Early research The first mechanical raster scanning techniques were developed in the 19th century for
facsimile, the transmission of still images by wire.
Alexander Bain introduced the facsimile machine in 1843 to 1846.
Frederick Bakewell demonstrated a working laboratory version in 1851. The first practical facsimile system, working on telegraph lines, was developed and put into service by
Giovanni Caselli from 1856 onward.
Willoughby Smith discovered the
photoconductivity of the element
selenium in 1873, laying the groundwork for the
selenium cell, which was used as a pickup in most mechanical scan systems. As a 23-year-old German university student,
Paul Julius Gottlieb Nipkow proposed and patented the
Nipkow disk in 1884. This was a spinning disk with a spiral pattern of holes in it, so each hole scanned a line of the image. Although he never built a working model of the system, Nipkow's spinning-disk
image rasterizer was the key mechanism used in most mechanical scan systems in both the transmitter and receiver. In 1885,
Henry Sutton in
Ballarat, Australia designed what he called a
telephane for transmission of images via telegraph wires, based on the
Nipkow spinning disk system,
selenium photocell,
Nicol prisms and
Kerr effect cell. Sutton's design was published internationally in 1890. An account of its use to transmit and preserve a still image was published in the
Evening Star in Washington in 1896.
Constantin Perskyi had coined the word
television in a paper read to the International Electricity Congress at the
International World Fair in
Paris on August 24, 1900. Perskyi's paper reviewed the existing electromechanical technologies, mentioning the work of Nipkow and others. The first demonstration of the
instantaneous transmission of images was made by a German physicist
Ernst Ruhmer, who arranged 25 selenium cells as the picture elements for a television receiver. In late 1909 he announced the transmission of simple images over a telephone wire from the
Palace of Justice at Brussels to the city of
Liège in Belgium, a distance of . This announced demonstration was described at the time as "the world's first working model of television apparatus". The limited number of elements meant his device was only capable of representing simple geometric shapes, and the cost was very high; at a price of £15 (US$45) per selenium cell, he estimated that a 4,000 cell system would cost £60,000 (US$180,000), and a 10,000 cell mechanism capable of reproducing "a scene or event requiring the background of a landscape" would cost £150,000 (US$450,000). Ruhmer expressed the hope that the 1910 Brussels would sponsor the construction of an advanced device with significantly more cells, as a showcase for the exposition. However, the estimated expense of £250,000 (US$750,000) proved to be too high. The publicity generated by Ruhmer's demonstration spurred two French scientists, Georges Rignoux and A. Fournier in Paris, to announce similar research that they had been conducting. A matrix of 64
selenium cells, individually wired to a mechanical
commutator, served as an electronic
retina. In the receiver, a type of
Kerr cell modulated the light and a series of variously angled mirrors attached to the edge of a rotating disc scanned the modulated beam onto the display screen. A separate circuit regulated synchronization. The resolution in this proof-of-concept demonstration was just sufficient to clearly transmit individual letters of the alphabet. An updated image was transmitted "several times" each second. In 1911,
Boris Rosing and his student
Vladimir Zworykin created a system that used a mechanical mirror-drum scanner to transmit, in Zworykin's words, "very crude images" over wires to the Braun tube (
cathode-ray tube or CRT) in the receiver. Moving images were not possible because, in the scanner, "the sensitivity was not enough and the selenium cell was very laggy".
