Disadvantages
The resolution along a Nipkow disk's scanline is potentially very high, being an analogue scan.
However the maximum number of scanlines is much more limited, being equal to the number of holes on the disk, which in practice ranged from 30 to 100, with rare 200-hole disks tested.
Another serious disadvantage lay with reproducing images at the receiving end of the transmission which was also accomplished with a Nipkow disk. The images were typically very small, as small as the surface used for scanning, which, with the practical implementations of mechanical television, were the size of a postage-stamp in the case of a 30 to 50 cm diameter disk.
Further disadvantages include the non-linear geometry of the scanned images, and the impractical size of the disk, at least in the past. The Nipkow disks used in early TV receivers were roughly 30 cm to 50 cm in diameter, with 30 to 50 holes. The devices using them were also noisy and heavy with very low picture quality and a great deal of flickering. The acquisition part of the system was not much better, requiring very powerful lighting of the subject.
Disk scanners share a major limitation with the Farnsworth image dissector. Light is conveyed into the sensing system as the small aperture scans over the entire field of view. The actual amount of light gathered is instantaneous, occurring through a very small aperture, and the net yield is only a microscopic percentage of the incident energy.
Iconoscopes (and their successors) accumulate energy on the target continuously, thereby integrating energy over time. The scanning system simply "picks off" the accumulated charge as it sweeps past each site on the target. Simple calculations show that, for equally sensitive photosensitive receptors, the iconoscope is hundreds to thousands of times more sensitive than the disk or the Farnsworth scanner.
The scanning disk can be replaced by a polygonal mirror, but this suffers from the same problem — lack of integration over time.
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