Spatial variations of the spectral response of photomultiplier cathodes

Experiments show that the cathode of the R.C.A. 931-A type of photomultiplier gives a spatial variation of response which is not of the same form for all wavelengths. In particular the form of this response at 6800 Å is appreciably different from that at 5300 and 4800 Å. Thus accurate relative intensity measurements can only be obtained if the sources concerned are so optically aligned that they illuminate the same cathode area.