Green Eggs,
OK, first of all there are (at least) three classes of lamps:
1. No fill gas. Vacuum inside keeps the filament from burning.
2. Noble gas fill. This means Argon, Krypton, or Xenon, and maybe some Nitrogen as well (in the case of Argon fill). Krypton and Xenon have a higher atomic Z (protons plus neutrons) and work better at supressing the vaporization of the tungsten filament, but they are SIGNIFICANTLY more expensive than Argon/Nitrogen, especially Xenon. In fact, chances are very good that if a company uses a fill gas such a xenon or krypton, that they also add a halogen to the mix, but I'm getting ahead of myself. These lamps (without the halogen) will, in any case, gradually blacken over time and decrease in brightness. mini-mag lamps are a great example of this. That silvery grey coating on the inside of the glass is tungsten.
3. Halogen lamps. These lamps mostly use the same fill gases as 2 above, but to this is added a small amount of a halogen, such as bromine or chlorine. These are instrumental in effecting what is called "the halogen cycle" where tungsten atoms chemically combine with the halgoen at the glass wall and which are then redeposited onto the filament. Wild, huh? This keeps the glass clear throughout the life of the lamp, but at some point the filament will get thin in one spot and get too hot and melt and--poof--no more filament.
SO, onto your question. The life rating of different lamps is defined differently. For xenon/halogen lamps (or krypton/halogen) the life rating is the length of time it takes for HALF of a sample group to die at a given voltage.
For any given lamp the life rating is a function of voltage. Decrease the voltage and you get a longer life. Increase it and you get a shorter life.
THERE IS NOT A SINGLE RATING FOR AN ENTIRE CLASS OF LIGHTS. You can't say "Xenon's only last 5 hours."
Clear as mud?
In the case of the Dorcy lamps, I bet that they are xenon/halogens and that Dorcy probably isn't exaggerating the life rating. If anything, they are hedging their bets. Practically speaking, a halogen lamp can't be driven much harder than a life rating of 10 hours. But for the record, I'm just guessing about that. Could be they aren't halogens, in which case 5 hours may be the time until 85 percent brightness, or something like that.
