An error due to inhomogenous distribution of absorbing species in the analytical sample. It may be observed by probing the sample volume with an incident beam of radiation of reduced cross-section.
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)