The determination of the nonlinearity of spectrophotometers is essential for evaluating instrument performance. Nonlinearity is cause mainly by the detector system. Indirect methods, e.g., using calibrated filters or standard solutions, to determine these small deviations are not reliable for high-accuracy measurements. Typical uncertainties are in the order of ± 0.003 Abs. (0.5 to 1% of stated transmittance). The double aperture method is based on the principle of light addition. It is a direct method that requires no secondary transmission or concentration measurements. With the double aperture 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 correction term can be found.
Using the double aperture method, the instrumental bias can be determined directly. While the measured systematic instrument error does not comprise additional uncertainties associated with the measurement of real samples, a calculational method to minimize instrument nonlinearity is shown.
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