Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1929 Page 27 sheets of colored gelatine. One set has filters of an orange-red color which make the cells see things as the hypothetical red sensitive nerves of the retina see them ; another set has yellow-green filters to give the green signal, and the third set has greenishblue filters which perform a corresponding function for the blue constituent of vision. The scanning disc and the light source are the same as with the beam scanning arrangement use in monochromatic television. The only difference is in the photoelectric cells, and thanks to the tri-chromatic nature of color vision, it is only necessary to have three times the number of cells used previously to reproduce all colors. Three series of television signals, one for each set of cells, are generated instead of one and three channels are used for the transmission of the television signals. The photoelectric cell container, or "cage," has been built in a somewhat different form from that used in our first demonstration. There three cells were used arranged in an inverted "U" in a plane in front of the object. In the new photo-cell cage twenty four cells are employed, two with "blue" filters, eight with "green" filters, and fourteen with "red" filters. These numbers are so chosen with respect to the relative sensitiveness of the cells to different colors that the photoelec ■ trie signals are of about equal value for the three colors. The cells are placed in three banks, one bank in front of and above the position of the scanned object, one bank diagonally to the right, and another bank diagonally to the left, so that the cells receive light from both sides of the object and above. In placing the cells they are so distributed by color as to give no predominance in any direction liiiiiiiiiiiiniiiiiiiiiiiiiiiiiiiiiiiiiiiiiinii; The complete apparatus at the receiving end. On the left is the sync h ronizing panel and on the right are the amplifiers for three channels. In the center is the cabinet containing the scanning disc, the argon and neon lamps, and the color filters. Illlllllllllllllllllllllllllllllllllllllllllllllll to any color. In addition large sheets of rough pressed glass are set up some distance in front of the cell containers so that the light reflected from the object to the cells is well diffused. The television signals produced in the color sensitive photoelectric cells through the color filters are no different electrically from those used in monochromatic television. Three sets of amplifiers are required, one for each color, and three communication channels in place of one, but the communication channels are exactly similar to those which were used with the same scanning disc before. For color television the three images must be received in their appropriate colors, and viewed simultane Sido view of sending apparatus with cabinet doors removed. With the exception of the photoelectric cabinet at the left, the apparatus is identical with that used for monochromatic television. ously and in superposition. The first problem was to find light sources which, like the neon lamp previously used, would respond with the requisite fidelity to the high-frequency signals of television, and at the same time give red, green, and blue light. "With such lamps available a decision would have to be made as to how the three colors could best be combined to form a single image. Methods of Reception Several methods of reception are possible. For displaying the transmitted image to a large audience a grid2 could be employed similar to that used for the earlier demonstration but it would consist of three parallel tubes instead of a single one. Thus far the television images have been received in a manner similar essentially to our method for monochromatic television. The surface of a disc similar to that used at the sending end is viewed, and the light Prom the receiving lamp is focussed on the pupil of the observer's eye by suitable lenses. To combine the light cf the three lamps, they are placed at some distance behind the scanning disc and two semi-transparent mirrors are set up at righl angles to each other bul each at 45° to the line of Bight. | 'iic lamp is then viewed directly through both mirrors and one lamp is seen by reflection from each, as Illustrated by the accompanying diagram. The matter of suitable lamps to provide die red, green, ami bine light lias required a great deal of study. There is no difficulty about the red lighl because (he neon gj0w lump which has been used previously in television can be transformed into a suitable red light by Interposing a red Idler. For the sources of green and blue light nothing nearly so efflcienl i ll< II Rrmnl. Man. 1027, pnfjr 819.