Page "Image intensifier" Paragraph 8
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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 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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