Journal of the Society of Motion Picture and Television Engineers (1950-1954)

equipped with 4-in. lenses for iconoscope use, were refitted with 2-in. //1. 9 lenses to produce a 3f in. X 4j in. image on the screen. The 35mm projectors, originally equipped with 8^-in. lenses, were refitted with 5-in. //1. 9 lenses. Tests of many screen materials were made. Among the materials tested were tracing paper, standard rear-projection material, experimental translucent plastic, latex, ground glass and flashed opal. In view of the relatively small image size, most of these materials were discarded because of excessive grain. From the standpoint of minimum grain and minimum light dispersion, latex appeared to be superior to other materials. However, it aged rapidly, changed color, and was difficult to keep clean. Ground glass, which did not have these undesirable characteristics, was finally selected as satisfactory for practical use. A metal hood is used to prevent stray light from reaching the screen from either side. Figure 3 shows the hood from the projection-room side, and Fig. 4 from the camera side. Note in Fig. 4 the detent mechanism which permits the rapid movement and precise location of the cameras. Light Reduction As is well known, substitution of image orthicons for iconoscopes requires a substantial reduction of the light intensity to secure operation of the pickup tube at the proper point on the characteristic curve. Many means of accomplishing this have been suggested. We elected an extremely simple method : the substitution of a 300-w projection lamp for the usual 1000-w lamp. These lamps are operated at 90 v instead of the nominal 1 1 5 v. Screen brightness measurements, with no film in the gate, showed 125 ft-L, uniform within approximately ±10 ft-L. In order to reduce the light output of the 35mm projectors to equal that of the 16mm projectors, we dropped the arc current from 25 amp to 20 amp and added neutral density filters having 40% transmission. With the opening of the Du Mont Tele-Center in New York, new and less expensive light sources, now under investigation, will be used. Operation and Adjustment Image-orthicon cameras are equipped with vertical and horizontal saw-tooth controls for shading. We have found that these can be set for a particular pickup tube and specific light input, and no shading adjustments need be made during the running of a film. Figure 5 shows the operating position at WABD. One operator handles two cameras, plus remote control of a flying spot scanner and automatic slide changer. The Du Mont cameras incorporate a black peak clipper as well as a white clipper. The white clipper is set so that normal white is never saturated, but extreme highlights may be reduced in amplitude. The black peak clipper is set so as to maintain constant black level and thus maintain standard setup. With most film, and a Type 5820 Image Orthicon, the camera lens may be stopped down to //5.6. When using a 5826, a typical aperture is //3.5. These lens settings are average. We have observed that the sensitivity of image orthicons of equal age may vary from tube to tube by a factor of 5. Sensitivity may also change with age by a factor of as much as 10. These variations are equalized by changing the lens stops. As indicated above, we have used both 5820 and 5826 for film transmission. The 5826 provides improved signal-to-noise ratio, and a somewhat better gray scale. Under some circumstances, with very dark scenes or very dense film, some adjustment to the iris is made. Also on occasion video gain and target voltage may be varied in order to avoid excessive black saturation. However, R. D. Chipp: Image-Orthicon Film Pickup