Chopped-light lock-in experiment

The University of California, Berkeley guide LLS - Measuring the Light Signal from a Diode, in Experimental Setup and Experiments X–XI, documents an experiment using a red light-emitting diode amid room-light background. An optical chopper, controlled by an SR540, periodically interrupts its light. A PIN-10DP photodiode sends electric current to an SR570 preamplifier, which converts it to an electric potential difference for an SR760 spectrum analyzer and an SR830 lock-in amplifier. The SR540 supplies the external reference. Preparation includes aligning the photodiode and its shielding cone and choosing gain to avoid overload. One comparison first searches for the source frequency without the chopper reference, then checks against that external reference; switching off the LED tests the candidate spectral peak. The measurement procedure varies chopper frequency and preamplifier filtering, then records repeated lock-in readings with selected averaging settings. These are prescribed observations to make, not published numerical results.^[ref]

The guide notes nearly triangular chopping because slit and emitter sizes are comparable. Its interpretation requires detector conversion, preamplifier gain and waveform geometry to infer optical power from the selected component.^[ref]

The inference is therefore specific: synchronized electric potential difference supports the presence of a modulated optical component at the reference frequency. It does not by itself give total emitted optical power. The photodiode's spectral responsivity, collected fraction of the light, and conversion from the measured Fourier coefficient to the chopped waveform must enter that calibration. The finite-window lock-in response also explains why a frequency-search reading can drift when the reference is not synchronized. An LED-off comparison tests background at the same timing; it cannot guarantee that every synchronized disturbance disappears when the desired source is present. Overload before multiplication destroys information that later averaging cannot restore.