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stroboscopic fillament



On February 1 Leigh Palmer wrote:

> I tipped a 300 watt tungsten halogen "torchiere" lamp (cheap at the mall) 
> over on its side to illuminate the top .. the instantaneous lamp temperature 
> varied. My 1/10,000 second shutter speed reveals clearly the appreciable 
> variation in the tungsten filament temperature. Even this incandescent source 
> has a small stroboscopic effect, something that was certainly a big surprise 
> to me. The effect would be greater for a tungsten halogen lamp than it would 
> be for a conventional nitrogen filled bulb because the peak temperature of
> the former is greater, and it cools faster.

Very surprising. The time interval between the peaks of I^2 (at 60 Hz) is 
only 1/120 s. And this is sufficient to overcome the "thermal inertia" of
a tungsten fillament. Amazing. How thin must the fillament (a black body 
source) be to make this possible?
                                        Ludwik Kowalski