page_title: Mechanical television
section_title: Flying spot scanners
hierarchical_section_title: Mechanical television / Technical aspects / Flying spot scanners
caption_attribution_description: English: A flyins spot scanner, an early type of television camera used in mechanical scan television systems during the 1930s and its developer, Ulysis Sanabria. During the 1930s, before modern electronic-scan television broadcasting began after World War 2, hundreds of experimental stations broadcast television using this low resolution mechanical techniques. This example is designed to take "head shots" of performers talking or singing. Instead of a camera, the flying spot scanner projects an intense spot of arc light from the lens at center. The spot scans rapidly over the subject's face in a raster pattern. The light reflected from the subject's face is picked up by the eight phototubes in the dish reflectors around the periphery. The phototubes produce a signal which is a record of the brightness of the image where the spot strikes. This is the video signal which is broadcast to the receiver. Broadcasts during the 1930s had low resolution, typically a 60 line frame at 20 frames per second.
context_page_description: Mechanical television or mechanical scan television is a television system that relies on a mechanical scanning device, such as a rotating disk with holes in it or a rotating mirror drum, to scan the scene and generate the video signal, and a similar mechanical device at the receiver to display the picture. This contrasts with vacuum tube electronic television technology, using electron beam scanning methods, for example in cathode ray tube televisions. Subsequently, modern solid-state liquid-crystal displays are now used to create and display television pictures.
  Mechanical-scanning methods were used in the earliest experimental television systems in the 1920s and 1930s. One of the first experimental wireless television transmissions was by John Logie Baird on October 02, 1925, in London. By 1928 many radio stations were broadcasting experimental television programs using mechanical systems. However the technology never produced images of sufficient quality to become popular with the public.
context_section_description: The most common method for creating the video signal was the "flying spot scanner", developed as a remedy for the low sensitivity that photoelectric cells had at the time. Instead of a television camera that took pictures, a flying spot scanner projected a bright spot of light that scanned rapidly across the subject scene in a raster pattern, in a darkened studio. The light reflected from the subject was picked up by banks of photoelectric cells and amplified to become the video signal.
  In the scanner the narrow light beam was produced by an arc lamp shining through the holes in a spinning Nipkow disk.  Each sweep of the spot across the scene produced a "scan line" of the picture. A single "frame" of the picture was typically made up of 24, 48, or 60 scan lines. The scene was typically scanned 15 or 20 times per second, producing 15 or 20 video frames per second. The varying brightness of the point where the spot fell reflected varying amounts of light, which was converted to a proportionally varying electronic signal by the photoelectric cells. To achieve adequate sensitivity, instead of a single cell, a number of photoelectric cells were used. Like mechanical television itself, flying spot technology grew out of phototelegraphy (facsimile). This scanning method began in the 19th century.
  The BBC television service used the flying spot method until 1935. German television used flying spot methods as late as 1938. This year was by far not the end of flying spot scanner technology. The German inventor Manfred von Ardenne designed a flying spot scanner with a CRT as the light source. In the 1950s, DuMont marketed Vitascan, an entire flying-spot color studio system. Today, graphic scanners still use this scanning method. The flying spot method has two disadvantages:
  Actors must perform in near darkness;
  Flying spot cameras tend to work unreliably outdoors in daylight.
  In 1928, Ray Kell from the United States' General Electric proved that flying spot scanners could work outdoors. The scanning light source must be brighter than other incident illumination.
  Kell was the engineer who ran a 24-line camera that telecast pictures of New York governor Al Smith. Smith was accepting the Democratic nomination for presidency. As Smith stood outside the capital in Albany, Kell managed to send usable pictures to his associate Bedford at station WGY, which was broadcasting Smith's speech. The rehearsal went well, but then the real event began. The newsreel cameramen switched on their floodlights.
  Unfortunately for Kell, his scanner only had a 1 kW lamp inside it. The floodlights threw much more light on Governor Smith. These floods simply overwhelmed Kell's imaging photocells. In fact, the floods made the unscanned part of the image as bright as the scanned part. Kell's photocells couldn't discriminate reflections off Smith (from the AC scanning beam) from the flat, DC light from the floodlamps.
  The effect is very similar to extreme overexposure in a still camera: The scene disappears, and the camera records a flat, bright light. Use the camera in favorable conditions, though, and the picture comes out fine. Similarly, Kell proved that outdoors in favorable conditions, his scanner worked fine.
caption: Flying spot scanner in a television studio in 1931. This type was used for "head shots" of performers speaking, singing or playing instruments. A bright spot of light projected from the lens at center scanned the subject's face, and the light reflected at each point was picked up by the 8 phototubes in the dish-shaped mirrors.
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