The basis of night vision is a conversion of energy that occurs within a vacuum tube. An image intensifier tube collects photons through an objective lens, via a photo cathode converts them to electrons, increases the electrical energy by electron acceleration or multiplication, converting the electrical energy back to photons using a phosphor screen and presenting the image through an ocular lens.
A miniaturized power supply (PSU) is used to provide the voltages between the elements of the vacuum tube that allow for the energy conversion and amplification. All of the elements within the tube are closely spaced to minimize electron scatter.
In the night vision world, the word generation (Gen) refers to major leap in technology. The higher the generation, the more advanced the image intensification technology. The generation gap is the change in technology that drives the change in nomenclature.
Early snipers used image converters (sniper-scopes) that required an additional infrared light source to illuminate the scenery. Known as Gen 0, these image converters evolved from RCAs image converter tube developed in the mid-1930s for use in televisions. The Gen 0 image converter used an S-1 photo-cathode, a high-voltage electron acceleration electrostatic field and a phosphor screen. The S-1 cathode (AgOCs) did not have as much quantum efficiency as the cathodes used today, it heavily relied on the IR light used to illuminate the surrounding.
The process by which the image was intensified was quite simple in this generation. It was more a transformation then an intensification, transforming the invisible IR light in to visible light. The reflected IR illuminator light entered the tube and the photo-cathode converted the light to electrons. Electronic elements focused these electrons through a coneshaped anode and accelerated them using high voltage so they hit the phosphor screen with greater energy, creating a visible image. This did not produce much gain and caused distortion in the image. Also, tube life was not very good by todays standards.
The starlight scope, developed during the early 1960s and used during the Vietnam War, was made using Gen 1 image-intensifier tubes. In this scope, three image-intensifier tubes were connected in series, making the unit larger and heavier than todays night-vision goggles. This early generation produced a clear center image with a distorted periphery. The use of multiple tubes connected in series allowed for much greater overall light gain as the output of the first tube was amplified by the second and the second by the third. Due to the simple power supply design, the image was subject to instances of blooming momentary image washout due to an overload in the intensifier tube caused by bright light sources.
The primary difference between Gen 1 and Gen 0 was the more sophisticated chemical process employed to create the photo-cathode. The S-20 cathode, a multi-alkali antimonide process, enhanced the sensitivity as well as the spectral response. However, Gen 1 did have some of the same drawbacks of image distortion and decreased tube life as seen with Gen 0. Tubes built with Gen 0 and Gen 1 technologies are commonly found in many of todays civilian low budget night vision viewers.