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

  • Context: Undergrad 
  • Thread starter Thread starter quozzy
  • Start date Start date
  • Tags Tags
    Ac
Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of lightbulbs flickering due to alternating current (AC) and why this flickering is not observable in photographs. Participants explore the implications of shutter speed on capturing light fluctuations from different types of bulbs, including incandescent and fluorescent lights.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that light sources powered by mains fluctuate in intensity at 50Hz or 60Hz, but this flicker is not noticeable to the human eye.
  • One participant explains that the flicker is not an on-off phenomenon, as the luminance ratio for traditional tungsten lightbulbs is less than 1/2EV due to the heat capacity of the tungsten wire.
  • Another participant describes an experiment where the filament of a bulb glows briefly after being turned off, highlighting the persistence of light that is not perceived in normal conditions.
  • It is suggested that using a photodiode with an oscilloscope can reveal the flicker from fluorescent bulbs, indicating that faster shutter speeds may capture differences in light intensity.
  • Some participants argue that incandescent bulbs flicker less due to the filament's heat capacity, while compact fluorescent bulbs also exhibit minimal flicker because of their electronic ballasts operating at higher frequencies.
  • One participant mentions that fluorescent bulbs with magnetic ballasts do flicker significantly, but questions the prevalence of such bulbs in typical lighting scenarios.
  • A simple strobe disk is proposed as a method to visually detect flicker, although its effectiveness may vary with the type of fluorescent fixture used.

Areas of Agreement / Disagreement

Participants express varying views on the extent of flicker in different types of lightbulbs, with some agreeing on the minimal flicker of incandescent bulbs while others emphasize the flicker in fluorescent bulbs. The discussion remains unresolved regarding the visibility of flicker in photographs under different conditions.

Contextual Notes

Participants mention factors such as the heat capacity of filaments, the type of ballast used in fluorescent bulbs, and the shutter speed of cameras, which may influence the perception of flicker. However, these factors are not fully resolved or quantified in the discussion.

quozzy
Messages
15
Reaction score
0
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?
 
Science news on Phys.org
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.
 
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.
 
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.
 
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.
 
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:

Similar threads

How Does Flickering Light Influence Our Brain's Perception of Image Brightness?
  • · Replies 8 ·
Replies
8
Views
6K
How can you find people to photograph?
  • · Replies 16 ·
Replies
16
Views
4K