Nipkow disk

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A Nipkow disk (sometimes Anglicized as Nipkov disk) is a mechanical, geometrically operating image scanning device, invented by Paul Gottlieb Nipkow. This scanning disk was a fundamental component in mechanical television through the 1920s.

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[edit] Physical description

This schematic shows the circular paths traced by the holes, that may also be square for greater precision.
This schematic shows the circular paths traced by the holes, that may also be square for greater precision.

The device itself is nothing more than 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.

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, much like a gramophone record. The holes, when the disk rotates, trace circular ring surfaces, with inner and outer diameter depending on each hole's position on the disk and thickness equal to each hole's diameter. These surfaces may or may not partly overlap, depending on the exact construction of the disk.

[edit] How it works

A lens projects an image of the scene in front of it directly onto the disk [1]. Each hole in the spiral takes a horizontal "slice" through the image which is picked up as a pattern of light and dark by a sensor. If a light powered by a signal from the sensor is placed behind a second Nipkow disk rotating in synch at the same speed and direction, the image can be reproduced line-by-line, however it remains no larger than the one projected onto the original receiving disk.

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.

[edit] Usage and applications

One of the few 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, the Radiovision, 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.

[edit] Disadvantages

Unlike the line resolution provided by a Nipkow disk, which is potentially very high, the maximum number of scanlines is much more limited, and precisely, it's equal to the number of holes on the disk, which in practice was comprised between 30 and 100, with rare 200-hole disks tested.

Another serious disadvantage when reproducing images with the aid of a Nipkow disk is that the images were typically very small, as small as the surface used for scanning, and which on the practical implementations of mechanical television were the size of a postage-stamp for a 30 to 50 cm diameter disk.

Further disadvantages include the previously illustrated non-linear geometry of the scanned images, and the sheer size of practical implementations of the disk, at least in the past.

In fact, the Nipkow disks used in early TV were roughly 30 cm to 50 cm in diameter, with 30 to 50 "holes". The devices using them were also noisy, heavy and picture quality was very low, with a lot of flickering. Things weren't better regarding the acquisition part of the system, which required very powerful lighting of the subject.

[edit] Applications

Apart from the aforementioned mechanical television, which never took off for the practical reasons mentioned above, a Nipkow disk is used in one type of confocal microscope, a powerful optical microscope. It is also sometimes used in the field of high speed photography, although in miniaturised and very high speed versions.

[edit] External Links

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