There are two different methods used by night vision equipment for visibility in darkness :
1) Thermal Imaging : This method uses the heat emitted by objects to its advantage and shows that to the viewer. The infrared light is detected by infrared-detectors and a detailed temperature pattern is created(Thermogram). The thermogram is converted into electric impulses, these impulses are sent to a signal processing unit, the processed signals are sent to a display and the image is shown.
2) Light Amplification : This is a more commonly used method. In this method, small amounts of light in the surrounding area are converted into electrical energy. Electrons pass through a thin disk, and are multiplied, these electrons bounce off a phosphor screen which converts them back to light. This light is what the viewer sees and enables the viewer to see in the dark.
Ali Memon, High School Student, Melbourne.
First, I'll assume by night vision you are referring to the way the military and others use equipment to generate those funny green-TV looking pictures. The way your eyes adapt to dim light is also very interesting, but we'll save that for later.
The military uses what is basically a gigantic photomultiplier tube to "see" at night. As an individual photon enters the NVG (night vision goggle), it strikes a highly charged cathode plate, which emits multiple electrons. That spray of electrons then strikes the phosphor plate (just like inside your TV) and what you "see" is an image at night that looks sort of like it does in the day. As a matter of fact, it is possible for things to be SO dark that you can't see even with NVG's. It's so dark that you can see the electronic "noise" from inside the tube. It's like visual static, and that phenomenon is called "scintillation".
Modern NVG systems use GaAs (Gallium Arsenide) chips for the cathode plate, and have a very special sensitivity to Infrared light. As a matter of fact, you have no way of knowing if a light you see is actually visible or if it's just a heat source. Also, because many factors that humans use for regular vision are removed (depth perception, visual acuity) the very best your sight can be with NVG's is about 20/25 to 20/40, even in "perfect" light. Also, they have only a 40 degree field of view. So imagine trying to fly an airplane fast, down low, with your two hands like binoculars over your eyes. Fun.
There's a great article I've attached here that explains in a little better detail:
Frank DiBonaventuro, B.S., Air Force officer, Physics grad, The Citadel
'One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery every day.'