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| © Eugene Brennan |
You can check what's between the inner and outer drums of a washing machine, or check for limescale build-up on the heating element, by spinning the drum and illuminating it with a bright torch. Normally, when you rotate the drum and the holes move, revealing various parts of the drum, you just see individual snapshots through the holes. However, when the drum is spun rapidly, visual persistence (the image remains briefly in the visual system), and temporal integration allows you to see what's behind it. Your brain effectively puts all the snapshots together as a composite image. This is similar to how the frames projected onto a screen from a reel of movie film, or the individual frames of a TV picture, are just static images but give the appearance of continuous motion when shown at a typical rate of 25 frames per second.
The critical flicker fusion frequency (CFF) threshold for humans is about 48 to 60 hertz (Hz), so although standard film is shot at 24 frames per second (fps), movie projectors flash each frame twice, giving a 48 Hz refresh rate, reducing flicker. TVs using cathode ray tubes (CRTs) displayed fields at a rate of 50 per second, using an interlacing technique in which the odd-numbered lines were displayed first, followed by the even-numbered lines. A complete picture was displayed at a rate of 25 fps.
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