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It was not until the development of the bialkali antimonide photocathodes ( potassium-cesium-antimony and sodium-potassium-antimony ) discovered by A. H. Sommer and his later multialkali photocathode ( sodium-potassium-antimony-cesium ) S20 photocathode discovered in 1956 by accident, that the tubes had both suitable infra-red sensitivity and visible spectrum amplification to be useful militarily.
The S20 photocathode has a sensitivity or around 150 to 200 µA / lm.
The additional sensitivity made these tubes usable with limited light such as moonlight, while still being suitable for use with low-level infrared illumination.

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