Night vision

Night vision is the ability to see, whether through biological or technological means, in a dark environment. Most instances, whether biological or technological, use a combination of two approaches: enhanced spectral range, and enhanced intensity range.

Enhanced spectral range
Enhanced spectral range techniques make the viewer sensitive to types of light that would be invisible to a human observer. Human vision is confined to a small portion of the electromagnetic spectrum called visible light. Enhanced spectral range allows the viewer to take advantage of non-visible sources of electromagnetic radiation (such as near-infrared or UV radiation).

Enhanced intensity range
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 a few tens of photons, even in ideal conditions. Some animals have evolved better night vision through the use of a larger optical aperture, improved retina composition that can detect weaker light over a larger spectral range, more photoefficient 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, gain multiplication CCD, or other very low-noise and high-sensitivity array of photodetectors.

Biological night vision
In biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as light is absorbed by them. The peak rhodopsin build-up time for optimal night vision in humans is 30 minutes. Rhodopsin in the human rods is insensitive 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.

Night glasses
Night glasses are telescopes or binoculars with a large diameter objective. Large lenses can gather and concentrate light, thus intensifying light with purely optical means and enabling the user to see better in the dark than with naked eye alone. Often night glasses also have a fairly large exit pupil of 7 mm or more to let all gathered light into the user's eye. However, many people can't take advantage of this because of the limited dilation of the human pupil. To overcome this, soldiers were sometimes issued atropine eyedrops to dilate pupils. Before the introduction of image intensifiers, night glasses were the only method of night vision, and thus were widely utilised, especially at sea. Second World War era night glasses usually had a lens diameter of 56 mm or more with magnification of seven or eight. Major drawbacks of night glasses are their large size and weight.

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 that 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 pit vipers and boas. However, this is not actual "vision", but more of a system of thermosensitive pits in the 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 introduced a far-infrared night vision as an option on the 2000 Deville. It sold well initially, but sales fell and the option was dropped early in the 2005 model year. Toyota Motor Company's Lexus marque was next with a night vision system in their LX470 SUV. The 2007 BMW 5 Series will also offer night vision as an option.

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.

Night vision goggles
Night vision goggles typically use an image intensifier to convert weak light from the visible and near-infrared spectrum to visible light. Most night vision goggles display a green image, because the peak sensitivity of human color vision is around 530 nm.

Patents

 * US D248860 - Night vision Pocketscope
 * US 4707595 - Invisible light beam projector and night vision system
 * US 4991183 - Target illuminators and systems employing same
 * US 6075644 - Panoramic night vision goggles