= Chopped-light lock-in experiment
The University of California, Berkeley guide \i[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.<LLS - Measuring the Light Signal from a Diode, University of California, Berkeley>[LLS - Measuring the Light Signal from a Diode, Experimental Setup; Block Diagram I; Experiment X, steps 2 and 9–13; Experiment XI, steps 1–8]{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.<LLS - Measuring the Light Signal from a Diode, University of California, Berkeley>[Experimental Setup, waveform discussion; Interpreting the Data, points 1–3 and collection-geometry discussion]{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.
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