A spherical integrating source is a hollow sphere with the entire inner surface uniformly coated with a material which has a high diffuse reflectance. The basic concept behind the spherical shape is that light from the internal source has a chance to perform multiple bounces thereby randomizing its original direction before it exits a small aperture. The sphere's interior coating is designed to have a very high degree of diffuse reflectance. A perfect diffuse reflector can behave like a perfect (i.e. Lambertian) diffuse source which means energy is distributed in all directions equally. A Lambertian source is a source whose radiance is independent of viewing angle. Radiance is defined as the energy flux per unit projected area per unit solid angle leaving a source, or a surface.

Each SIS is calibrated by SBRS to National Institute of Standards and Technology (NIST) traceable standards of spectral irradiance. In addition, EOS cross calibration activities include comparison of the SBRS radiometric scale to the NIST, University of Arizona, NASA's Goddard Space Flight Center, and Japan's National Research Laboratory of Metrology (NRLM) radiometric scales.

The Landsat Transfer Radiometer (LXR), a visible and near infrared radiometer designed for stability by NIST, is used to monitor the output of each sphere during each calibration and calibration check. This radiometer has also been calibrated by NIST to provide an independent check on the radiometric calibration of the two sources.

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Last Update: September 1, 1998