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In the measurement of absorption, a sample and a reference are measured each time to compare these measurement results.
Since a single beam system has only one position for measurement, the sample and the reference should be replaced, and 100% adjustment (or 0 Abs adjustment) should be conducted whenever measurement wavelength is changed.
Although this system is used mainly for quantitative measurement, and offers a small and inexpensive device configuration, changes (drift) in the light source cannot be adjusted.
A double beam system has two dedicated positions for the sample and the reference, in contrast to the single beam system with only one measurement position. The double beam system measures sample based on this signal because the light of reference enters into Detector.
The change of the light source can be corrected, and it is possible to measure the system with stability.
A monochrome (monochromator, spectroscope) plays the role of a sorter that picks up the light of the desired wavelength out of the light containing various wavelengths.
But, however carefully they are checked, wavelengths other than the wavelength of interest may slip in (this light that was lost or slipped in is called stray light).
This stray light may affect the accuracy of the measurement. A double monochrome system conducts a second check on the light that passed the first check. The system that checks only once is called a single monochrome system.