A Nipkow disk also known as scanning disk, is a mechanical, geometrically operating image scanning device, invented by Paul Gottlieb Nipkow

Paul Gottlieb Nipkow

Paul Julius Gottlieb Nipkow was a German technician and inventor.-Beginnings:Nipkow was a German of born in Lauenburg in Pomerania. While at school in Neustadt , West Prussia, Nipkow experimented in telephony and the transmission of moving pictures. After graduation, he went to Berlin in order to...

. This scanning disk was a fundamental component in mechanical television

Mechanical television

Mechanical television was a broadcast television system that used mechanical or electromechanical devices to capture and display video images. However, the images themselves were usually transmitted electronically and via radio waves...

 through the 1920s.

Design

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

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 drilled in it. The holes may also be square for greater precision.

These holes are positioned to form a single-turn spiral

Spiral

In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...

 starting from an external radial point of the disk and proceeding to the center of the disk, much like a gramophone record

Gramophone record

A gramophone record, commonly known as a phonograph record , vinyl record , or colloquially, a record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove...

. The holes, when the disk rotates, trace circular ring surfaces, with inner and outer diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 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.

Process

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 a light is controlled 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

Circular sector

A circular sector or circle sector, is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. In the diagram, θ is the central angle in radians, r the radius of the circle, and L is the arc length of the...

 of the disk (the viewport

Viewport

A viewport is a rectangular viewing region in computer graphics, or a term used for optical components. It has several definitions in different contexts:- Computing :...

), 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

Cinematography

Cinematography is the making of lighting and camera choices when recording photographic images for cinema. It is closely related to the art of still photography...

, and capturing of motion

Motion (physics)

In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...

 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 curve

Curve

In mathematics, a curve is, generally speaking, an object similar to a line but which is not required to be straight...

s.
So the ideal Nipkow disk should have either a very large diameter, which means smaller curvature

Curvature

In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...

, or a very narrow angular

Angle

In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...

 opening of its viewport. Another way to produce acceptable images would be to drill smaller holes (millimeter or even micrometer

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 scale) closer to the outer sectors of the disk, but technological evolution has favoured electronic

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 means of image acquisition.

Advantages

One of the few advantages of using a Nipkow disk is that the image sensor

Sensor

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

 (that is, the device converting light to electric signals) can be as simple as a single photocell or photodiode

Photodiode

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....

, since at each instant only a very small area (a pixel

Pixel

In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

) 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

Timing

Timing is the time when something happens or the spacing of events in time. Some typical uses are:* The act of measuring the elapsed time of something or someone, often at athletic events such as swimming or running, where participants are timed with a device such as a stopwatch...

, and very high scanline resolution

Angular resolution

Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

. 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

Mechanical television

Mechanical television was a broadcast television system that used mechanical or electromechanical devices to capture and display video images. However, the images themselves were usually transmitted electronically and via radio waves...

, the Radiovision

Radiovision

Radiovision can refer to:* An early name used by Charles Francis Jenkins for technology now known as television.* The name of a BBC radio educational program for schools that was distributed with slides and films...

, accomplished by the Scottish inventor John Logie Baird

John Logie Baird

John Logie Baird FRSE was a Scottish engineer and inventor of the world's first practical, publicly demonstrated television system, and also the world's first fully electronic colour television tube...

, as well as the first "TV-Enthusiasts" communities and even experimental
image radio broadcasts in the 1920s.

Disadvantages

The resolution along a Nipkow disk's scanline is potentially very high, being in effect 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 comprised between 30 and 100, with rare 200-hole disks tested.

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

Mechanical television

Mechanical television was a broadcast television system that used mechanical or electromechanical devices to capture and display video images. However, the images themselves were usually transmitted electronically and via radio waves...

, 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

Image dissector

An image dissector, also called a dissector tube, is a video camera tube in which photocathode emissions create an "electron image" which is then scanned to produce an electrical signal representing the visual image...

. 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 incident energy.

Iconoscope

Iconoscope

The Iconoscope was the name given to an early television camera tube in which a beam of high-velocity electrons scans a mosaic of photoemissive isolated granules...

s (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 scanner.

The scanning disk can be replaced by a polygonal mirror, but this suffers from the same problem — lack of integration over time.

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

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

. It is also sometimes used in the field of high speed photography

High speed photography

High speed photography is the science of taking pictures of very fast phenomena. In 1948, the Society of Motion Picture and Television Engineers defined high-speed photography as any set of photographs captured by a camera capable of 128 frames per second or greater, and of at least three...

, although in miniaturised and very high speed versions.

External links

• http://zeiss-campus.magnet.fsu.edu/articles/spinningdisk/introduction.html, 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 for experimentation.
• "Will 'camera-boxes' help catch Whitechapel Ripper?" A fictional piece about the use of Nipkow Disks in 1888 London, at Skeptic Friends Network.