Process
A lens projects an image of the scene in front of it directly onto the disk. Each hole in the spiral takes a "slice" through the image which is picked up as a pattern of light and dark by a sensor. If the sensor is made to control a light behind a second Nipkow disk rotating synchronously at the same speed and in the same direction, the image will be reproduced line-by-line. The size of the reproduced image is again determined by the size of the disc; a larger disc produces a larger image.
When spinning the disk while observing an object "through" the disk, preferably through a relatively small circular sector of the disk (the viewport), for example, an angular quarter or eighth of the disk, the object seems "scanned" line by line, first by length or height or even diagonally, depending on the exact sector chosen for observation. By spinning the disk rapidly enough, the object seems complete, in a way similar to cinematography, and capturing of motion becomes possible.
This can be intuitively understood by covering all of the disk but a small rectangular area with black cardboard (which stays fixed), spinning the disk and observing an object through the small area.
Here arises one of the drawbacks of the Nipkow disk as an image scanning device: the scanlines are not straight lines, but rather curves. So the ideal Nipkow disk should have either a very large diameter, which means smaller curvature, or a very narrow angular opening of its viewport. Another way to produce acceptable images would be to drill smaller holes (millimeter or even micrometer scale) closer to the outer sectors of the disk, but technological evolution has favoured electronic means of image acquisition.
Read more about this topic: Nipkow Disk
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