This effect arises when the
luminescence detector does not see a portion of the luminescent volume where the excitation beam
enters the sample. Thus the exciting beam flux is reduced by absorption by the analyte
and interfering impurities before it enters the volume observed by the detection system.
Source:
PAC, 1984, 56, 231
(Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI:
molecular luminescence spectroscopy)
on page 244