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

on the photosurface to produce the recommended 0.35-//a value of highlightsignal current. It shows that the light required varies almost inversely with the cube of the signal-electrode voltage. In terms of foot-candles these figures may seem rather high, but actually only f 1m is needed to produce approximately 250 ft-c of illumination on the photosurface because the useful photo-area of the vidicon is only 0.875 sq in. (i X f in.). Experience in operating the new vidicon in film cameras has shown that the optimum value of light on the photosurface is an average illumination of about 250 ft-c. Since there is some difference in sensitivity between individual film-pickup vidicons operated at a given signal-electrode voltage, the fixed parameter of a system will be the light level used, with the signal-electrode voltage adjusted to produce the proper output-signal level. The signalelectrode operating potential at this point will lie between 15 and 25 v, which is slightly less than half the voltage that would be used if maximum sensitivity were the only requirement. Other Characteristics The spectral sensitivity of the photoconductive layer used in the RCA-6326 vidicon is essentially panchromatic. It follows closely the spectral response of the image orthicon, having its lowest sensitivity in the red regions and practically no infrared response. This makes the vidicon highly suitable for the reproduction of color film on a monochrome system. The low infrared response of the tube makes an infrared filter in the optical system unnecessary from this standpoint, although such a filter may be required to reduce heating of the photoconductive layer at high illumination levels. Image storage on the vidicon presents no problem at all for motion-picture work. The storage characteristics are excellent. The length of time that an image can be stored on the photosurface without loss of amplitude or resolution is at least several seconds. The vidicon can operate with any television film projector which has a 3-2 "pulldown ratio" or light-application rate. (In this type of projector the standard film speed of 24 frames/sec is converted to the 60-field/sec rate required for television by the use of a special shutter or pulsed light source which project five images of equal duration over each interval of 1/12 sec. The pulldown mechanism or drive holds successive frames of the film in the light gate for unequal lengths of time, so that the first frame of each pair is illuminated by three light pulses and the second by two.) In addition to this, one attractive feature of the vidicon is its ability to suppress the effect of the light-application bar of the film projector. If the light-application time is in the order of 30% of the active scanning time for a single television field, the light-application bar will be entirely unnoticeable. This is a very desirable characteristic because the projector meeting this requirement need not be synchronized with the field repetition rate of the television signal when used with the vidicon film camera. The tube will not operate satisfactorily in a system that does not have a projector with a 3—2 pulldown ratio or light-application rate because of the low-frequency flicker resulting from variations in the illumination of successive frames. Exhaustive data on the life of the film-pickup vidicon have not been accumulated at this time due to the fact that the tube has only recently been made available for commercial use. However, indications are that it will be possible to obtain life in excess of 1000 hours in film-camera operation. Conclusion The behavior of the RCA-6326 filmpickup vidicon shows that this tube has all the basic performance character 150 February 1954 Journal of the SMPTE Vol. 62