An error which arises if the two beams in a double-beam spectrometer are not fully equivalent in transmitted power nor are corrected to be so (e.g. due to unmatched cells).
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 1456 (https://doi.org/10.1351/pac198860091449)