Night vision is a term that refers to the ability to produce visual imagery under low lighting conditions. Many animals (mostly nocturnal predators) have better night vision than that of humans, thanks to a wider spectral range of their visual sensors (extending beyond visible light into the infra-red and ultraviolet parts of the spectrum), or differences in the physiological structure of the eye, allowing for better photon capture and absorption. By applying the same principles to existing visual sensors, night vision devices were created.
The most common type of night vision device (NVD), mostly prominent in military use, is based on image intensification. Simply speaking, the device (also called Night Optical Device or NOD) uses a very sensitive photocathode as a sensor, which, by making use of the photoelectric effect, releases an electronic signal that is amplified by the device, and finally converted to a clearer picture with a higher contrast then seen by the naked eye. The technology has been in development since WWII, achieving in its latest generation light amplification of 30,000-50,000 times, and impressive resolution.
In some cases, when the ambient light is not enough, the sensor is switched to IR range, and active illumination is added – the environment is illuminated by IR light, thus avoiding detection by visible range sensors and enabling more light to be reflected off of objects, turning them visible to the NOD. This method is mostly used in night vision cameras, enabling photography and video in little-to-no-light conditions, such as moonless nights.
The other common method used in night vision technology is that of infrared thermography (IRT), also called thermal imaging. This method uses the natural infrared radiation, produced by all objects with a temperature above absolute zero, to create an image without the need for visible light. Usually the sensor used in this type of NVD is perceptive in the 9–14 µm range. This method makes external illumination and direct line of sight unnecessary for creating an image, and used most effectively in locating and tracking humans and other warm-blooded animals – which produce considerable (and visible) heat. Many security cameras include an IRT mode for low light conditions.
Outside of its use as an NVD technology, thermal imaging is also used by firefighters to assess situation and locate victims in a fire through the smoke, by engineers to locate infrastructure and thermal insulation faults, and by certain types of medical diagnosis equipment – since some health conditions manifest a change in body temperature of warm-blooded animals as a symptom.
Night vision equipment is extremely sensitive to stray light (since its main function is to amplify immensely any light it receives), most sensitive being the image-intensifier tube. The challenge lies in producing a "protective layer" inside the tube, to absorb stray light, improving resolution and contrast, without absorbing so much as to reduce the brightness and clarity of the image.