Night Vision Devices have been in use for almost more than 80 years now. With its initial debut in WWII. Back then the devices where heavy and bulky, most often only rifle mounted. These units consisted of a simple imaging tube, which didnt amplify any light, it only converted it.
Being able to pick up light in the invisible IR light spectrum a power consuming infrared light source was always necessary for it to be of any advantage over the enemy. This only worked as long as the enemy didnt have any night vision, because otherwise a so called active night vision system (any NVD using a additional light source, most often IR) would completely defeat its purpose to operate undetected at night.
The light source would appear as a bright spotlight and the opponent would be easily able to pinpoint the location of any opposing forces using active night vision. These devices, which where of no advantage without IR illumination and only converted light instead of amplifying it, are called Generation 0 devices or simply Gen 0.
As often in history big steps in technology came along with wars. In this case the real step forward to passive night vision systems came in the 1960s with the Vietnam war.
In 1958 a company called ITT (nowadays known as Exelis) began manufacturing NVDs for the US military and soon after that the U.S. Department of Defense itself founded laboratories to further research the technology.
In the first half of the 1960 a lot of improvements were made in the Image Intensifier Tube (IIT) technology such as bi-alkali photocathodes.The S-20 photocathode (actually discovered by accident in 1958) greatly improved IITs sensitivity compared to earlier Gen 1 IITs.
These new Gen 1 IITs were able to use light sources like moonlight and low-level IR illumination instead of relying on external IR illumination.
This big step with photocathodes was literally just one side, the other side was the invention of fiber optics as the output window. Without this, IITs were too bulky to be used on i.e. a rifle. Fiber optics enabled manufacturers to size down tubes to a usable size. The next step was the development of cascade tubes. These are stacked Gen 1 tubes that intensify the image of the tube before. These kind of tubes saw extensive use during the Vietnam war in PVS-1 and PVS-2 Starlight scopes. They where the very first truly passive night vision systems, even though these systems worked best with at least 1/3 of a full moon.
Around the end of the Vietnam war the first Gen 2 IITs entered service. These used the S-25 photocathodes, which were essentially the same materials as used in S-20 photocathodes, but a thicker layer. They had higher gain which made them more suitable for military applications. Also, the most significant development was the shift from accelerating electrons to multiplying them using a microchannel plate (MCP). Significantly improving performance and reduction in size. These MCPs where invented around the year 1964, a new era had begun.
Optic Electronic Corporation of Dallas, Texas, was one of the first companies to use the new Gen 2 technology. In 1975 they developed the famous PVS-4 that was the replacement for the Vietnam War era PVS-2 Starlight Scope. 1972 the PVS-5 was developed and was one of the very first useful head-mounted passive Night Vision System. The U.S. Army used it until the 1990 when it was replaced by the PVS-7.