THE DIRECTIONAL RESPONSE ERROR IN INTEGRATING-SPHERE TRANSMITTANCE MEASUREMENTS AT SOLAR WAVELENGTHS

To correctly measure the solar transmittance of a material that scatte rs light, a detector with uniform directional response must be used, a nd the integrating sphere is often used in combination with an optical detector for this purpose. However, no real optical detector system h as a completely uniform directional response, thus a directional respo nse error will always exist. This note analyzes the directional respon se error in directional-hemispherical transmittance measurements at so lar wavelengths and determines this error for transmittance measuremen ts on materials that scatter light in different ways, made with a stat e-of-the-art integrating-sphere-spectrophotometer system. It is shown that for certain types of materials, the directional response error is the dominant error in the measurement of directional-hemispherical tr ansmittance.