night vision (thing)

Night vision units are Electro-optical devices which do not rely on their own light source. They simply amplify (or intensify) available light. Night vision units are sensitive to a wide range of the spectrum, from visible to infrared. (You can even purchase infrared beam casters that dramatically increase infrared sensitivity, without displaying anything visible to the naked eye. Cool Stuff. You'd be amazed at how it looks through the lens. It looks like you're shining a spotlight, but when you look at it with the naked eye, you see nothing. )

When you look through something equipped with night vision, you're not really looking "through" it per se - you're actually looking at the amplified image which is being displayed in real time on a phosphor screen, which has been colored green. The green color is intentional, as the human eye can discern a greater number of shades of green than any other phosphor colors. The fact that you're actually looking at a display rather than the actual physical world through a lens is why looking through a night-vision device severely impairs depth perception.

This is a (quite crude) drawing I did of how a light vision unit works:

         ._______.
         |  |____|__
Light -> |  |____|__|
         |__|_|__| |
         `    |  ` |
              |   Eyepiece
              |
          Image Intensifier


                  |------Vacuum------|       
                  |                  |
    (Light -> [ ] | -> [ ] -> [ ] -> | [ ])
               |        |      |        |
             Lens       |      |        |
                 Photo Cathode |    Eyepiece
                               |
                        Phosphor Screen

The top diagram shows the construction of a typical night vision unit. Light enters the unit through the objective lens, and strikes the photo cathode. The photo cathode has a high energy charge from the built-in power supply, usually consisting of a rechargable battery unit. The energy charge from the power supply accelerates across a vacuum inside the image intensifier, and strikes the phosphor screen where the image is displayed. The eyepiece simply magnifies the image displayed on the screen.

The bottom diagram shows the layout of the image intensifier. The phosphor screen works just like the screen in your television or monitor - except there's only one photon gun in the night vision unit, as opposed to three in your TV or monitor.