Television demonstrations . This schematic shows the circular paths traced by the holes, which may also be square for greater precision. The area of the disk outlined in black shows the region scanned. It was the 1907 invention of the first
amplifying vacuum tube, the
triode, by
Lee de Forest, that made the design practical. Scottish inventor
John Logie Baird in 1925 built some of the first prototype video systems, which employed the
Nipkow disk. On March 25, 1925, Baird gave the first public demonstration of televised
silhouette images in motion, at
Selfridge's Department Store in London. Since human faces had inadequate contrast to show up on his primitive system, he televised a ventriloquist's dummy named "Stooky Bill" talking and moving, whose painted face had higher contrast. By January 26, 1926, he demonstrated the transmission of images of a face in motion by radio. This is widely regarded as being the world's first public television demonstration. Baird's system used the
Nipkow disk for both scanning the image and displaying it. A brightly illuminated subject was placed in front of a spinning Nipkow disk set with lenses that swept images across a static photocell. The
thallium sulfide (thalofide) cell, developed by
Theodore Case in the USA, detected the light reflected from the subject and converted it into a proportional electrical signal. This was transmitted by AM radio waves to a receiver unit, where the video signal was applied to a neon light behind a second Nipkow disk rotating synchronized with the first. The brightness of the neon lamp was varied in proportion to the brightness of each spot on the image. As each hole in the disk passed by, one
scan line of the image was reproduced. Baird's disk had 30 holes, producing an image with only 30 scan lines, just enough to recognize a human face. In 1927, Baird transmitted a signal over of telephone line between London and
Glasgow. In 1928, Baird's company (Baird Television Development Company/Cinema Television) broadcast the first transatlantic television signal between London and New York, and the first shore-to-ship transmission. In 1929, he became involved in the first experimental mechanical television service in Germany. In November of the same year, Baird and
Bernard Natan of
Pathé established France's first television company, Télévision-
Baird-Natan. In 1931, he made the first outdoor remote broadcast of
The Derby. In 1932, he demonstrated
ultra-short wave television. Baird's mechanical system reached a peak of 240 lines of resolution on
BBC television broadcasts in 1936 though the mechanical system did not scan the televised scene directly. Instead, a
17.5 mm film was shot, rapidly developed and then scanned while the film was still wet. An American inventor,
Charles Francis Jenkins also pioneered television. He published an article on "Motion Pictures by Wireless" in 1913, but it was not until December 1923 that he transmitted moving silhouette images for witnesses, and it was on June 13, 1925, that he publicly demonstrated synchronized transmission of silhouette pictures. In 1925, Jenkins used
Nipkow disk and transmitted the silhouette image of a toy windmill in motion, over a distance of from a naval radio station in Maryland to his laboratory in Washington, D.C., using a lensed disk scanner with a 48-line resolution. He was granted the U.S. patent No. 1,544,156 (Transmitting Pictures over Wireless) on June 30, 1925 (filed March 13, 1922). On December 25, 1926,
Kenjiro Takayanagi demonstrated a television system with a 40-line resolution that employed a Nipkow disk scanner and
CRT display at Hamamatsu Industrial High School in Japan. This prototype is still on display at the Takayanagi Memorial Museum in
Shizuoka University, Hamamatsu Campus. By 1927, he improved the resolution to 100 lines, which was unrivaled until 1931. By 1928, he was the first to transmit human faces in half-tones. His work had an influence on the later work of
Vladimir K. Zworykin. In Japan, he is viewed as the man who completed the first all-electronic television. His research in creating a production model was halted by the US after Japan lost
World War II. In 1928,
General Electric launched their own experimental television station
W2XB, broadcasting from the GE plant in Schenectady, New York. The station was popularly known as
WGY Television, named after the GE-owned radio station
WGY. The station eventually converted to an all-electronic system in the 1930s and in 1942, received a commercial license as
WRGB. The station is still operating today. Meanwhile, in the
Soviet Union,
Léon Theremin had been developing a mirror drum-based television, starting with 16 lines resolution in 1925, then 32 lines and eventually 64 using
interlacing in 1926, and as part of his thesis on May 7, 1926, he electrically transmitted and then projected near-simultaneous moving images on a square screen. A few systems ranging into the 200-line region also went on the air. 180-lines broadcast tests were carried out by the
Reichs-Rundfunk-Gesellschaft in 1935, with a transmitter in
Berlin. Transmissions lasted 90 minutes a day, three days a week, with sound/visions frequencies being . Likewise, a 180-line system that Compagnie des Compteurs (CDC) installed in
Paris was tested in 1935, and a 180-line system by
Peck Television Corp. started in 1935 at station VE9AK in
Montreal, Quebec, Canada.
Color television ) can just be seen through the lens on the right. John Baird's 1928 color television experiments had inspired Goldmark's more advanced
field-sequential color system. The CBS
color television system invented by
Peter Goldmark used such technology in 1940. In Goldmark's system, stations transmit color saturation values electronically; however, mechanical methods are also used. At the transmitting camera, a mechanical disc filters hues (colors) from reflected studio lighting. At the receiver, a synchronized disc paints the same hues over the CRT. As the viewer watches pictures through the color disc, the pictures appear in full color. Later, simultaneous color systems superseded the CBS-Goldmark system, but mechanical color methods continued to find uses. Early color sets were very expensive: over $1,000 in the money of the time. Inexpensive adapters allowed owners of black-and-white
NTSC television sets to receive color telecasts. The most prominent of these adapters is Col-R-Tel, a 1955 NTSC to field-sequential converter. This system operates at NTSC scanning rates but uses a disc like the obsolete CBS system had. The disc converts the black-and-white set to a field-sequential set. Meanwhile, Col-R-Tel electronics recovers NTSC color signals and sequences them for disc reproduction. The electronics also synchronize the disc to the NTSC system. In Col-R-Tel, the electronics provide the saturation values (chroma). These electronics cause chroma values to superimpose over brightness (luminance) changes of the picture. The disc paints the hues (color) over the picture. A few years after Col-R-Tel, the
Apollo Moon missions also adopted field-sequential techniques. The lunar color cameras all had color wheels. These
Westinghouse and later
RCA cameras sent field-sequential color television pictures to Earth. The Earth receiving stations included electronic equipment that converted the raw color video signals into the NTSC standard.
