HID is light emitted from an arc of electricity that is maintained between an anode and cathode. There are no phosphors involved here as there are in fluorescent lights. The color temperature is determined by a few factors that I do not entirely understand so I'm not going to go there. (all I know is it has something to do with materials used in construction and a proper chemical balance)
The word Xenon is commonly thrown around as a buzz word. Xenon is a heavy fill gas used in many types of bulbs, both arc and incan. Often times, incan bulbs that are "xenon filled," are not purely filled with just xenon (in fact, that's probably pretty rare), the xenon is in addition to various halogens and they can work together to further improve bulb life. Xenon having a high atomic weight acts as a barrier that reduces the amount of tungsten evaporating from the surface of the filament.
Sodium is a form of HID or arc lamp. Again, no phosphors involved that I am aware of.
Being able to focus many arc lamps into a tight beam is more of a byproduct of the design, rather than their intended use. There are far more arc lamps in the world in use as overhead flood lighting than there are in spotlights. Next time you are in home depot, look up.
The temperature that a bulb runs at has little to nothing to do with whether or not it has a halogen fill or not. The halogens in the fill gas helps to redeposit evaporated tungsten back on to the filament, which increase lamp life, and at the same time helps keep the envelope clear (less tungsten deposits on the envelope). If you were to compare a 100W long life halogen bulb to a 100W typical household bulb (argon filled $0.30 bulb), both would be generating about the same amount of heat.
As marduke said, the closest thing to a fluorescent is a white LED. The LED itself is not actually emitting white light, they are typically blue emitters, that excite carefully selected phosphors to produce an average output that is close to "white."
Eric
Now I feel all defensive!
In response to firefly99's friendly response, I not only said I was no expert, I also said that "it's my understanding that......" I don't think that is handing out misinformation. I'm always glad to learn and I'm happy to be corrected where my understanding was wrong.
I was wrong about sodium lights and HID being fluorescent....... I stand corrected. My dictionary says fluorescence is the process of "radiation being taken in and given out in the form or light". Aren't there other kinds of fluorescent lights besides mercury vapor? Now that I think about it, ordinary street lights, as well as sodium street lights, are much the same shape and size as each other, and very different in shape from ordinary fluorescent lights. What are they?
I remember seeing TV programs about deep sea fish that fluoresce to attract prey....... is that true fluorescence or something else? Even fish like flashlights!
Cars' Xenon headlamps are HID, like flashlights, but I've seen a lot of after-market bulbs that are labeled xenon-this and xenon-that but you only have to look at them to see that they are incandescent.
Halogen bulbs are also incandescent and give out more light because they run hotter. I remember now about the halogen gas in the bulb preventing the evaporated filament tungsten being deposited on the inside of the glass, gradually reducing the output.......but it had nothing to do with the temperature, so I'm corrected there, too.
The filaments are able to run hotter because the quartz glass is more heat-resistant than conventional bulb envelopes. I remember when they first appeared, in Europe, in the early '60s, they were often called quartz-iodine ......they had even more impact that xenons do now, because they weren't self-leveling except on the big Citroens. I don't know what halogen gas they use now...... still iodine?
So, to revert to the original topic, the light sources are incandescent, HID, fluorescent and LED. Are there any others? And are there any fluorescent flashlights? Do I remember seeing fluorescent lanterns?
