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 in 1885 by Paul Gottlieb
Nipkow. This scanning disk was a fundamental component in
mechanical television through the 1920s and 1930s.
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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. 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 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.
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
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 variable light source, driven by the signal provided by the
acquisition device. Some means of synchronizing the disks on the two
devices must also be devised (several options are possible, ranging
from manual to electronic control signals).
These facts helped immensely in building the first mechanical
television accomplished by the Scottish inventor John Logie Baird, as
well as the first "TV-Enthusiasts" communities and even experimental
image radio broadcasts in the 1920s.
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
Another drawback 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 favoured electronic
means of image acquisition.
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
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
The scanning disk can be replaced by a polygonal mirror, but this
suffers from the same problem — lack of integration over time.
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 optical
^ "Das erste deutsche Fernsehpatent von Paul Nipkow". PC Magazin. 30
June 2015. Retrieved 28 April 2017.
^ "The Nipkow disk". Users.swing.be. Archived from the original on
2012-10-18. Retrieved 2010-03-02.
, Introduction Article to Spinning Disk Microscopy*Paul Nipkow,
biography includes a description and drawing of the Nipkow disc.
The Invention of Television: Early Pioneers
Nipkov disc, instructions on creating a cardboard Nipkov disc fo