front surface geometry in luminescence

A system where excitation and observation are from the same face of the sample ( $α ∠ 90 °$ , $β ∠ 180 °$ where $α$ = angle of incidence of the exciting beam on the plane surface of the sample, and $β$ = angle between the exciting direction and observation direction.)

Source:

PAC, 1984, 56, 231 (Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy) on page 244