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{{Short description|Image scanning device}}
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[[Image:Nipkow disk.svg|thumb|300px|Schematic showing the circular paths traced by the holes in a Nipkow disk]]
A '''Nipkow disk''' (sometimes Anglicized as Nipkov disk; patented in 1884), also known as '''scanning disk''', is a mechanical, rotating, geometrically operating [[image scanning]] device, patented
== Operation ==
The device is a mechanically spinning disk of any suitable material (metal, plastic, cardboard, etc.), with a series of equally
▲The device is a mechanically spinning disk of any suitable material (metal, plastic, cardboard, etc.), with a series of equally distanced circular holes of equal [[diameter]] drilled in it. The holes may also be square for greater precision.These holes are positioned to form a single-turn [[spiral]] starting from an external radial point of the disk and proceeding to the center of the disk. When the disk rotates, the holes trace circular ring patterns, with inner and outer [[diameter]] depending on each hole's position on the disk and thickness equal to each hole's diameter. The patterns may or may not partially overlap, depending on the exact construction of the disk. A lens projects an image of the scene in front of it directly onto the disk.<ref>{{cite web |url=http://users.swing.be/philippe.jadin/nipkowdisk.htm |title=The Nipkow disk |publisher=Users.swing.be |date= |accessdate=2010-03-02 |deadurl=yes |archiveurl=https://www.webcitation.org/6BV693iba?url=http://users.swing.be/philippe.jadin/nipkowdisk.htm |archivedate=2012-10-18 |df= }}</ref> 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 and capturing of [[motion (physics)|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.
== Advantages ==
One of the advantages of using a Nipkow disk is that the image [[sensor]] (that is, the device converting light to electric signals) can be as simple as a single [[photocell]] or [[photodiode]], since at each instant only a very small area
▲One of the advantages of using a Nipkow disk is that the image [[sensor]] (that is, the device converting light to electric signals) can be as simple as a single [[photocell]] or [[photodiode]], since at each instant only a very small area (a [[pixel]]) is visible through the disk (and viewport), and so decomposing an image into lines is done almost by itself with little need for scanline timing, and very high scanline [[Angular resolution|resolution]]. A simple acquisition device can be built by using an electrical motor driving a Nipkow disk, a small box containing a single light-sensitive (electric) element and a conventional image focusing device (lens, [[dark box]], etc.).
Another advantage is that the receiving device is very similar to the acquisition device, except that the light-sensitive device is replaced by a
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== 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.
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So the ideal Nipkow disk should have either a very large diameter, which means smaller [[curvature]], or a very narrow [[angle|angular]] opening of its viewport. Another way to produce acceptable images would be to drill smaller holes (millimeter or even [[micrometre|micrometer]] scale) closer to the outer sectors of the disk, but technological evolution favoured [[electronics|electronic]] means of image acquisition.
Another
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.
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[[Iconoscope]]s (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 &
== Applications ==
Apart from the aforementioned mechanical television, which
▲Apart from the aforementioned mechanical television, which never became popular for the practical reasons mentioned above, a Nipkow disk is used in one type of [[confocal microscope]], a powerful [[microscope|optical microscope]].
== References ==
{{Reflist}}
== External links ==
* [http://zeiss-campus.magnet.fsu.edu/articles/spinningdisk/introduction.html Introductory article on Spinning Disk Microscopy]
* [
* [http://www.teletronic.co.uk/pioneers.htm The Invention of Television: Early Pioneers]
* [https://web.archive.org/web/20091026194656/http://geocities.com/davidvwilliamson/nipkov.html Nipkov disc]
[[Category:Television technology]]
[[Category:History of television]]▼
[[Category:German inventions]]
[[Category:1885 in science]]
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