A Nipkow disk (sometimes Anglicized as Nipkov disk), also known as scanning disk, is a mechanical, geometrically operating image scanning device, invented by Paul Gottlieb Nipkow Paul Julius Gottlieb Nipkow B was a German technician and inventor. This scanning disk was a fundamental component in mechanical television Mechanical television was a television system that used mechanical or electromechanical devices to capture and display images. However, the images themselves were usually transmitted electronically and via radio waves. The reason for this dual nature of mechanical television lies in the history of technology. Mechanical television mechanics came through the 1920s.
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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 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. The word "diameter" derives from Greek διάμετρος , "diagonal of a circle", from δια- (dia-), "across, through& drilled in it. The holes may also be square for greater precision.[citation needed]
These holes are positioned to form a single-turn spiral In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point starting from an external radial point of the disk and proceeding to the center of the disk, much like a gramophone record A gramophone record, commonly known as phonograph record , vinyl record (when made of polyvinyl chloride), or simply record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove. The groove usually starts near the periphery and ends near the centre of the disc. Phonograph records are generally. The holes, when the disk rotates, trace circular ring surfaces, with inner and outer 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. The word "diameter" derives from Greek διάμετρος , "diagonal of a circle", from δια- (dia-), "across, through& 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.[1] 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 A circular sector or circle sector, is the portion of a circle enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. Its area can be calculated as described below 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 Cinematography , is the making of lighting and camera choices when recording photographic images for the cinema. It is closely related to the art of still photography. Many additional issues arise when both the camera and elements of the scene may be in motion, though this also greatly increases the creative possibilities of the process, and capturing of motion In physics, motion is change of location or position of an object with respect to time. Change in motion is the result of an applied force. Motion is typically described in terms of velocity also seen as speed, acceleration, displacement, and time. An object's velocity cannot change unless it is acted upon by a force, as described by Newton's 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 In mathematics, a curve is, generally speaking, an object similar to a straight line but which is not required to be straight. Often curves in two-dimensional or three-dimensional (space curves) Euclidean space are of interest. So the ideal Nipkow disk should have either a very large diameter, which means smaller 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. There is a key distinction between extrinsic, or a very narrow angular In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. The magnitude of the angle is the "amount of rotation" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide with the other opening of its viewport. Another way to produce acceptable images would be to drill smaller holes (millimeter or even micrometer A micrometre is one millionth of a metre, or equivalently one thousandth of a millimetre or one thousand nanometres. It can also be written in scientific notation as 1×10−6 m, meaning 1⁄1000000 m scale) closer to the outer sectors of the disk, but technological evolution has favoured electronic Electronics is the branch of science and technology which makes use of the controlled motion of electrons through different media and vacuum. The ability to control electron flow is usually applied to information handling or device control. Electronics is distinct from electrical science and technology, which deals with the generation, means of image acquisition.
Advantages
One of the few advantages of using a Nipkow disk is that the image 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 glass tube. A thermocouple converts temperature to an (that is, the device converting light to electric signals) can be as simple as a single photocell Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties: or photodiode A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation, since at each instant only a very small area (a pixel In digital imaging, a pixel is a single point in a raster image. The pixel is the smallest addressable screen element; it is the smallest unit of picture that can be controlled. Each pixel has its own address. The address of a pixel corresponds to its coordinates. Pixels are normally arranged in a 2-dimensional grid, and are often represented) 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 Angular resolution or 'spatial resolution' describes the resolving power of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye. 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 was a television system that used mechanical or electromechanical devices to capture and display images. However, the images themselves were usually transmitted electronically and via radio waves. The reason for this dual nature of mechanical television lies in the history of technology. Mechanical television mechanics came, the Radiovision, accomplished by the Scottish inventor John Logie Baird John Logie Baird was a Scottish engineer and inventor of the world's first working television system, also the world's first fully electronic colour television broadcast. Although Baird's electromechanical system was eventually displaced by purely electronic systems (such as those of Vladimir Zworykin and Philo Farnsworth), his early successes, as well as the first "TV-Enthusiasts" communities and even experimental image radio broadcasts in the 1920s.
Disadvantages
The resolution along a scanline provided by a Nipkow disk is potentially very high, being virtually an "analogue" scan.
However 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 lay with reproducing images at the receiver's end of the transmission, also via the use of a Nipkow disk. The images were typically very small, as small as the surface used for scanning, and which on the practical implementations of mechanical television Mechanical television was a television system that used mechanical or electromechanical devices to capture and display images. However, the images themselves were usually transmitted electronically and via radio waves. The reason for this dual nature of mechanical television lies in the history of technology. Mechanical television mechanics came 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 receivers 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 much better regarding the acquisition part of the system, which also required very powerful lighting of the subject.
Disk scanners share a major limitation with the Farnsworth Image Dissector. Light is only conveyed into the sensing system as the small aperture scans over the entire field of view. The actual amount of light gathered is instantaneous, though a very small aperture, and the net yield is only hundredths or thousandths of the incident energy.
Iconoscopes (and their successors) continuously accumulate energy on the target, 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 equal-sensitivity photosensitive receptors, the iconoscope is naturally hundreds to thousands of times more sensitive than the disk or Farnsworth scanners.
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 A microscope is an instrument to see objects too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope. It is also sometimes used in the field of 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 consecutive frames. High speed photography can be, although in miniaturised and very high speed versions.
References
- ^ "The Nipkow disk". Users.swing.be. http://users.swing.be/philippe.jadin/nipkowdisk.htm. Retrieved 2010-03-02.
External links
- [1], 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.
Categories: Television technology Categories: Broadcast engineering | Television | Electronic engineering | History of television | Film and video technology | History of television
