Nipkow disk

The Nipkow disk is a mechanical,geometrically operating image scanning device (by itself, it performs neither image acquisition or reproduction ) invencted by Paul Gottlieb Nipkow, which was primarily used as a fundamental component in mechanical television.

Table of contents

1 Physical Description
2 How it works
3 Usage and Applications
4 Disadvantages
5 Applications

Physical Description

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 trace form a single-turn spiral starting from an external radial point of the disk and proceding to the center of the disk, much like a vinyl 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.

How it works

When spinning the disk while observing an object "through" the disk, preferably through a relatively small circular sector of the disk (from here on called a "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 would be that of drilling smaller holes (millimeter or even micrometer scale) closer to the outer sectors of the disk, but technological evolution privileged electronic means of image acquisition.

Usage and Applications

One of the few, in not the only advantage of using the Nipkow Disk is that the image sensor (that is, the device converting light to electric signals) can be very simple, as simple as a single photocell or photodiode, since 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 almost no need for scanline timing, and almost infinite (or rather, very high) scanline resolution. A simple acquisition device can be built by using an electrical motor driving the 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, only that 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 by means of the Scottish invenctor John Logie Baird, as well as the first "TV-Enthusiasts" communities and even experimental image radio broadcasts, back in the 20s!

Disadvantages

Unlike the line resolution provided by the 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 are typically very small, as small as the surface used for scanning, and which on the practical implementations of mechanical television was typically postage-sized, for a 30 to 50 cm sized 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 to 50 centimeter 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.

Applications

Apart from the aforementioned mechanical television, the Nipkow Disk has had little other practical use. This doesn't mean it's useless, as it's still used sometime in the field of high speed photography, although in miniaturized and very high speed versions.