Revived Spot

Primitive television systems used a "flying spot." A thin beam of light scanned the scene, and its reflected brightness as it crossed light and dark areas was turned by a photosensitive cell into an electrical signal. Changed back into light, the signal produced an image that bore a vague resemblance to the original scene.

Such a system had many failings, most of which came from the clumsiness of the mechanical moving parts. They were replaced by the modern television camera tube, which has no moving parts except a beam of quickly obedient electrons. But the flying spot did not die entirely. Last week Allen B. Du Mont Laboratories described a new system of televising indoor scenes in color by means of a thoroughly modernized flying spot of light.

Educated Floodlight. Du Mont's "Vitascan" system uses what looks like a TV camera with a conventional lens, but instead of taking light in, the "camera" shoots light out, acting like a highly educated floodlight. The source of light behind the lens is a luminescent surface that glows brightly when struck by a hair-thin beam of electrons. The electron beam scans back and forth, covering the whole surface in a small fraction of a second. The spot of light that it creates is projected on the scene by the lens, scanning it like the beam of a small searchlight. There are no moving parts except electrons and light, so the spot can move as fast as necessary.

Reflected light from the flying spot is picked up by a group of phototubes, each of which "sees" only one of the primary colors--red, green or blue. The electrical signals from the tubes are turned into a colored picture by a conventional color TV receiver.

Except for the small and fleeting spot, the scene must be shot in darkness, because even faint light from any other source upsets the phototubes. But actors and other performers cannot work in darkness; they have to have a little light to see their way around.

Sight in Darkness. Du Mont's solution is to utilize the brief periods, 60 of them per second, when the flying spot has finished scanning the scene and is waiting for an instant before scanning it again. The phototubes are "blind" during this period, so the Du Mont engineers provide stroboscopic lights that flash brightly 60 times per second. To the slow-acting human eye they seem steadily luminous, but as far as the TV apparatus is concerned, they are not shining at all. All it can see is a dark room scanned by a flying spot.

Du Mont claims that this system, which sounds complicated, is much cheaper and easier to handle than the three-tube cameras normally used for broadcasting color TV. Its disadvantage is that it will not work outdoors. It requires an enclosed space where there is no natural light to compete with the flying spot.

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