Some samples are excited to fluorescence by the incident radiation beam. When added to the transmitted radiation beam this fluorescence results in an erroneous reading called the fluorescence error. It is particularly noticeable if the photodetector is situated close to the sample.
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)