Critical Fusion Frequency

Take a light and switch it on and off. When the switching is slow, the individual flashes are distinguishable. Speed the flashes up, and it becomes harder to distinguish the individual flashes from each other.

The graph below, taken from an old text book, shows the typical vision system's response.

As the frequency increases, our sensitivity (or ability to distinguish between individual flickers) decreases. At the critical fusion frequency (CFF), the flashes are indistinguishable from a steady light of the same average intensity. Interestingly, the CFF is different for each of us, as demonstrated here for patients with diseases of the eye. (a side note: do autistics have a higher CFF? This might explain why fluorescent lights bother autistic people so much ... No, Sarge, I'd rather not turn on the lights in here).

This same phenomenon also explains why we can generally see the stutter in video when the frame rate drops below 20 frames per second. Frames per second can thus be seen as the frequency of video, yes? Is it a coincidence that interlaced video is 60 fields per second (NTSC) and power line frequency is 60Hz? PAL is 50 fields in a country with a power line frequency of 50Hz? Hmmm.