Reflectometer measurement of roofing aggregate albedo

A solar reflectometer is commonly used to determine the albedo of roofing products. This study validates against pyranometer measurements of albedo three new methods for solar reflectometer measurement of the albedo of the irregular surface presented by a bed of roofing aggregate. Method A determines the albedo of an aggregate bed by averaging many reflectometer readings of a small sample of aggregate. Method B relates the albedo of the aggregate bed to reflectometer measurements of the albedo of an opaquely thick pile of finely crushed aggregate. Method C relates the albedo of the aggregate bed to reflectometer measurements of the albedo of a faux roofing shingle surfaced with finely crushed aggregate. When applied to the 17 specimens tested in this study, Method A worked well for all but the largest aggregates; Methods B and C worked well for all aggregates. The absolute mean error of each method was less than 0.01, and the RMS error of each method did not exceed 0.021.

As an ancillary note, we find that beds of mineral particles have albedos that decrease with increasing particle size, up to sizes at which the particles become opaque to sunlight.