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

its performance in two ways. The first and most pronounced improvement is a reduction in dark current. It will be remembered that the photoconductive layer is a rather good insulator in the dark. However, there is always some current flow through the material even in total darkness. A uniform dark current would not be objectionable but variations in the dark current over the useful area of the photoconductive layer are highly undesirable. This dark-current variation, if too prominent, will be exhibited as a flare signal, not unlike the signal from an iconoscope in the dark. It is logical to assume that if the average value of the dark current is reduced, the amplitudes of variations of the dark current will be reduced. Operation of the signal electrode at lower than normal voltage reduces the dark current to a very small value and, as a result, the dark-current flare in the picture is reduced to an imperceptible level. The sensitivity of the photosurface also decreases with this reduction in signal-electrode voltage, although not as rapidly as the dark current. This is illustrated by Fig. 5, which shows the ratio of signal current to dark current at various signal-electrode voltages, under constant illumination. However, for film-pickup applications this loss of sensitivity is a minor problem, since the vidicon has very high inherent sensitivity and the illumination needed for its small photosurface area is low. Signal Lag Reduction of the applied signalelectrode voltage improved the performance of the vidicon in still another manner. One of the characteristics of the photoconductive surface used in the vidicon is a lag in its response to changing illumination. This produces a slight smearing of moving objects in normal operation. However, a reduction of the applied signal-electrode voltage and the resulting necessary increase in light on the photosurface considerably reduces RATIO OF SIGNAL OUTPUT CURRENT TO DARK CURRENT 2 £ 8 S CONSTANT ILLUMINATION OVER 'X2 x 3/&' SCANNED AREA OF PHOTO-CONDUCTIVE LAYER N \ \ N "X \ DAF K Cl RRE >4T = .< )2*ia ^ 20 30 40 50 SIGNAL-ELECTRODE VOLTS Fig. 5. Ratio of signal-output current to dark current in the 6326 vidicon at various signal-electrode voltages. the lag of the vidicon signal. The yardstick that has been adopted to measure or rate the lag is the amount of residual signal developed by the tube after 1/20 sec in the dark, which is slightly longer than the time necessary to discharge a complete frame. Figure 6 shows the lag of the vidicon signal at various signal-electrode voltages. It can be seen from this curve that the lag drops almost directly with the signal-electrode voltage. When operated at 25 v (onehalf normal signal-electrode voltage), the residual signal after 1/20 sec is down to only slightly more than 10%, or less than half its normal value. By way of comparison, this is very close to the lag characteristic of an image orthicon operated at full storage. At this operating point, the lag of the signal produced by the vidicon is entirely unobjectionable in film reproduction. Vidicon Gamma The vidicon has several other very desirable characteristics that present no problems and require no special operating techniques. Perhaps the most important of these is its light-transfer characteristic. If the light-transfer characteristic of a transducer such as a camera tube Neuhauser: Vidicon for Film Pickup 147