Night vision |
Night vision is used to describe both biological and technological methods used for visual perception in an environment with very low light. Most instances use a combination of two approaches: Enhanced spectral range, and enhanced intensity range.
Enhanced spectral range is where the viewer is sensitive to light beyond what would be considered normal by a human observer. Human vision is confined to a very small portion of the electromagnetic spectrum which is considered light. This allows the viewer to take advantage of additional sources of light, or make more effective use of the available sources of light.
Enhanced intensity range is simply the ability to see with very small quantities of light. Although the human visual system can, in theory, detect single photons under ideal conditions the neurological noise filters limit sensitivity to where a few tens of photons are required even in ideal conditions [http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html]. Some animals have achieved improved night vision evolutionarily though the use of a larger optical aperture, improved retina composition which can detect weaker light over a larger spectral range, more photoefficent optics in the eye, and improved neurological filtering which is more tolerant of noise. Enhanced intensity range is achieved via technological means through the use of an image intensifier, photo-multiplier tube, or other very low noise and high sensitivity array of photo-detectors.
= Biological night vision =
In biological night vision, rhodopsin in the rods of the eye breaks as light hits it. The peak rhodopsin build up time for optimal night vision in humans is 30 minutes. Rhodopsin in the human rods isn t sensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye s rhodopsin stores in the rods and instead is viewed by the cones.
Some animals, such as cats, dogs, and deer, have a structure called the tapetum in the back of the eye that reflects light for even better night vision than humans in which only 10% of the light that enters the eye falls on photosensitive parts of the retina. Their night vision likely falls between a Generation 1 and Generation 2 image intensifier.
= Thermal vision =
Far infrared, or thermal, sensing is generally not considered night vision because it is constructed with mechanisms substantially different from the methods used to sense visible light. It is possible to construct an imaging device with microwave energy, sound, or any other signal which is reflected or radiated by objects and can be focused and sensed, but these are also not generally considered night-vision.
Some organisms have the ability to sense far infrared energy which we perceive as heat. This is prevalent in some snakes such as the pit viper and boas. However, this is not vision per se, but more of a system of thermo-sensitive pits in their face that can detect the amount of heat and the distance to the heat source. There is still some debate as to what degree this information is perceived as feeling heat, and to what degree it is processed as an image by the snake s brain. Since these sense structures lack imaging optics for focus the spatial resolution of such sensing is necessarily very poor.
Cadillac (automobile) introduced a far-infrared night vision as an option on the 2000 Cadillac Deville. It sold well initially, but sales dropped and the option was dropped early in the 2005 model year.
See Thermographic camera.
=Image intensifier=
The image intensifier is a vacuum-tube based device that amplifies visible light from an image so that a dimly lit scene can be viewed by a camera or by eye. For more information, see image intensifier.
=Night vision goggles=
See main article night vision goggles
Night vision goggles typically use an image intensifier to convert weak light from the visable and near-infrared spectrum to visable light. Most night vision output green colored light because the peak sensitivity of the color human visual system is around 530nm.
= See also =
= Patents =
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