Television, Color, Electromechanical

Prior to 1861 photographs in color could only be reproduced by hand painting daguerreotypes, which were direct-image photographs produced on a silver-coated copper plate. In 1855 James Clerk Maxwell suggested a method for creating a color image.

If a scene or object were photographed through red, green, and blue filters separately to obtain three negatives and the three positives prepared from them were projected with the images in alignment onto a screen by means of three lanterns, fitted with the appropriate red, green, and blue filter in front of the projection lens, a colored image would result. Maxwell demonstrated the correctness of his notion at a meeting of the Royal Institution in London in May 1861.

John Logie Baird adapted Maxwell’s concept to display colored television images on 3 July 1928, for the first time anywhere. His sending and receiving apparatus utilized Nipkow disk scanners, each having three spiral sets of apertures. The three sets were covered separately by red, green, and blue filters. At the sending end of the television link the scene or object was analyzed, sequentially, into its three primary color components, and at the receiving end the video signals corresponding to these were used to reconstitute three primary color images in register. Because of the persistence of vision, the final image was colored.

On 27 June 1929 Bell Telephone Laboratories (BTL) gave a demonstration in New York of color television using Nipkow disk scanners and transmitting stills. However, whereas Baird used a single scanner having three spirals, at each end of his television link and a single transmission channel, BTL employed a single scanner, having a single spiral of apertures, at each end of the television link, and three transmission channels. The red, green, and blue color contents of the scene or object being televised were transmitted simultaneously and not sequentially. An advantage of this method was that the same scanning disks and motors, synchronizing equipment, and circuits were applicable as in the monochrome television scheme. Neither scheme led to color television broadcasting services.

On 4 February 1938 Baird gave a public demonstration of television in the Dominion Theatre, London, at which high-definition images of about 3 by 2.7 meters, were shown in color, the television signals being received by radio, using a wavelength of 8.3 meters, from the Crystal Palace transmitter, about 16 kilometers away

The transmitting apparatus consisted of a 20.3- centimeter-diameter mirror drum, provided with 20 mirrors rotating at 6000 revolutions per minute (rpm). These mirrors reflected the scene to be transmitted, through a lens, onto a 500-rpm Nipkow type disk provided with 12 concentric slots positioned at different distances from the disk’s axis. Each of the slots was covered by a color filter, blue-green and red being used alternately. By these means the fields given by the 20-mirror drum were interlaced six times to give 120-line picture signals repeated twice for each revolution of the disk.

At the receiver a similar system of rotating mirror drum and disk was employed together with a high-intensity arc lamp source. The light intensity was modulated; that is, made to fluctuate, in exact conformation to the variations from the transmitting end, as it passed through a modified Kerr cell connected to the television receiver.

Another demonstration was given by Baird, in his private London laboratory, on 27 July 1939. However, Baird utilized a cathode ray tube (CRT) at the receiver in place of the mirror drum and slotted disk. A rotating disk having 12 circular filters, alternately red and blue-green, was positioned in front of the screen of the CRT.

The following year Baird, on 17 September, patented a method of color television that enabled him to demonstrate, in April 1941, 600-line color television. Again the two-color principle was adopted but the sending-end scanner was now a CRT of the type that had been employed as a projection unit for cinema television. Both the sending-end and receiving-end CRTs had two- color filter disks rotating in front of their screens.

Further development of this system allowed Baird to demonstrate colored stereoscopic images to the press on 18 December 1941 (Figure 5). At the transmitter, the primary scanning beam of light, after having passed through one of the color filters and the projection lens, was divided by a system of two pairs of parallel mirrors into two secondary beams spaced apart by a distance equal to the average separation of an observer’s eyes. By means of the revolving shutter disk, the scene was scanned alternately by each secondary beam.

Figure 5. Schematic diagrams showing the layout of the apparatus that Baird used to show stereoscopic color television

The receiver included a color disk identical to that of the transmitter and a revolving shutter, both synchronized to the corresponding disks at the sending end of the system. Hence each eye of the viewer alternately observed red, green, and blue images. The shutters used differed at the transmitting and receiving ends to minimize flicker.

Baird’s only competitor in the color television field by 1940-1941 was the Columbia Broadcasting System (CBS). On September 4, 1940 in New York, CBS engineer Peter Goldmark demonstrated equipment that comprised a Farnsworth image dissector tube camera and associated three-color filter disk rotating at 1200 rpm at the sending end; and a 23-centimeter CRT and another identical rotating filter disk at the receiving end. A 343-line image 14 by 18.5 centimeters was displayed.

From early 1942 Baird began to consider nonmechanical methods of color television and patented one version on 13 May 1942. He adapted the Thomas system of color cinematography in which an optical unit in the camera automatically produces images on a black and white film of the red, green, and blue components of a scene. At the receiver an identical optical unit combines the three cine film images to display a colored image. In the television version of this system the film camera and projector were replaced by an electronic camera and a projection CRT.

Two years later, on 16 August 1944, Baird gave a demonstration to the press of his telechrome tube—the world’s first multigun, color television display tube. The tube employed either double or triple, separate and independent, electron guns and multiple fluorescent screens depending upon whether two- or three-color reproduction was required. Only the two-gun version of the telechrome tube was demonstrated.

In 1944 it was clear that electromechanical methods of color television would not be viable in a domestic situation—although CBS persisted with its scheme—and by 1950 all electronic frame-, line-, and dot-sequential color television systems were being considered.