The image intensifier is a vacuum-tube based device that converts visible light from an image so that a dimly lit scene can be viewed by a camera or the naked eye. While many believe the light is 'amplified,' it is not. When IR light strikes a charged photocathode plate electrons are emitted through a vacuum tube that strike the microchannel plate that cause the image screen to illuminate with a picture in the same pattern as the IR light that strikes the photocathode, and is on a frequency that the human eye can see. This is much like a CRT television, but instead of colour guns the photocathode does the emitting. The image is said to become 'intensified' because the output visible light is brighter than the incoming IR light, and this effect directly relates to the difference in passive and active night vision goggles. Amplification of light is the result of LASERs (Light Amplification through Stimulated Emission of Radiation). Lasers are quite intense and can destroy an imagetube in a fraction of a second. Currently, the most popular image intensifier is the drop-in ANVIS module.

Night vision goggles refer to an IR or thermal night vision device with dual eyepieces; the device can utilize either one intensifier tube with the same image sent to both eyes, or binocular eyepieces with an image intensifier tube for each eye. This is opposed to a monocular night vision device which uses only one eyepiece, such as a weapon mounted sight glass. Binocular night vision has the advantage of individually focusing the image to each eye to maintain depth perception, which is not possible with monocular night vision.

An image intensifier is a device that amplifies visible and near-infrared light from an image so that a dimly lit scene can be viewed by a camera or by eye. Unlike a thermographic camera, an image intensifier does not work in the total absence of visible (or near infra-red) light. It does, however, create a more realistic image, because the intensities it shows are related to true optical intensity and not to temperature. This realism makes it more suitable for use by untrained operators and can be used to view objects not visible by a difference in temperature alone. Image intensifiers are also much less expensive than thermal imaging.

HOW IT WORKS
Image intensifiers work by having an objective lens focusing an image into a vacuum tube with a photocathode at one end that releases electrons by the photoelectric effect on the incidence of incoming photons. From there, the photoelectron is accelerated through around 5000 volts into a tilted microchannel plate. The high energy electron releases some micro channel plate electrons, which further release other electrons, in a process called secondary cascaded emission. The MCP is tilted to encourage more electron collisions, thus increasing the rate of emission of secondary electrons.
The electrons all move together due to the potential difference across the tube, and where one or two electrons entered, thousands may emerge. A separate (lower) charge differential in the tube accelerates the secondary electrons until they hit a phosphor screen at the other end, releasing a photon for every electron. Provided the electric field is uniform, the electrons have a linear path, so correspond to the exact incident image. The image is focused by conventional optics using an ocular lens. The only multiplicative stage is in the secondary cascaded emission. The phosphor is usually green, as the human eye is more sensitive to green than other colours (hence the soldiers' nickname 'green TV' for image intensification devices).