A flying-spot scanner (FSS) uses a scanning source of a spot of light, such as a high-resolution, high-light-output, low-persistence cathode ray tube (CRT), to scan an image. Usually the image to be scanned is on photographic film, such as motion picture film, or a slide or photographic plate. The output of the scanner is usually a television signal.
In the case of the CRT-based scanner, as an electron beam is drawn across the face of the CRT it creates a scan that has the correct number of lines and aspect ratio for the format of the signal. The image of this scan is focused with a lens onto the film frame. Its light passes through the image being scanned and is converted to a proportional electrical signal by photomultiplier tube(s), one for each color (red, green, blue), that detects the variations in intensity of the beam spot as it scans across the film, and are converted to proportional electrical signals, one for each of the color channels.
Telecines that use a monochrome CRT as the light source can be referred to as flying-spot scanners. The advantage of the FSS technique is that as colour analysis is done after scanning; simple dichroics may be used to split the light to each photomultiplier —and there are no registration errors, as would have been introduced by early electronic cameras.
Historically, flying-spot scanners were also used as primitive live-action studio cameras at the dawn of electronic television, in the 1920s. A projector equipped with a spinning perforated Nipkow disc created the spot that scanned the stage. Scanning a subject this way required a completely dark stage, and was impractical for production use, but gave early researchers a way to generate live images before practical imaging pickup tubes were perfected.
Flying-spot scanner technology was later implemented by DuMont Laboratories in the Vitascan color television system, released in 1956. Vitascan produced NTSC color video using a "camera" that acted in reverse by housing the flying-spot CRT which was projected through the "camera"'s lens and illuminated the subject in a special light-tight studio. The light from the CRT "camera" was then picked up by special "scoops" housing 4 photomultiplier tubes (2 for red, 1 for green, and 1 for blue), which then would provide video of the talent in the studio. Unlike earlier FSS systems that relied on the studio being entirely darkened, Vitascan used a special strobe light would illuminate the studio for the talent's convenience, and would turn on during the photomultiplier scoop's blanking interval pulses, so as not to interfere with the scanning.
- Science Newsletter, April 16, 1927 (reproduced at Science News Online) "How New Television Process Works" with Gray's flying-spot scanner innovation