As a starting point attention is called to an empirical relation between the detectivity and the time constant of lead sulfide photoconductive cells. This relation, found by E. D. McAlister, states that the ratio /τ of the detectivity in the reference condition C to the time constant τ is approximately constant
This relation holds for tests with radiation from a 500°K blackbody, and holds for both cooled and uncooled cells. The relation is based on data concerning the detectivity and time constant of about eighty different cells of domestic and foreign manufacture with time constants in the range from 3 microseconds to 3 milliseconds.
It is shown that this relation has a number of important consequences. For cells that accord with McAlister’s relation, one draws the following conclusions: 1. Lead sulfide cells are Class II detectors. 2. The value of the figure of merit M2 may be determined. 3. It is disadvantageous to match the time constant of the cell to the duration of the signal pulse; rather, one should always use a cell with a time constant large compared with the signal duration. 4. Cooling the cell is of no advantage for signal frequencies greater than the crossover frequency; cooling the cell is merely one method of increasing the time constant of the cell. 5. Lead sulfide cells are remarkably versatile detectors.
McAlister’s relation is purely empirical, and has no known theoretical basis. It is therefore possible that the relation, and the consequences just described, may be modified in the future. In the meantime, the relation provides a working basis of considerable practical importance.
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