Differential method of lag compensation in photoconductive devices

The slow response time often observed in devices utilizing photoconductivity may be caused by the slow rise and decay of conductivity in the photoconductor, or by the RC time constant of the photoconductor in its associated circuit. In imaging devices, such as television camera tubes and light amplifiers, the lag is manifested by smearing of the image for moving scenes. This paper describes a method for reducing the effective response time of the device regardless of the source of lag. By taking the difference of the signals from two photoconductive elements having unlike transient responses, a resultant signal can be obtained having a faster response than either element alone. Measurements have been taken on pairs of vidicons set up to view the same scene simultaneously. The lag-corrected video signal formed by the external combination of the two outputs showed improved speed of response with a moderate loss in signal. Similar results have been obtained with pairs of photoconductive cells. The method can also be applied to light amplifiers and to experimental camera tubes designed to yield a lag-corrected signal directly.