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Re: stroboscopic fillament

I too found it surprising that a tungsten filament showed a strobe
effect. So, I pulled out an MBL rig and did an experiment. Using the
Vernier light probe and ULI with a PC, I immediately was able to see
the 70Hz refresh rate of my computer monitor by holding the probe to
the screen. While I did not do it now due to no such lights in my
office, I know from prior work that I can easily see the 120Hz
flicker from fluorescent lights with this rig.

Now, to an incandescent desk lamp. I expected to see only a steady
light, but indeed there is a clear 120Hz flicker. However, the
variation in intensity was only about 15% from min. to max. This was
using a standard 60W (damn, I should have turned it off before
looking...) frosted light bulb. I'm sure that more exotic lamps might
have thinner filaments, and so cool still more quickly.

I was surprised, but yes, incandescent lamps show the 120Hz flicker.
Whether that's enough to _see_ a strobe effect by eye seems to be
borne out by the experiment of others.

JEG






On 2 Feb 97 at 18:26, kowalskil@alpha.montclair.edu wrote:

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


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John E. Gastineau (304) 296-1966
900 B Ridgeway Ave. gastineau@badgerden.com
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www.badgerden.com/~gastineau