Nipkow disk

Scheme of the Nipkow disk

The Nipkow disk is the basis of the " electrical telescope " invented and named by Paul Nipkow (1860–1940) , an early form of television. The patent with the number 30105 was published by the Imperial Patent Office on January 15, 1885 and retrospectively dated January 6, 1884. The spiral perforated disk proposed by Paul Nipkow had 24 holes, which should write a picture with 24 lines. With their help, it could break down images into light-dark signals and put them back together again. To do this, the rotating disk moves line by line past the image (when dismantling) or the projection surface (when combining).

Scheme of the electric telescope: a scene is scanned line by line through a mask of a rotating Nipkow disk and converted into an electrical signal. This signal, connected to a lamp behind the mask of a second synchronously rotating Nipkow disk, generates the image again.

technology

functionality

The holes of the Nipkow disk are made along concentric circles. The individual holes thus scan one hole width from the outside to the inside. So that there is always only one hole within the interesting section, there may only be one hole in a circle segment. In other words, the disk is divided into as many segments of a circle as it has to scan lines. By doing this, the image is scanned sequentially. As a result, images created with a Nipkow disc can be recognized (e.g. on old photos) by the slightly curved lines. The geometric solution to the challenge of getting only one point with a constant distance Δr from the center is achieved with an Archimedean spiral . This achieves the homogeneity of the image, i.e. the uniform distribution of the scanned points per area.

Use at high resolutions

In 1928, John Logie Baird succeeded in England for the first time in transmitting a 30-line color image with the Nipkow disk and RGB color filters.

As the number of lines increased, the Nipkow disk proved to be less and less useful, as the following consideration shows:

The mechanical difficulties were also the minor ones. The real problem was the extremely low light output. It could be calculated that even with studio recordings with an illumination of 70,000 lux one could only expect a luminance that was four orders of magnitude lower than that of chemical film. 70,000 lux, the maximum value that occurs outdoors, was no longer practicable for permanent studio lighting.

Mechanical scanning reached its technical limit with 441 lines as early as 1939, and it was already very costly. The higher number of lines in the 1940s could only be achieved by purely electronic scanning.

However, the low resolution had the advantage that it only required a low bandwidth; TV pictures could even be broadcast over medium or short wave. In amateur radio , mechanical television , mainly implemented with Nipkow disks due to its simplicity, was therefore able to survive in the form of narrow bandwidth television as a niche application.

Improved Nipkow disks

When recording TV

Very early on, attempts were made to circumvent the technical limits of the Nipkow disk. John Logie Baird built a Nipkow disk as early as 1927, in which the holes were exchanged for significantly larger lenses in order to achieve a greater light output. Since this design made the device much more expensive, especially for the receiver, and the neon lamps became better and stronger, this design was rarely found.

Attempts to use an endless belt instead of a disk were quickly discarded due to the high mechanical stress on the belt.

For higher definition television (180 lines and more), several rows of holes were made on the Nipkow disk, at the same time a screen rotated with the Nipkow disk so that only one hole was free at any one time. In this way, the disk could be made much smaller. Since the number of revolutions had to be increased accordingly, the Nipkow disk was in a vacuum container.

In film scanning

From 1938, so-called perforated rings or lens rings were used specifically for film scanning. In terms of functionality, these are also Nipkow disks, but with the difference that the scanner was drum-shaped and the holes were punched into the side. This eliminates the typical curvature of the image lines generated with a Nipkow disk. At the same time, it was possible to get by with very few holes, which were only made on one level. The film was not jerked like in a cinema projector, but moved smoothly in front of the perforated wreath , so that the image was scanned line by line in this way. In this way it was even possible to scan films in the standard of that time with 441 lines.

distribution

For low resolution

Between 1928 and 1935, there were ready-made devices and even kits to buy, especially in the USA and Great Britain . The popular “Televisor” with a resolution of 30 lines by Baird cost £ 27 in Great Britain in 1932, for example , in Germany around 1930 a kit was sold for around 30 Reichsmarks . At more than 60 lines, however, the technical limits were reached, where receivers became disproportionately expensive. However, the image quality of only 30 lines is only sufficient for the transmission of portraits, and the images flickered considerably at initially only 12.5 images per second, so that commercial success was not achieved. Up to 1938, Nipkow discs with the German television standard of that time with 180 lines were occasionally used in television stations. After 1938, Nipkow disks were still used in television intercom (a test service for television telephones) and in film scanning, and they were used in high-definition television with 441 lines until the 1940s.

