Night visionFrom Wikipedia, the free encyclopediaFor other uses, see Night vision (disambiguation).Two American soldiers pictured during the2003 Iraq War seen through an image intensifierNight vision is the ability to see in low light conditions. Whether by biological or technological means,night vision is made possible by a combination of two approaches: sufficient spectral range, andsufficient intensity range. Humans have poor night vision compared to many animals, in part becausethe human eye lacks atapetum lucidum. Contents [hide] 1 Types of rangeso 1.1 Spectral rangeo 1.2 Intensity range 2 Biological night vision 3 Night vision technologies 4 Night vision goggles 5 Active infrared 6 Laser range gated imaging 7 Thermal vision 8 Image intensifier 9 Night vision devices 10 Automotive night vision 11 See also 12 Patents
13 References 14 External linksTypes of rangesSpectral rangeNight-useful spectral range techniques can sense radiation that is 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 ofelectromagnetic radiation (such as near-infrared or ultraviolet radiation). Some animals can see usingmuch more of the infrared and/or ultraviolet spectrum than humans.Intensity rangeSufficient intensity range is simply the ability to see with very small quantities of light. Many animals have better night vision than humans do, the result of one or more differences in themorphology and anatomy of their eyes. These include having a larger eyeball, a larger lens, a largeroptical aperture (the pupils may expand to the physical limit of the eyelids), more rods than cones (orrods exclusively) in the retina, and a tapetum lucidum.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 visionFor more details on this topic, see Adaptation (eye).In biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape asthey absorb light. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light.Exposed to a spectrum of light, the pigment immediately bleaches, and it takes about 30 minutes toregenerate fully, but most of the adaptation occurs within the first five or ten minutes in the dark.Rhodopsin in the human rods is less sensitive to the longer red wavelengths of light, so traditionallymany people use red light to help preserve night vision as it only slowly depletes the eyes rhodopsinstores in the rods and instead is viewed by the cones. However the US submarine force ceased usingred lighting for night adaptation after studies found little significant advantage of using low level redover low level white lighting.Many animals have a tissue layer called the tapetum lucidum in the back of the eye that reflects light
back through the retina, increasing the amount of light available for it to capture. This is found inmany nocturnal animals and some deep sea animals, and is the cause of eyeshine. Humans lack atapetum lucidum.Nocturnal mammals have rods with unique properties that make enhanced night vision possible. Thenuclear pattern of their rods changes shortly after birth to become inverted. In contrast tocontemporary rods, inverted rods have heterochromatin in the center of their nucleiand euchromatin and other transcription factors along the border. In addition, the outer nuclearlayer (ONL) in nocturnal mammals is thick due to the millions of rods present to process the lower lightintensities of a few photons. Rather than being scattered, the light is passed to each nucleusindividually. In fact, an animals ability to see in low light levels may be similar to what humans seewhen using first- or perhaps second-generation image intensifiers.A larger size of pupil relative to the rest of the eye, also aids night vision. Night vision technologiesFilm about the development of military night vision technologyNight vision technologies can be broadly divided into three main categories:Image intensification Image intensification technologies work on the principle of magnifying the amount of received photons from various natural sources such as starlight or moonlight. Examples of such technologies include night glasses and low light cameras. Active illumination Active illumination technologies work on the principle of coupling imaging intensification technology with an active source of illumination in the near infrared (NIR) or shortwave infrared (SWIR) band. Examples of such technologies include low light cameras. Thermal imaging
Thermal imaging technologies work by detecting the temperature difference between thebackground and the foreground objects. Some organisms are able to sense a crude thermalimage by means of special organs that function as bolometers. This allows thermal infraredsensing in snakes, which functions by detection of thermal radiation. Night vision goggles This section does not cite anyreferences or sources. (May 2012) Binoculars (night vision goggles on flight helmet) Note: the green color of the objective lenses is the reflection of the Light Interference Filters, not a glow. 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 the 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 users eye. However, many people cant take advantage of this because of the limited dilation of the human pupil. To overcome this, soldiers were sometimes issued atropine eye drops to dilate pupils.[when?] Before the introduction of image intensifiers, night glasses were the only method of night vision, and thus were widely utilized, 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. Active infrared
Imaging results with (top) and without (bottom) active-infrared.Active infrared night vision combines infrared illumination of spectral range 700–1,000 nm (just below the visible spectrum of the human eye) with CCD camerassensitive to this light. The resulting scene, which is apparently dark to a human observer,appears as a monochrome image on a normal display device.Because active infrared night vision systems can incorporate illuminators that producehigh levels of infrared light, the resulting images are typically higher resolution than othernight vision technologies. Active infrared night vision is now commonly found incommercial, residential and government security applications, where it enables effectivenight time imaging under low light conditions. However, since active infrared light can bedetected by night vision goggles, there can be a risk of giving away position in tacticalmilitary operations.Laser range gated imagingLaser range gated imaging is another form of active night vision which utilizes a highpowered pulsed light source for illumination and imaging. Range gating is a techniquewhich controls the laser pulses in conjunction with the shutter speed of the camerasdetectors. Gated imaging technology can be divided into single shot, where thedetector captures the image from a single light pulse to multi-shot, where the detectorintegrates the light pulses from multiple shots to form an image.
One of the key advantages of this technique is the ability to perform target recognition asopposed to detection with thermal imaging.Thermal visionSee also: thermographic camera and Forward_looking_infraredThermal imaging cameras are excellent tools for night vision. They detect thermalradiation and do not need a source of illumination. They produce an image in the darkestof nights and can see through light fog, rain and smoke. Thermal imaging cameras makesmall temperature differences visible. Thermal imaging cameras are widely used tocomplement new or existing security networks, and for night vision on aircraft, wherethey are commonly referred to as "FLIR" (for "forward-looking infrared".)Image intensifierMain article: Image intensifierThe image intensifier is a vacuum-tube based device that converts invisible light from animage to visible light so that a dimly lit scene can be viewed by a camera or the nakedeye. While many believe the light is "amplified," it is not. When light strikes achargedphotocathode plate, electrons are emitted through a vacuum tube that strike themicrochannel plate that cause the image screen to illuminate with a picture in the samepattern as the light that strikes the photocathode, and is on a frequency that the humaneye can see. This is much like a CRT television, but instead of color guns thephotocathode does the emitting.The image is said to become "intensified" because the output visible light is brighter thanthe incoming IR light, and this effect directly relates to the difference in passive andactive night vision goggles. Currently, the most popular image intensifier is the drop-inANVIS module, though many other models and sizes are available at the market.Night vision devicesMain article: Night vision deviceA night vision device (NVD) is a device comprising an image intensifier tube in a rigidcasing, commonly used by military forces. Lately, night vision technology has becomemore widely available for civilian use. For example, enhanced vision systems (EVS) havebecome available for aircraft to help pilots with situational awareness and avoidaccidents. These systems are included in the latest avionics packages frommanufacturers such as Cirrus and Cessna.
A specific type of NVD, the night vision goggle (NVG) is a night vision device with dualeyepieces. The device can utilize either one intensifier tube with the same image sent toboth eyes, or a separate image intensifier tube for each eye. Night vision gogglecombined with magnification lenses constitutes night vision binoculars. Other typesinclude monocular night vision devices with only one eyepiece which may be mounted tofirearms as night sights. NVG and EVS technologies are becoming more popularproducts for helicopter operations to improve safety. The NTSB is considering EVS asrecommended equipment for safety features.