Photoelectric devices: Spectral sensitivity
A range of photocathodes is available covering the spectrum from the ultraviolet to about 10 000 A^˚ . The cut-off in the ultraviolet is controlled by the glass envelope of the cell and, by using a quartz window above the cathode, the ultraviolet sensitivity can be extended. The various spectral responses are designated by a number (e.g. S^–1) and three such sensitivity curves are depicted in figure 1.

Figure 1. The spectral sensitivity of various photocathodes.