A @[email protected] inside an evacuated envelope with a transparent window, the photocathode, and the @[email protected] The photocathode can be opaque or semitransparent.
Source: PAC, 1995, 67, 1745. (Nomenclature, symbols, units and their usage in spectrochemical analysis-XI. Detection of radiation (IUPAC Recommendations 1995)) on page 1752 [Terms] [Paper]