The average life expectancy of incandescent lamps is measured in two different ways. Some lamps have a fill gas, such as xenon or krypton or argon, but do NOT have a halogen added, such as idodine or bromine. These lamps blacken over time, and have a continually decreasing output. (The stock mini-mag bulbs are a good example of this). IIRC, their life rating is the number of hours, on average, that it takes for the lamp to decline to 85 percent brightness.
Other incan lamps have traces of a halogen added, and are called "halogen lamps". It should be noted that almost all "xenon lamps" are in fact halogens, because I know of no company that would go to the expense of using a mainly xenon fill gas mixutre, without adding a trace of a halogen. It would make no sense. So in general, xenon and halgoen are not competing types of incandescent lamps, although it is theoretically possible to make a xenon lamp without adding a halogen.
Anyway, the halogen in the fill gas actually transports tungsten atoms which have been deposited on the glass wall of the lamp to the filament, where the intense heat causes the tungsten atom to be released from the iodine atom and be re-deposited on the filament. The Brinkmann Nexstar 2AA T1 xenon 2.4 V lamps do in fact have a halogen in the fill gas, because they do NOT blacken over time. Here at work we have been using the same Nexstar lamps in our mini-mags for 6 months straight, an average of 8 hours a week, and they are no dimmer than they were at the beginning.
So, halogen lamps life rating is the AVERAGE number of hours of life until the filament burns out. Welch Allyn does this using a controlled, constant, DC voltage source, which would be the ideal scenario. Anyway, this is a statistical thing. If you took 100 halogen lamps and ran them all continuously at the same voltage, at some point 50 would have burned out and 50 would still be running. THAT is the life rating in hours. But it should be obvious that many (half) will have burned out sooner, and half later. The spread, or standard deviation, depends on a number of factors which I am unaware of, but I can tell you that it is significant. It is more than 10 percent, I would guess.
This means that while unlikely, it is quite possible that someone got 100 hours from a P60. Entirely possible. It's also possible that some poor bastid only got 5 hours.
In any case, there is no standard life rating for all halogens or any other type of incandescent lamp. For a given lamp, the hours of life depends directly (but not linearly) on the applied voltage. Increase the voltage by only 10 percent and you reduce the life by more than a factor of 3. Thus a 6.0 volt lamp rated for 30 hours of life driven at 6.6 volts will only have a 10 hour life. (on average). The formula for halogen lamp life re-rating is as follows:
Lr = (Vd/Va)^12*Ld, where Lr is re-rated life, and Ld is design life, and Vd is design voltage and Va is applied voltage.
As you can see the re-rated life goes as the twelfth power of the ratio of design to applied voltages. That's HIGHLY non-linear.
One consequence of this is that when direct driven from batteries lamps take the biggest hit on their filament lives at the beginning of the battery capacity, when the voltage is highest. So those who are always changing thier batteries (or recharging) early will be running thier lamps harder than those who try to get the last drop of juice from thier batteries.
Also, a cold filament has a very large inrush current at start-up, which is hard on it and reduces the life. This is why regulated incandescents, such as the SF A2, or my M6-R, have a soft-start feature which limits the current for the first 50 to 100 mili-seconds or so. This is claimed to prolong filament life, and it certainly doesn't hurt, but by far the most important factor in getting the most life from a filament is precise voltage control, or at least prevention of volage spikes. Just a few seconds near the melting point of the filament takes many minutes off of the life of the lamp.
What I'm getting at, is that filament life also depends on HOW you use your flashlight. It's just an average, best-guess kind of number.
And, yes, the P60 and P61 and P90 and P91 lamps are all officially rated by SureFire at 30 hours. Going by the CCT (i.e. beam-color or "whiteness") and by known over-drive limits, I would guess that this is a touch on the conservative side. It's probably actually more like 35 or 40 hours.
For those of you who really want more punishment, I present the "rule of threes" from halogen lamp design:
At a CCT of 3330 K a lamp has an efficiency of 30 lumens/watt, (which corresponds roughly to 30-35 hour life depending on the fill gas and other factors). Every 3 lumens/watt increase or decrease in efficiency raises or lowers the CCT by 100 K. 37 lumens/watt is the practical upper limit of efficiency and overdrive, which would correspond to a CCT of 3550 K. Unless you are a modder I doubt very much that you will have seen an incandescent lamp pushed to this level of over-drive. It is an almost religious experience (or it was for me). Most professional photographic type lamps, which require high CCT, run at 3200 to 3400 K, and this is quite white (for an incan). The SureFire P60 and P61 are not as white as this, but are close (with fresh batteries), so I'd say, going by beam color, that this corresponds to a 40 hour life. But it's just a guess, and there are factors which affect this, most especially whether or not xenon was used as the fill gas. Xenon can increase luminous efficiency by about 5-10 percent, which corresponds to a CCT increase of 100 K. This can only be used in "low" voltage (less than 50 volts) lamps, otherwise you'd turn your incan into a poor excuse for an HID. hehe.
OK. Sorry if it seemed like I was playing professor. Just thought that maybe some people might be interested, being flashaholics and all.
