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What waveform does the light coming off different types of artificial light sources have and (of course) what are the underlying physical reason for them?


I was inspired to ask this question because of several things I have been hearing lately.

Firstly, there was a recent TED talk about how LED bulbs can be used to transmit data. Researchers have been working on the problem for a little while now and they've made a fairly impressive product, but it was only recently made possible by the LED technology, which has such a fast response that it can easily compare to the transfer rates of electric information condiuits that we normally use.

My understanding is that incandescent bulbs flicker because they have 60 Hz AC power being pulsed through them. In fact, I think 60 Hz was chosen in the first place because it was a threshold that was unnoticeable to humans. Looking at a high speed youtube video of such a bulb demonstrates that it has a very significant lag time responding to both the normal electrical phasor and being turned off. In other words, it flickers but it doesn't go dark at any point in the AC waveform.

Looking for more information, however, I was a little disappointed, since nothing seemed to give a graph of lumens emitted over time for all different kinds of bulbs. Of course, there isn't just one relevant time scale. A light could flicker with 60 Hz and also flicker with 2000 Hz (I just made that up). I'm interested in the complete picture of what lights exhibit what patterns.

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4 Answers 4

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Old fashioned (or very cheap) fluorescent bulbs flicker at twice the mains frequency if they use magnetic ballasts driven directly from the AC. Modern fluorescent bulbs use an electric ballast that runs at 5khz - 40khz (for CFLS), then the electrons stay excited for longer than the switching frequency and the light output is constant.

LEDs are constant, unless you count the individual electrons moving over the diode junction! High power LEDs normally have their own driver chip which carefully controls the current and voltage of the diode so they are less sensitive to the supply power.

Solid state lasers are essentially the same as LEDs.

edit: some very cheap LED lights, eg christmas decoration are driven directly from AC - using the diodes themselves to rectify it - these will flicker at the 1/2 mains frequency.

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LED light is constant? Not according to this article. –  Cees Timmerman Jan 29 at 19:13
    
If LEDs and diode lasers are constant, then how is it possible that they can be used to transmit information at gigahertz rates through an optical fiber? –  james large 2 days ago
    
@jameslarge - you can turn them on and off very quickly. Because of their small size and high efficency the amount of energy in the diode is small and so you can change it's state much more quickly than eg. a filamanet bulb –  Martin Beckett 2 days ago

LEDs are often driven so that they flicker at whatever frequency the driver designer chose. Some types of LEDs are prone to burning out when driven with a constant current and it is possible to get a higher average brightness by turning them on for short pulses. LED dimming is also achieved by pulse width modulation, rather than changing the peak current.

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In support of this, LED flicker is a problem for cameras. As I understand it, even at "normal" shutter speeds, you can capture the flicker from consumer-grade lights. –  horatio Jan 28 '14 at 17:18

Incandescent light bulbs flicker at twice the AC frequency, because the filament grows a bit hotter each time the current peaks. So: 50 Hz AC => 100 Hz flicker; 60 Hz AC => 120 Hz flicker.

Fluorescent bulbs do the same, for the same reason. Their flicker has a higher amplitude than incandescent bulbs, because there's no thermal inertia.

LEDs... I don't know. I'm fairly sure there must be at least some residual flicker, even if internally they are being fed DC, because that DC is never going to be 100% filtered. But whether any filtering occurs, and if so then how much, I don't know. Or maybe they have some kind of switching invertor and their flicker is at a much higher frequency than the AC mains.

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According to this article LED bulb light is about 3 times more stable than that of incandescents. –  Cees Timmerman Jan 29 at 19:16

Incandescent lamps have a lot of thermal inertia. Because they are hot metal, the filament can't heat up and cool down 120 times per second, so they don't flicker.

Flourescent lamps work by a glowing gas (Mercury) that gets phosphor glowing on the inside of the tube. White phosphor is pretty fast, so a cheap flourescent can flicker. Radar and sonar displays used to use a very slow green phosphor that would not flicker, but who wants a green light bulb?

LEDs are super fast. If you drive them with DC, they will not flicker. But, LEDs use very low voltage (around 1 volt) and converting 12 or 120 volts to a small DC using resistors or transistor in old-fashioned "linear" regulation wastes most of the power. So, folks usually pulse width modulate (PWM) LEDs. This turns them on and off at a very fast rate. It wastes very little power and allows you to vary the brightness by changing the % of the time it is on. If the on/off frequency is high enough, you can't see any flicker. One issue is that if you move your head quickly, the LED will look like a dotted line. Modern car tail lights have this problem.

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