The majority of artificial light sources flicker, but they do so with different amplitudes and frequencies.
If you have lights powered by direct current (DC) from a battery:
- An incandescent bulb will not flicker.
- An LED driven directly or with a resistor will not flicker. But an LED driven with a buck/boost DC-to-DC converter might flicker, possibly at a high frequency.
- Fluorescent lamps are usually not powered by DC.
If you have lights powered by DC that was converted from AC, they might flicker a little bit due to ripples in the power conversion. Simple converters based on a transformer and filter might flicker at 100 or 120 Hz (twice the power frequency). Better converters based on SMPS might have essentially invisible flicker in the tens of kilohertz.
If you have lights powered by alternating current:
- An incandescent bulb will flicker a tiny bit at 100 or 120 Hz. The thermal mass of the tungsten will keep the light output pretty steady, but there are still subtle fluctuations of heating and cooling.
- An LED hooked up directly to power will flicker strongly at 100 or 120 Hz. Cheap Christmas lights, which are strings of LEDs, tend to be like this.
- Other LED bulbs have AC-to-DC conversion circuitry, possibly with inductors and capacitors to smooth out the power, and possibly use SMPS high-frequency conversion.
- Old, big fluorescent lamps will flicker moderately at 100 or 120 Hz. The color may shift between orange-ish and blue-ish during each cycle.
- New compact fluorescent lamps (CFLs) have onboard power converters that will produce invisible flicker in the tens of kilohertz.
CRT televisions/monitors are a great example of strong flicker. You can use them to test how your eyes react, and easily adjust the frequency through video output settings. For example, in my peripheral vision I can see flicker at 50 Hz but not at 60 Hz.
If the flicker frequency is too low (let's say below 50 Hz), it could potentially cause eye strain, fatigue, and disorientation.
Moderate flicker frequencies (say 100 Hz) are generally fine, but can produce slightly annoying artifacts when the light source moves or when objects move. For example, moving your hand under a flickering light source can produce ghost copies of the hand. For example, a flickering car tail light can produce many copies of the light against a dark background.
Another consideration is that in machine shops with spinning equipment, a flickering light source can produce the aliasing illusion that a fast-spinning machine is stopped or going slowly; this can be a safety hazard.
And finally, if the flicker causes visible problems in the photos or videos that you take, then by definition the flicker is bad for that particular use case.
Overall, non-flickering light sources are the most natural to our eyes and cameras, but moderate flicker frequencies (around 100 Hz) can be tolerated by our eyes very well.