According to R. Reiman author of Who Invented Television: The Nipkow disk was a rotating disk with holes arranged in a spiral around its edge. Light passing through the holes as the disk rotated produced a rectangular scanning pattern or raster which could be used to either generate an electrical signal from the scene for transmitting or to produce an image from the signal at the receiver. As the disk rotated, the image was scanned by the perforations in the disk, and light from different portions of it passed to a selenium photocell. The number of scanned lines was equal to the number of perforations and each rotation of the disk produced a television frame.
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In the receiver, the brightness of the light source would be varied by the signal voltage. Again, the light passed through a synchronously rotating perforated disk and formed a raster on the projection screen. Mechanical viewers had the serious limitation of resolution and brightness.