Measurement of Frequency Response of Photoelectric Converter by Flyingspot Scanning

Abstract

Speed of response is one of th most important characteristics of photoelectric converters. Usual method of estimating this response is to use a chopped light through a rotating sector. But, for high speed, the response cannot be measured by such a rotating sector method because unre-alizably high speed rotation becomes necessary. To make up for this, a circular disc with a num-ber of slits along its periphery is being used. Yet, the lack in uniformity of shapes and spacing of the slits limits the measuable maximum frequency to about 10 K. C. This limit is extended by the following flying-spot scanning method.
(1) A part of a photosensitive surface is covered by an opaque mask so as to get sharp boundaries of illuminated and unilluminated regions.
(2) This photosensitive surface is scanned by a flying spot across the boundary lines and its response is amplified and reproduced on a cathode ray oscillograph against time scale synchronized with the period of the scanning flying spot.
(3) From the traced or photographed curve taken on oscillograph, the frequency response of the photosensitive surface can be calculated by the use of one or two derived equations. Two more methods, both experimentally easily realizable, are descrided, by which the same results as above can be obtained. Measurements and analyses made on a gold evaporated germanium photo-diode and a selenium photovoltaic cell are given as examples, the results obtained agreeing satis-factorily with the speed of response estimated by other methods.