Reflectance is measured according to strict standards. A perfect reflector would have a value of 100 percent for all wavelengths of light. When just one value of reflectance is given, it usually represents an average value weighted according to intensity across the solar spectrum.
With the growing recognition of reflectance as an important design parameter, reflectance values are being thoroughly measured across the entire solar spectrum (including infrared wavelengths) and accurate values for specific materials and coatings are becoming readily available.
The beauty of a white roof coating like those from Conklin is that the pigments, such as zinc oxide (ZnO) or titanium dioxide (TiO2), scatter a large fraction of the incident photons back into the sky, optimized for reflection in the visible and nearinfrared bands. Metal-oxide pigments used in white coatings scatter light by the same mechanism that light is scattered with by the water droplets in a cloud, or fog; the scattering is due to the tiny particle size. Except for the small particle size, these materials would be transparent to light, because the metal atoms are oxidized, i.e., the valence electrons of the metals are held by the oxygen and do not interact with the incoming photons.
Most white coatings have a very high reflectance across all visible wavelengths and also have a high reflectance for near-infrared radiation (NIR). Typical reflectance values averaged over these wavelengths are in the neighborhood of 70 percent or 80 percent.
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