Cascade (Passive) Image Intensifier Tubes
Although originally experimented with by the Germans in World War One, it wasn't until the 1950s that the US began conducting early experiments using multiple tubes in a "Cascade", by coupling the output of an inverting tube to the input of another tube, which allowed for increased amplification of the object light being viewed. These experiments worked far better than expected and night vision devices based on these tubes were able to pick up faint starlight and produce a usable image. However, the size of these tubes, at 17 inches long and 3.5 inches in diameter were too large to be suitable for military use. These tubes, known as "Cascade" tubes provided the capability to produce the first truly passive night vision scopes. With the advent of fiber optic bundles in the 1960s, it was possible to connect smaller tubes together which allowed for the first true Starlight Scopes to be developed in 1964. Many of these tubes were used in the AN/PVS-2 rifle scope which saw use in Vietnam.
