Night vision is the ability of see in low light circumstances. There are basically three different approach for night vision. All approaches use same working principle as human eye. Light hits an object and reflect from it. A detector takes it and turn it into a vision. Human eye or detector should take sufficient energy (light or heat) to see.
Thermal cameras can see without needed light in places where the objects containing or emitting heat. Actually, many objects emitting energy, in other words heat. If the temperature of the object is high, the energy it emits will be more. Each object store and emits the energy in different rates. Therefore, it will be easy to distinguish the objects. Thermal cameras can detect small heat difference at its view angle. Therefore, the visual of the area can be easily seen. The heat of the human body creates a sharp contrast in a relatively cold background. So, thermal cameras are used to see human and animals.
Thermal cameras are actually a sensor rather than a camera. The light that a human is able to see is just a small part of electromagnetic spectrum. Thermal cameras are focusing on the radiation beyond the visible light. Thermal cameras use the heat instead of light to create the visual. Here, the so-called heat is actually infrared.
Night vision devices takes the low amount of light, makes it bigger in certain scale and creates the visual. Night vision devices has same limitations as human eye. Yet it is capable of sensing more sensitively than the human eye. Thus, it can provide high resolution visual even under the moon light. The visual in night vision devices are deliberatively green. Because human eye is more sensitive to green light. Night vision devices can operate in extremely low light. If there is no light literately, then this kind of devices will not work.
Infrared illuminated cameras try to create its own light by emitting infrared rays. Infrared rays hit the objects within its view range and reflect back. Cameras use the reflected rays to create visual. In this technique, the ability to see is depend on reflected rays. This approach is usually used in security cameras.
Each technology has advantages and disadvantages in different conditions. For this reason, the evaluation must be made according to the application, appropriate technology should be selected.