For higher resolution

Images of up to 441 lines could be scanned with rapidly rotating disks in a vacuum. They were used by the Reichspost and the TV station Paul Nipkow .

Alternative mechanical methods

In addition to the Nipkow disc, there were numerous similarly working processes: The Weiller's mirror wheel had mirrors instead of holes, it made it possible to deflect a light beam line by line. This requires a dark studio in which a camera works without a deflection unit. The image points are broken down by the fact that the light beam scans the scene one after the other and thus ensures the image breakdown. The lens wreath scanner works the other way round , here lenses instead of mirrors are attached to the wheel, which ensure the image is broken down in the camera. However, this method requires enormous precision.

The ' mirror screw ' works completely differently , where mirrors arranged in a helix reflect the light from a light source, giving the viewer the impression of a picture. This system was used by the company TeKaDe from 1930 to 1935 up to a resolution of 180 lines. The mirror drum system, also developed by Baird, was much simpler. A mirror is attached to a rapidly rotating drum for each line, each mirror is slightly offset so that the image is scanned line by line. This scanning is much brighter, so that larger images could also be realized, but the maximum resolution is also limited to low-line television.

The system was improved by the company Scophony from England , which ran two mirror drums against each other, one for the horizontal and one for the vertical deflection. Resolutions of up to 405 lines were achieved with this system, allegedly up to 525 lines in test operation in the USA . Although these devices achieved picture quality, brightness and size unmatched in 1939, the receivers were many times more expensive than a television with a cathode ray tube . The development was stopped during the war and not taken up again after the war.

End of the opto-mechanical distraction

As early as 1906, Max Dieckmann succeeded in displaying a television picture with the help of a Braun tube , but a Nipkow disk was used to generate the picture . At the radio exhibition in 1931 Manfred von Ardenne presented a fully electronic television system. The reproduction was made possible by a high-vacuum cathode ray tube ( Braun's tube ) with a 25 cm × 28 cm screen. A second cathode ray tube with constant brightness served as the camera (scanning tube, was also used together with the integrating sphere for color slide and film scanners). In front of this there was a photocell, the signal of which was used to modulate the brightness of the receiver tube. A slide or various objects like scissors were held between the “camera tube” and the photocell. Its excellent quality was unmatched by any mechanical deflection system. So this procedure finally prevailed in 1932/33: At the 12th German Radio Exhibition in 1935, there were 20 different television sets on display, 18 of which were already working electronically, the other two with moving mirrors. In 1938 the last Nipkow disk finally disappeared from German television studios, although mechanical scanners were still used for film scanning until around 1941.

Usage today

Space probes

A special form of the Nipkow disk is used in space travel today: Many space probes use only a single photocell instead of a camera. By rotating around its own axis, an image line is scanned. Since the spacecraft is moving forward at the same time, a complete image can be transmitted. However, this is not television in the true sense of the word, as an "endless image" of the travel route of the probe is transmitted in the manner described, not a series of moving images.

Microscopes

In addition to scanning for image transmission, the Nipkow disk was also used in the invention of the confocal microscope by Mojmir Petran . The so-called Yukokawa disk uses a microlens pattern to focus the collimated illumination radiation . This significantly reduces the bottleneck of the low power, because on the one hand the brilliance of the laser enables high luminance and on the other hand the microlenses greatly increase the efficiency of the optics. In recent years in particular, microscopes with Nipkow disks have been used more and more in confocal microscopy, since they achieve a significantly higher imaging speed than conventional laser scanners with the necessary restriction due to the confocal pinhole diaphragm.

Individual evidence

1. Patent DE30105 : Electric telescope. Registered January 6, 1884 , published January 15, 1885 , applicant: Paul Nipkow.
2. Kirschstein, Krawinkel: Fernsehtechnik , Chapter 3.1: The uselessness of the mechanical image field splitter with high numbers of lines , S. Hirzel Verlag, Zurich, 1952
3. Helmut Schönfelder: TV Technology in Transition , Chapter 1.7: Limits to Mechanical Scanning , Springer-Verlag, Berlin-Heidelberg, 1996
4. Wide Field of View | Yokogawa Europe. Retrieved May 4, 2020 .