How to read the dust level of a concentrated-solar-power heliostat off the polarization of the sky it reflects, instead of off its brightness. Rayleigh scattering leaves skylight partially linearly polarized in a band pattern across the sky dome whose geometry is fixed by the sun's position and is therefore computable for any site, date and time. A clean mirror reflects that light specularly and the reflection Mueller matrix, built from the Fresnel coefficients through the Jones-to-Stokes transformation, largely preserves the degree of linear polarization; dust grains on the glass are comparable to or larger than a visible wavelength, so they scatter in the Mie regime and, through repeated events modelled by Monte Carlo including light that enters the glass and returns off the silvered back, depolarize what comes out. Degree of linear polarization therefore falls monotonically with soiling and can be inverted for relative reflectance against a clean reference facet in the same frame. Students learn why a polarization signal survives conditions that defeat an intensity measurement, how a single-shot division-of-focal-plane camera makes the measurement fast enough to carry on a drone flying precalculated waypoints over a live plant, and what the technique's real limits are: a clean reference must be visible, structures reflected in the mirror corrupt the region of interest, and the inversion inherits every error in the sky model. Demonstrated as PIMS at the Sandia National Solar Thermal Test Facility with relative reflectance error near two percent.
A concentrated solar power plant aims sunlight at a central tower with a field of tracking mirrors called heliostats, and dust settling on those mirrors scatters light that should have been specularl…