelopes which has very low thermal expansion and therefore good resistance to heat cracking. Salt from your fingers will be absorbed into the glass matrix which makes it much weaker at that point.
Halogen lamps are one of the least efficient kind, which is why they get so hot. More energy lost to heat = less energy converted to light.
Switch to fluorescent. They have different color types now, and the electronic ballasts replace the buzz of the magnetic kind.
Also, if you have a solid shade on a desklamp you will get too much contrast.
Your pupil will reduce due to the bright spot, then enlarge as you look at something outside the bright spot, and then you have trouble when you look back at the bright spot again.
Halogen lamps are actually considerably more efficient than conventional incandescents, precisely because they get so hot. Strictly speaking, halogens are incandescent, in that the light they produce is a direct consequence of their temperature. For incandescent light sources, the higher the temperature, the more the peak of the light output is moved towards the blue end of the spectrum. An ordinary incandescent has its peak well into the infrared, so most of the light produced is infrared light for which we have little use. A halogen light is hotter, so the peak is shifted into the visible range. So more of the light produced is of the type we can use, so it’s more efficient.
Of course, there are other types of lighting which aren’t incandescent at all, such as fluorescent lights and LEDs. These can in principle be very efficient, since they need not output any infrared or ultraviolet at all. And in actual practice, they often are more efficient than halogens. But such lights generally only have a few sharp lines of color represented, rather than a smooth continuum as from any sort of incandescent, which can carry its own drawbacks (for instance, some things will appear slightly different colors under incandescant and spectral-line lighting).
