Spectrophotometer linearity testing using the double-aperture method.

The double-aperture method has been used to determine the nonlinearity correction for a new spectrophotometer having a precision of +/-4 x 10(-5) transmittance units. The random and systematic errors of the method are discussed, and techniques are described that yield the additive nonlinearity correction to the high level of precision required for this spectrophotometer. The correction was found to be independent of source polarization, free from interference errors, but slightly dependent on wavelength.