optical-beam error

in spectrochemical analysis
https://doi.org/10.1351/goldbook.O04305
For wavelength-scanning spectrometers, an error may be introduced if the image position on the photodetector changes. Reflection of the incident radiation, e.g. between the cell walls, is another source of error and results in a measured absorbance slightly higher than the true absorbance. These errors are collectively termed optical-beam errors.
Source:
PAC, 1988, 60, 1449. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - VII. Molecular absorption spectroscopy, ultraviolet and visible (UV/VIS) (Recommendations 1988)' on page 1457 (https://doi.org/10.1351/pac198860091449)