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540 XVI. TELEVISION AND FILM
of 30/24 would be needed; the (motion picture lines) resolving power required is 33 lines per millimeter. 16-mm films can readily provide resolving power of this order ; release print black-and-white raw stock is currently rated at 90 lines per millimeter. As in the case of films for 16-mm projection, care in film processing is necessary if the required resolving power is to be retained in release prints to be used for television transmission.
Special Transmission Characteristics. Commercial television transmission equipment is capable of using film with negative picture images and transmitting it in such manner that conventional positive picture images are produced at the receivers. This can be accomplished just as readily as conventional positive transmission ; it is called negative transmission. There is no technical equivalent in films; the nearest similar non-electrical arrangement is the dark-field vs. light-field microscope illumination systems common in the microscopy of biology and metallography.
Density and Contrast Characteristics. The density and contrast characteristics required of release prints of picture for television transmission seem to differ but little in a practical way from good motion picture prints projected with standard screen illumination levels in the order of 10 ft.-lamberts. Theoretical considerations derived from the known characteristics of pickup tubes, such as the iconoscope, and receiving tubes, such as the kinescope, would appear to show some contrast deviations. At the present state of the art, these do not seem to be of much practical significance. Just as in the case of motion pictures, some stations prefer light prints where the projector illumination level is low, or similar factors peculiar to the particular equipment obtain.
In the case of negative transmission, conventional motion picture negative and duplicate negative raw stocks are not well suited to television transmission due to the contrast characteristics of the pickup and the receiving tubes. The special raw stocks supplied by films manufacturers especially for this purpose should be used ; should any major change occur in the contrast characteristics of the receiving system, such as a radical change in the contrast of the tube phosphors used, it might be well to review once again the over-all contrast characteristics of the television transmission system. Such major change does not appear to be on the horizon at present, but the possibility of it should not be overlooked.
Photographing Television Images and Transmitting Film Images. The use of different frame frequencies for television and for motion