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

characteristics of the input stage of the camera amplifier. High-performance, low-noise cascode amplifiers are used for this application. C. The possibility of aperture response correction is particularly inviting with the vidicon because of the excellent signal-to-noise relation. For example, it is possible to raise the aperture response from 35% to 100% at 350 lines resolution by suitable techniques and still maintain a signal-to-noise ratio of 100 to 1. This improves horizontal resolution, but does not affect vertical resolution. Overall tests indicate that the process is definitely necessary for all pickup tubes, but can be used only where signal-to-noise performance is not sacrificed. D. The gray-scale or transfer characteristic, which is inherent in the vidicon surface itself, has a log-log slope of 0.65 when signal output current is plotted against light on the photoconductive target. (This gamma is complementary to the kinescope transfer gamma characteristic, requiring no further correction in the video amplifiers.) A dynamic range of 150 to 1 or more in the usual gray-scale logarithmic test wedge can readily be demonstrated. With the iconoscope, 50 to 1 represents a value which can be attained only with special precautions. The slope is constant over a wide range of lighting and does not have the "rubbery" or variable gamma handicap of the iconoscope. E. Film-reproduction range and latitude are wide, due both to the low gamma and to the constant character of the signal output light input characteristic. Normal shifts in print density produce very little change in quality since these can be compensated by either a change of video gain or projector light output. The high signal-to-noise ratio initially available makes this possible. F. Light source requirements under typical conditions, and using commercially available lenses, are of the order of 300 ft-c, average, measured at the film gate. Since practically all intermittent-type television motion-picture film projectors used with the iconoscope have an exposure shutter opening of approximately 7%, phased under blanking, this 300-ft-c average corresponds to about 4000-ft-c peak. Optimum vidicon results are obtained using approximately -J of the maximum light output available in standard television projectors designed for use with the iconoscope. Sensitivity is deliberately sacrificed for improved performance by the use of a low signal-electrode voltage. The decrease in light requirements nevertheless prolongs projector lamp life greatly. G. Since the vidicon tube is essentially an orthicon or low-velocity device as far as the scanning process is concerned, there is inherently no spurious shading signal developed. This contrasts very favorably with the iconoscope where elaborate precautions in edge lighting and waveform cancellation are necessary to minimize a normally large spurious signal. In the vidicon, no electrical shading cancellation signals are required, thus resulting in equipment and operational simplifications. In early models of vidicons, there were problems of maintaining uniform sensitivity of the photoresistive signal-electrode, so there was unequal signal output at the edges as compared to the center of the raster. Improvements in production techniques have made such variations negligible. By operating the vidicon signal-electrode at low voltages for motion-picture film use, the decreased dark current of the device and the improvement in lag and burn characteristics greatly outweigh the loss of light sensitivity. High light sensitivity is vital for direct pickup cameras, but is of only casual interest in motion-picture reproduction. Kozanowski: Vidicon Cameras 155