Night Vision Devices are electronic optical devices which collect and amplify existing light through the objective lens which is then focused on the image intensifier. These devices are very sensitive to the wide spectrum of light - from visible to infrared. Auxiliary illuminator can increase existing light in infrared spectrum by directing light beam invisible to man's eyes. You do not see “through” a device. You see the amplified electronic image on the screen with luminophore coating. Therefore the image is in green colours. Inside the intensifier, a photocathode is "excited" by the light and converts the photon energy in to electrons. These electrons are accelerated across an electrostatic field inside the intensifier and strike a luminophore screen (like in a TV set), which emits an image that you can see. It is the acceleration of electrons, which provides gain and enhances the image. The eyepiece amplifies the image. Image converter (a tube) is a main part of a night vision device. It directs and accelerates photoelectronic stream. Basically the quality of NV devices depends on characteristics of a tube and quality of optical parts. According to practice accepted in the world tubes are classified as follows: Generation I, II, III (with some intermediate variants Gen. I+, II+).