Editing Image intensifier (section)

===Cascade (passive) image intensifier tubes===
[[Image:Gen1C-2-Comparison.jpg|thumb|right| A photographic comparison between a first generation cascade tube and a second generation wafer tube, both using electrostatic inversion, a 25mm photocathode of the same material and the same F2.2 55mm lens. The first generation cascade tube exhibits pincushion distortion while the second generation tube is distortion corrected. All inverter type tubes, including third generation versions, suffer some distortion.]]

Although originally experimented with by the Germans in World War Two, it was not until the 1950s that the U.S. 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 in (43&nbsp;cm) long and 3.5 in (8.9&nbsp;cm) in diameter, were too large to be suitable for military use. Known as "cascade" tubes, they 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 [[night vision device#Generation 1 (GEN I)|Starlight scope]]s to be developed in 1964. Many of these tubes were used in the [[AN/PVS-2]] rifle scope, which saw use in Vietnam.

An alternative to the cascade tube explored in the mid 20th century involves [[optical feedback]], with the output of the tube fed back into the input. This scheme has not been used in rifle scopes, but it has been used successfully in lab applications where larger image intensifier assemblies are acceptable.<ref>[[Martin L. Perl]] and [[Lawrence W. Jones]], Optical Feedback Image Intensifying System, {{US patent|3,154,687}}, Oct. 27, 1964.</ref>