Compliance of Si photodiodes with Talbot's law

For accurate measurements of chopped radiation, it is important that the photoelectric instrumentation complies with Talbot's law, that is, measures the time average. Two different photoelectric measuring systems were investigated for their compliance with Talbot's law: (1) Si photodiode with an integrating digital voltmeter (IDVM) system, and (2) Si photodiode with a lock-in amplifier. The test consisted in comparing values predicted from the amplitude and the duty factor of the chopper with values measured by both systems. It was found that the IDVM system showed satisfactory compliance with Talbot's law, whereas the lock-in system did not. However, a Fourier analysis of the signal wave shape produced correction factors for the lock-in system data which then yielded satisfactory compliance with Talbot's law.