NVIS goggles consist of sometimes one, usually two image intensifier tubes and look much like binoculars mounted to a head strap or helmet. An NVIS intensifier tube (figure 5) is a vacuum tube device similar to a miniature video camera and screen packaged together. The stages in the intensifier tube are:
- a photocathode receptor, which converts visible and IR light energy into electrons.
- a microchannel plate, which multiplies the number of electrons emitted by the photocathode.
- a green phosphor screen, which converts the electrons into a visible image.
Figure 5. Diagram of an NVIS intensifier tube
All three elements are packaged close to each other to minimize blurring of the converted visible image. An electron accelerating voltage is applied between each of the three stages to produce amplification and maintain image quality.
The two main types of intensifier tube in use are classified as Gen 2, which uses a multialkali photocathode, and Gen 3, which uses the far more sensitive gallium arsenide photocathodes developed in the 1970s. The improvement is obvious by comparing typical Gen 2 and Gen 3 response curves (figure 6). Gen 3 tubes cost approximately three times more than Gen 2, but their useful life is also three times higher.