Determination and calculational minimization of the nonlinearity of a vis-NIR-spectrophotometer using the double aperture method

The determination of the nonlinearity of spectrophotometers is essential fo r evaluating instrument performance. Nonlinearity is caused mainly by the d etector system. Indirect methods, e.g., using calibrated filters or standar d solutions, to determine these small deviations are not reliable for high- accuracy measurements. Typical uncertainties are in the order of +/-0.003 A bs. (0.5 to 1% of stated transmittance). The double aperture method is base d on the principle of light addition. It is a direct method that requires n o secondary transmission or concentration measurements. With the double ape rture mask, two light beams are produced that can be measured separately or additively. In an ideal linear system, the sum of the separate readings is equal to the additive reading. The deviation in a real system represents a degree of nonlinearity. Through simple mathematical considerations a corre ction term can be found. Using the double aperture method, the instrumental bias can be determined d irectly. While the measured systematic instrument error does not comprise a dditional uncertainties associated with the measurement of real samples, a calculational method to minimize instrument nonlinearity is shown.