A night vision device works by taking existing light, whether infra-red, moonlight or starlight, and changing it into visible light. This visible light is amplified to the point where it may be viewed in the device's eyepiece. The process, where the existing light (made up of photons) moves through a photocathode tube, changes to electrons that are electro-chemically amplified and then driven against a phosphorus screen to return them to visible light, gives the final image a green hue. Night vision devices are referred to by what "Generation" they are. Each generation has a different type of intensifier tube.
1st Generation amplifies light several thousand times. The image will generally be sharp in the center and slightly blurry around the edges, allowing wildlife observation or for general security use. A 1st generation will hum when on, it may continue to glow for some time after it is turned off, but is the least expensive type of night vision device.
2nd Generation has an extra process because a micro-channel plate is directly behind the photocathode tube and amplifies the light many times more than a 1st generation. The image is sharper and brighter.
3rd Generation has a chemical added to the photocathode and a film is added to increase tube life. These changes give excellent low light performance.
4th Generation does not use the film in the 3rd generation but uses a radically different way in which power is supplied to the unit. Image resolution is increased and there is a significant decrease in interference from bright light sources. Range is increased. The US Military does not use the 4th generation designation. These devices are referred to as "Filmless & Gated".
Regardless of generation, the image may have some black spots on it. These do not affect the performance or reliability of the device and should not be considered defects.