Multiple-integrating sphere spectrophotometer for measuring absolute spectral reflectance and transmittance

A spectroreflectometer/transmissometer is described that permits determination of absolute optical characteristics in the 300-2600-nm wavelength region (which is essentially the complete solar spectrum). The uniqueness of the instrument derives from use of three rapidly interchangeable 20-cm (8-in.) integrating spheres to measure (1) absolute hemispherical spectral reflectance as a function of angles of incidence from -40 to +40 degrees employing an Edwards-type integrating sphere with a center-mounted sample [using small 2.5-cm (1-in.) diam specimens], (2) absolute hemispherical and absolute diffuse spectral reflectance at an angle of incidence of 20 degrees employing a sphere with a wall-mounted sample (for large specimens) and a screened detector, and (3) absolute hemispherical and absolute directional (near-normal exitance) transmittance employing a complete integrating sphere with the only ports being for the sample and reference beams. Data are presented that demonstrate the ability to measure the spectral reflectance of nonmirror surfaces to an absolute accuracy of 0.995 (an uncertainty of +/-0.005 reflectance units) in both reflectance spheres and of highly specular mirrors to an absolute accuracy of 0.993 (an uncertainty of +/-0.007 reflectance units). Spectral transmittance can be measured to an absolute accuracy of better than 0.995 (an uncertainty of +/-0.005 transmittance units).