Nipkova Disk

The device is a simple rotating disk of any opaque material (metal, plastic, cardboard, etc.) with a series of holes of the same diameter at an equal angular distance from each other.

The holes are arranged in a spiral in one revolution, starting from the outer edge of the disc and ending in the center, as is done in a gramophone record . When the disk rotates, the holes move along circular paths, depending on the location of a particular hole on the disk.

These paths may partially overlap in some disk designs.

Basically, the Nipkova disk is used in the construction of mechanical TVs both when scanning an image and for displaying it. The lens in front of the disc projects the image of the subject directly onto the disc ^[4] . Each hole of the spiral during movement forms an almost horizontal (in a separate section of the disk) hole through which light passes from a certain section of the object and enters the photodetector. If this receiver is connected to a light source (in practice, neon lamps were often used, and nowadays super-bright LEDs ) are placed behind the second Nipkov disk, rotating at the same speed and direction as the first, then you can see the original image reproduced line by line.

If you observe an object through a rotating Nipkova disk, through a relatively small sector (no more than 90 °), you will notice that the visible object is scanned line by line from top to bottom. Typically, the disk is almost completely covered by an opaque material, leaving only a hole in the form of a sector of the disk or rectangular for viewing. With very fast disk rotation, the observed object can be seen completely.

Since a limited number of holes can be placed on the disk, the resolution of the image transmitted using the disk was quite low - most often about 30 lines, occasionally up to 120. There were several decomposition standards that used a scan of up to 200 lines. One of these high-resolution systems (180 lines) was used in Canada by Peck Television at the VE9AK station.

One of the few advantages of the Nipkova disk is that the photodetector located behind the disk can be quite simple, for example, one photoresistor or a photodiode . This advantage follows from the principle of operation of the disk - at any given moment in time, only one point ( pixel ) passes through the disk and the image is decomposed into separate lines automatically, and with a fairly high horizontal resolution.

The simplest device for scanning an image can be assembled from an engine rotating a Nipkov disk, a small container with one photoelectric element and a conventional lens for projecting an image.

Another advantage of devices using the Nipkova disk is the similarity of a device for acquiring an image (camera) and a device for displaying an image. In fact, they differ only in the element located behind the disk: in the first case it is a photoelectric element, in the second - a light source controlled by the camera. Of course, in addition to this, means for synchronizing disk rotation are desirable (from manual tuning to electronic circuits).

Thanks to its merits, Nipkova’s disk formed the basis for the design of John Byrd’s mechanical television in the 1920s .

Unlike horizontal resolution, which is potentially very high for Nipkow disks, vertical resolution is limited by the total number of holes on the disk, which are usually from 30 to 100, less often up to 200.

Another serious drawback was the small size of the reproduced images, which in height was no more than the width of the surface of the disk used for scanning. In practice, in mechanical television, a disc with a diameter of 30-40 cm was used to reproduce an image the size of a postage stamp.

Any hole, even on a relatively small portion of the visible screen, does not move horizontally, but along a radial path. This is the cause of geometric distortions of the transmitted image, which is also a drawback of the Nipkova disk. Part of this problem can be solved using disks of a sufficiently large diameter, or by reducing the screen size - in this case, the curvature of the trajectories will decrease. Another solution to the problem is to make the holes in the disk smaller and closer to the outer edge of the disk.

In fact, Nipkova discs used in the first televisions had a diameter of 30-50 cm and 30-50 holes. Devices that used disks were noisy, heavy. Image quality was very poor with frequent flickering.

For the transmitting side, the situation was not better - due to the low sensitivity of the used photovoltaic cells, very strong illumination of the subject was required.

In addition to the already mentioned mechanical television, Nipkova discs are used in powerful optical microscopes - confocal microscopes .

Sometimes miniature and high-speed discs are used in high-speed photography .

• Antonina Shmelyova. Where did the Shchepanik telescope disappear? (Russian) // Technique - Youth : Journal. - 1980. - No. 9 . - S. 48-49 . - ISSN 0320-331X . (inaccessible link)
• B. Schaefer “Home-made TV” Detizdat, 1937, 34 pp .

• Biography of Paul Nipkov , as well as a description, drawings and diagrams of Nipkov's disk (English)
• The Invention of Television: Early Pioneers