This effect arises when the exciting beam does not fill the cell completely and luminescence is absorbed by the analyte and interfering impurities in the non-illuminated region facing the detector.
Source:
PAC, 1984, 56, 231. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy' on page 244 (https://doi.org/10.1351/pac198456020231)