The limiting sensitivity of an alternating-current system of amplification is considered. The practical limit is determined theoretically and experimentally for measured circuit parameters. A particular circuit using an electrometer-tube and a cesium-antimony cell is shown to be capable of detecting 4.5×10−15 watt in 30 seconds. Data on various input-tubes and photoelectric cells are given.
E. A. JOHNSON, W. H. MOCK, and R. E. HOPKINS, "The Limiting Sensitivity of the Alternating-Current Method of Photo-Cell-Current Amplification," J. Opt. Soc. Am. 29, 506-511 (1939)