Reflectance measurements that are made on a scale that is not relative to an arbitrary standard are often called “absolute” measurements. The method presented here uses an auxiliary sphere with a double-beam integrating-sphere spectrophotometer to make measurements on an absolute basis. The basic requirements are: (1) The auxiliary sphere must be uniformly coated with a highly-reflecting, highly-diffusing material; (2) a flat plate must be coated in an identical manner to provide a measure of reflectance of the coating; (3) the interior-surface area of the sphere and the area of the entrance port must be measured.

The theory of the method is discussed and an error analysis is made. Reflectance data are reported for specimens of smoked MgO and pressed powders of MgO and BaSO₄.

The precision of repeatability has been evaluated from measurements of a Vitrolite reference standard. More than a dozen measurements at each of eight wavelengths made over a 3-year period exhibited a standard deviation of 0.003 for the spectral reflectance.

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