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Lightbulbs flicker due to AC. Why does this not show up in photographs?

  1. Sep 27, 2011 #1
    This problem just occurred to me while I was taking photos indoors:

    Light sources powered by mains fluctuate in intensity due to alternating current. In other words, a lightbulb will actually flicker on and off at 50Hz/60Hz (or, really, 100Hz/120Hz, since I guess it should be the absolute value) in Europe/US respectively. This is not noticeable to humans because it is faster than the refresh rate of our eyes, so we perceive a constant stream of light.

    So far, so good.

    But what if we take a camera and adjust its shutter speed to something significantly shorter, such as, say, 1/1000 seconds. We should be able to observe the fluctuation in that, in a succession of photos taken indoors, some will be brighter than others. I just tried this a number of times by taking photos of a lightbulb, and there is no visible difference (either in the photo or on the histogram) between any of the photos. Why is that?
     
  2. jcsd
  3. Sep 27, 2011 #2

    xts

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    It is not an on-off flicker - the luminance ratio between max and min is quite small: for typical traditional tungsten lightbulbs it is less than 1/2EV.

    The reason is heat capacity of the tungsten wire - it won't radiate all energy from previous half-cycle before the next cycle heats it up again.

    There is more flicker in fluorescent lamps - you may spot it on photos made with high speed shutters.
     
  4. Sep 28, 2011 #3
    You may find the following interesting:
    In an otherwise dark room, turn on a light. Then, put your hand over your eyes, and look in the direction of that light. Then, as simultaneously as possible remove your hand and turn the light off. You'll notice the filament still glowing. The first time I did this I was rather surprised how bright the light was a second after being turned off. You just don't notice it normally because your eyes don't adjust to darkness fast enough. It's worth doing with both traditional and florescent bulbs.
     
  5. Sep 29, 2011 #4
    If you hook up a simple photodiode to an oscilloscope and point it at the overhead fluorescent bulbs, you can clearly see the 120 Hz sine wave. If that's the only light in the room I'm sure you could notice differences between photos taken at different times in the cycle if your shutter speed is much faster than 1/120 second, like say 1/1000 second.
     
  6. Sep 29, 2011 #5

    cjl

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    Fluorescent bulbs do flicker significantly if they are using a magnetic ballast. However, incandescent bulbs don't flicker very much due to the filament's heat capacity, and compact fluorescent bulbs don't flicker significantly either because they use an electronic ballast that has a much, much higher switching frequency (several kilohertz). In most cases, I doubt that a large percentage of the illumination of a room is from magnetic-ballasted fluorescent lights, so the flicker should not be a terribly significant effect.
     
  7. Sep 29, 2011 #6
    It is possible to visually detect the flicker with a simple http://www.google.com/search?q=strobe+disk". As mentioned above, many fluorescent fixtures now use frequencies much higher than the line's 50 or 60 Hz, so a standard strobe disk may not always work.
     
    Last edited by a moderator: Apr 26, 2017
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