Solar spectral optical properties of pigments—Part I: model for deriving scattering and absorption coefficients from transmittance and reflectance measurements
The suitability of a pigment for inclusion in "cool" colored coatings with high solar reflectance can be determined from its solar spectral backscattering and absorption coefficients. Pigment characterization is performed by dispersing the pigment into a transparent film, then measuring spectral transmittance and reflectance. Measurements of the reflectance of film samples on black and white substrates are also used. A model for extracting the spectral backscattering coefficient S and absorption coefficient K from spectrometer measurements is presented. Interface reflectances complicate the model. The film's diffuse reflectance and transmittance measurements are used to determine S and K as functions of a wavelength-independent model parameter σ that represents the ratio of forward to total scattering. σ is used to estimate the rate at which incident collimated light becomes diffuse, and is determined by fitting the measured film reflectance backed by black. A typical value is σ=0.8. Then, the measured film reflectance backed by white is compared with a computed value as a self-consistency check. Measurements on several common pigments are used to illustrate the method.