February 3rd, 2010
|03:21 pm - I'd always wondered...|
"In all mammalian eyes, rods and cones make electrical activity out of light waves by means of a change of pigment in the cells. The rate at which cells do this is called the 'flicker-fusion' rate: the number of snapshots of the world that the eyes take in every second. The flicker-fusion rate of humans is 60 images every second. Dogs have higher rate: 70 or 80 cycles a second.
"Like film, the image on your (non-digital) TV is really a sequence of still shots sent quickly enough to fool our eyes into seeing a continuous stream. But it's not fast enough for dog vision. They see the frames and the dark spaces in between them, too."
—extracted from Inside of a Dog by Alexandra Horowitz
Wow. The science fact about dogs is interesting, but what really leaped out at me in that snippet was a weird and presumably unrelated coincidence: The standard frequency for North American AC house current is also 60 Hz.
I don't think it is entirely
unrelated, actually: US pre-digital (NTSC) television refreshed the screen at 60 Hz partly because it was enough to make you see motion, but also partly to match the current frequency...
"The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen."
...and UK (PAL) television refreshed at 50 Hz, matching the frequency of UK current.
Actual film in a cinema is usually only 48 cycles/second: there are only 24 exposed frames per second, but each one is shown twice by a shutter flip in the projector, so the flicker rate is 48 Hz:
"...virtually all modern projector shutters are designed to add additional flicker periods, typically doubling the flicker rate to 48 Hz ... which is less visible."
—http://en.wikipedia.org/wiki/Persistence_of_visionEdited at 2010-02-04 03:28 am (UTC)
That makes sense (also a common refresh rate for computer monitors), but what I meant was that it's odd that the frequency for house current happens to be 60 Hz. Since house current was invented before television, it seems unlikely that whoever chose that frequency did so because he knew it would eventually match the human eye's flicker-fusion rate and make TV work. :)