A perforated hemispherical reflector made of half of a Pyrex flask has been employed to collect diffusely reflected light from a surface to be measured on a photo-cell or thermopile placed at a focal point of the hemisphere conjugate to the position of the sample, thus providing adequate amounts of energy on the thermopile to permit spectral-reflectance measurements to longer wave-lengths than with usual reflectance spectrophotometers. Relative diffuse spectral-reflectance curves extending to 2.5µ are given for a number of “infra-red reflecting” paints, showing qualitatively the superiority of high index of refraction, large particle size, and low extinction coefficient in producing layers of pigments having high scattering coefficients in the near infra-red.
The use of the hemispherical integrator with a tungsten source and a Wratten 87 filter and with either a thermopile or photo-cell receiver is illustrated by measurements which show that the thermopile-Wratten 87 system, embracing wave-lengths 0.76 to 2.6µ produced reflectance factors which more nearly approximated integrated reflection factors for sunlight between 0.76 and 2.4µ and more truly indicated the effect of variations in physical characteristics of the reflecting material than did photo-cell measurements embracing wave-lengths 0.76 to 1.2µ.
The use of a caesium oxide cathode, infra-red image tube in making rapid inspections of surfaces in the region of wave-length shorter than 1.2µ is described.
J. A. SANDERSON, "The Diffuse Spectral Reflectance of Paints in the Near Infra-Red," J. Opt. Soc. Am. 37, 771-774 (1947)