The particle size distribution and the reflectance properties of silvered glass mirrors exposed in an outdoor environment have been measured to determine the separate effects of natural soiling and natural cleaning (wind and rain). The wavelength dependent diffuse reflectance was correlated with the particle size distribution function for mirrors exposed to increasing periods of outdoor exposure. These measurements have indicated the significant role of small particles (radii ≲ 5 μm) in the soiling of exposed mirrors and in the subsequent reflectance loss over the solar spectrum.

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