= Radiation-pressure laser power measurement
{title2=$\Phi=cF_\perp/[(a+2R)\cos\theta]$}
A 2013 experiment by <Paul A. Williams> and collaborators mounted a 25-mm dielectric mirror on a commercial <force-restoration balance>. Removing its pan-weight compensation spring allowed horizontal <force> sensing with a horizontal <laser> path. They measured mirror <optical reflectance> and varied incidence from about $7^\circ$ to $45^\circ$. A 1071-nm <laser>, capable of 530 W, was switched at 0.1 Hz. Digitized <mass>-equivalent readings were converted to <force> using $g$; <quadrature demodulation> isolated the switched signal from drift. <Use of radiation pressure for measurement of high-power laser emission by Paul A. Williams and others>[Williams and collaborators, pp. 2–3, Figures 1–3 and equation (2): preparation, apparatus and readout]{ref}
<Measurement calibration> used a 20-g <mass>, with checks down to 1 mg. The optical <forces> lay below that checked range, limiting the accuracy claim. An angle scan tested the predicted <cosine> dependence at fixed intercepted <power>. Replacing the mirror with a <carbon nanotube> coated absorber increased absorbed <power> while reducing the predicted optical <force>; no modulation peak was detected. This bounded gas-heating artifacts under those conditions, rather than proving their universal absence. The fitted slope was about 7% below $2/c$. Inferring <laser> <power> requires calibrated <force>, <optical reflectance>, <radiometric absorptance> and <angle of incidence>, with <measurement uncertainty> for each. <Use of radiation pressure for measurement of high-power laser emission by Paul A. Williams and others>[Williams and collaborators, pp. 2–4, Figures 3, 5–7: calibration limits, angle and absorber controls, and reported slope discrepancy]{ref}
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