Decline The advancement of
vacuum tube electronic television (including
image dissectors and other camera tubes and CRTs for the reproducer) marked the beginning of the end for mechanical systems as the dominant form of television. Mechanical TV usually only produced small images. It was the main type of TV until the 1930s. Vacuum tube television, first demonstrated in September 1927 in
San Francisco by
Philo Farnsworth, and then publicly by Farnsworth at the
Franklin Institute in
Philadelphia in 1934, was rapidly overtaking mechanical television. Farnsworth's system was first used for broadcasting in 1936, reaching 400 to more than 600 lines with fast field scan rates, along with competing systems by
Philco and
DuMont Laboratories. In 1939,
RCA paid Farnsworth $1 million for his patents after ten years of litigation, and RCA began demonstrating all-electronic television at the
1939 World's Fair in
New York City. The last mechanical television broadcasts ended in 1939 at stations run by a handful of public universities in the United States.
'Scophony' mechanical display receiver Early
Cathode-Ray Television tube displays were small in size. The 'Scophony' television receiver of 1938, an advanced television receiver that used a mechanical display, was capable of displaying a 405-line picture (compatible with the then
405-line television system used in the United Kingdom) on a display that was wide and high. A version intended for theater audiences had a wide display. It was also capable of being set up for the US
441-line television system. For 405 lines, it used a high-speed scanner running at and a low-speed mirror drum running at around , in conjunction with a
Jeffree cell to modulate a focused light beam from a
mercury lamp. It used 39 vacuum tubes in its electronic circuits and consumed around . Although it produced impressive results and reached the marketplace, the receiver was very expensive, costing around twice as much as a cathode-ray television. It was not a commercial success, and television transmissions in the UK were suspended for the duration of the Second World War, sealing its fate. No complete receiver survives, although some components do.
Modern applications of mechanical scanning Since the 1970s, some
amateur radio enthusiasts have experimented with mechanical systems. The early light source of a
neon lamp has now been replaced with super-bright
LEDs. There is some interest in creating these systems for
narrow-bandwidth television, which would allow a small or large moving image to fit into a channel less than wide (modern TV systems usually have a channel about wide, 150 times larger). Also associated with this is
slow-scan TV – although that typically used electronic systems utilising the P7 CRT until the 1980s and
PCs thereafter. There are three known mechanical monitor forms: two fax printer-like monitors made in the 1970s, and in 2013 a small drum monitor with a coating of glow paint where the image is painted on the rotating drum with a
UV laser.
Digital light processing (DLP) projectors use an array of tiny ()
electrostatically-actuated mirrors selectively reflecting a light source to create an image. Many low-end DLP systems also use a
color wheel to provide a sequential color image, a feature that was common on many early color television systems before the
shadow mask CRT provided a practical method for producing a simultaneous color image. Another place where high-quality imagery is produced by opto-mechanics is the
laser printer, where a small rotating mirror is used to deflect a modulated laser beam in one axis while the motion of the
photoconductor provides the motion in the other axis. A modification of such a system using high-power lasers is used in laser video projectors, with resolutions as high as 1,024 lines and each line containing over 1,500 points. Such systems produce, arguably, the best quality video images. They are used, for instance, in
planetariums. Mechanical techniques are also used in long-wave
infrared cameras used in military applications such as night vision for fighter pilots. These cameras use a high-sensitivity infrared photo receptor (usually cooled to increase sensitivity), but instead of conventional lenses, these systems use rotating prisms to provide a 525 or 625-line standard video output. The optical parts are made from germanium because glass is opaque at the wavelengths involved. Similar cameras have also found a role in sporting events where they are able to show (for example) where a ball has struck a bat.
Laser lighting display techniques are combined with computer
emulation in the LaserMAME project. It is a
vector-based system, unlike the
raster displays thus-far described.
Laser light reflected from computer-controlled mirrors traces out images generated by classic arcade software, which is executed by a specially modified version of the
MAME emulation
software. == Technical aspects ==