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 infrared sensitivity and visible spectrum amplification to be useful militarily. The S20 photocathode has a sensitivity of 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.