Projection engineering (Sept 1929-Nov 1930)

Page 26 Projection Engineering, September, 1929 Television in Color Natural Color Television Accomplished Through the Use of Three Sets of Special Photoelectric Cells and Color Filters By Herbert E. Ives* OVER two years ago Bell Telephone Laboratories demonstrated a practical system of television. For the first time successful representations of objects at rest or in motion were transmitted electrically — over wires or through the ether — for considerable distances. The reproduction of the scene then transmitted was in monochrome — the orange-red color of the neon lamp. Recent developments of the labora sium. Its active surface is sensitized by a complicated process using sulphur vapor and oxygen instead of by a glow discharge of hydrogen as with the former type of cell. The response of the new cell to color, instead of stopping in the bluegreen region, continues all the way to the deep red. Because the former potassium cells were responsive only to the blue end of the spectrum, objects of a yellowish color appeared darker ;ON TUBE -GREEN FILTER -SEMI-TRANSPARENT MIRROR r.=fc BLUE FILTER SCANNING DISC RED FILTER One semi-transparent mirror reflects red light from the neon tube; one reflects green light from one argon tube, and through both mirrors passes blue light from the other argon tube. tories, however, have made it possible to reproduce scenes with their true color values. The appearance of reality in the reproduced scene is thus greatly enhanced. One of the most significant features of this new achievement is that it does not require completely new apparatus. The same light sources, driving motors, scanning discs, synchronizing systems, and the same type of circuit and method of amplification are used as in the monochromatic system. The only new features are the type and arrangements of the photoelectric cells at the sending end, and the type and arrangements of the neon and argon lamps at the receiving end. The outstanding contributions that have made the present achievement possible are a new photoelectric cell, new gas cells for reproducing the image, and the equipment associated directly with them. New Type Photo-Cell To render the correct tone of colored objects, it was neecssary to obtain photoelectric cells which — like the modern orthochromatic or panchromatic plate — would be sensitive throughout the visible spectrum. This requirement has been satisfactorily met. Through the work of A. R. Olpih and G. R. Stilwell a new kind of photoelectric cell has been developed, which uses sodium in place of potas than they should have and the tone of the reproduced scene was not quite correct. This disadvantage applied particularly to persons of dark or tanned complexion. When the new cells are used in the original television apparatus and with yellow filters — similar to those used in photographing landscapes in order to make the blue sky appear properly dark — this defect is corrected and the images assume their correct values of light and shade no matter what the color of the object or the complexion of the sitter. It is the availability of the new photoelectric cells which makes color television possible ' by their use. The development of color television has been greatly simplified by the fact that as far as the eye is concerned any color may be represented by the proper mixture of just three fundamental colors — red, green, and blue. This fact was utilized in the development of color photography, and all the research that had been done in that field was available as background for color television. A host of methods of combining the three basic colors to form the reproduced image was available but, insofar as the sending or scanning end is concerned, a method was developed which has no counterpart in color photography. The method of "beam scanning"— used in the first television demonstration1 — has been employed. Color Filters To apply this method to color television, three sets of photoelectric cells are employed in place of the one set used before. Each of these sets is provided with color filters made up of 1 Bell Record, June, 1928, page 325. *Member of the Technical Staff, Bell Telephone Laboratories. The disc and motor drive for the color television apparatus are the same as for monochromatic television. The mirror and colored filters are In the small box behind the disc, at the right side of the cabinet. Note the disposition of the tubes.