The effect of flicker
According to studies about 1 in 4,000 people are highly susceptible to flashing lights cycling in the 3 to 70 Hz range. Such obvious flickering can trigger ailments as serious as epileptic seizures. Less well known is the fact that long-term exposure to higher frequency (unintentional) flickering (in the 70 to 160 Hz range) can also cause malaise, headaches, and visual impairment.
Unfortunately, unless a person is in natural daylight, they are likely to be exposed to this higher frequency flickering, because all mains-powered light sources, whether incandescent, halogen, fluorescent or LED, are subject to flickering. The source is the AC component of the power supply and the frequency of the flickering is typically either equal to the mains frequency (usually 50 or 60 Hz) or double the mains frequency.
Tests show that humans find it difficult to directly sense light flickering at these higher frequencies, but that seems to hardly matter. Scientists have conducted research that indicates the human retina is able to resolve light flickering at 100 to 150 Hz, even if the subject is not aware of it, leading to the conclusion that the brain may well be reacting.
The insidious effects of this so-called imperceptible flickering in the 100 to 150 Hz range are not just a function of frequency; physical and physiological factors also play a big part. For example, bright light is worse than dim, and the difference between "bright" and "dark" parts of the lighting pattern are important (a light that goes completely dark during the "off" part of the cycle is worse than one which only partially dims). Red light and alternating red and blue can be particularly troublesome, and the position of the light source on the retina is important, as light sensed by the center is worse than that falling on the periphery.
Some researchers even claim the retina can sense flickering up to 200 Hz, but tests have shown that above 160 Hz the health effects of flickering are negligible.¹