Variant of the dual-beam measuring method that makes it possible to shift the zone of higher resolution from the lower to the upper end of the absorption scale.
When transmission values measured by the photometer are converted into absorption units (spectral absorbance), formation of the logarithm produces higher resolution at lower concentrations. If one wishes to measure and control high concentrations, the substitution method can be used to shift the better resolution to the range of higher concentrations by optical means. The following Figure compares the resulting scale (1) with classical absorption measurement (2).
The way the substitution method operates is based on reversing the roles of the measurement and reference beams: A neutral density filter of known absorption (compensator K) is inserted into the measuring beam (M), the flow cell into the reference beam (V). As a result, the measured reading arises not directly from the sample's transmission, as is usually the case, but from the ratio of the transmissions of the the compensator and the sample. The logarithmic conversion into absorption units yields the mirror image of the scale, thus shifting the range of higher resolution.
The absorbance of the compensator inserted into the measurement beam represents the highest possible measurable absorption of the sample. The highest absorbance that can still be detected photometrically is 3.