Page "Photomultiplier" Paragraph 17

In the early 1940s the JEDEC ( Joint Electron Devices Engineering Council ), an industry committee on standardization, developed a system of designating spectral responses.

The philosophy included the idea that the product's user need only be concerned about the response of the device rather than how the device may be fabricated.

Various combinations of photocathode and window materials were assigned " S-numbers " ( spectral numbers ) ranging from S-1 through S-40, which are still in use today.

For example, S-11 uses the caesium-antimony photocathode with a lime glass window, S-13 uses the same photocathode with a fused silica window, and S-25 uses a so-called " multialkali " photocathode ( Na-K-Sb-Cs, or sodium-potassium-antimony-caesium ) that provides extended response in the red portion of the visible light spectrum.

No suitable photoemissive surfaces have yet been reported to detect wavelengths longer than approximately 1700 nanometers, which can be approached by a special ( InP / InGaAs ( Cs )) photocathode.