Night vision

Two American Soldiers pictured during the 2003 Iraq War seen through an Image Intensifier

Night vision is the ability to see in a dark environment. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range. Humans have poor night vision compared to many animals, in part because the human eye does not have a tapetum lucidum.

Night vision approaches

Spectral range

Night-useful 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 ultraviolet radiation). Some animals can see well into the infrared and/or ultraviolet compared to humans, enough to help them see in conditions humans cannot.

Intensity range

Sufficient 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.[1]

Many animals have better night vision than humans do, the result of one or more differences in the morphology and anatomy of their eyes. These include having a larger eyeball, a larger lens, a larger optical aperture (the pupils may expand to the physical limit of the eyelids), more rods than cones (or rods exclusively) in the retina, a tapetum lucidum, and improved neurological filtering.

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. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. Exposed to white light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully, but most of the adaptation occurs within the first five or ten minutes in the dark. 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.

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 in many nocturnal animals and some deep sea animals, and is the cause of eyeshine. Humans do not have a tapetum lucidum and, moreover, only 10% of the light that enters the human eye falls on photosensitive parts of the retina. An animal's ability to see in low light levels may be similar to what humans see when using first or perhaps second generation image intensifiers.

Large size of the eye, and large size of the pupil relative to the eye, also contribute to night vision.

Night glasses

Binoculars (night vision goggles on flight helmet)

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 eye drops to dilate pupils. 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.

Infrared filters

Infrared (IR) filters are made of polysulphone plastic that blocks over 99% of the visible light spectrum from any “white” light source. Infrared filters allow a maximum of infrared output while maintaining extreme covertness. Currently in use around the world, infrared filters are used in military, law enforcement, industrial and commercial applications.

Active Infrared

File:Extreme-CCTV-Active-Infrared-Night-Vision.jpg

Imaging results with and without active-infrared.

Active infrared night vision combines infrared illumination of spectral range 700nm-1000nm - just beyond the visible spectrum of the human eye - with special CCD cameras sensitive to this light. The resulting scene, which is apparently dark to a human observer, appears as a monochrome image on a normal display device.[2]

Because active infrared night vision systems can incorporate illuminators that produce high levels of infrared light, the resulting images are typically higher resolution than other night vision technologies[3][4]. Active infrared night vision is now commonly found in commercial, residential and government security applications, where it enables effective night time imaging under low light conditions. However, since active infrared light can be detected by night vision goggles, it is generally not used in tactical military operations.

Thermal vision

Thermal imaging cameras are excellent tools for night vision. Contrary to other technologies they do not need any light at all to produce a clear image. They produce an image in the darkest of nights and can see through light fog, rain and smoke. Thermal imaging cameras make small temperature differences visible. They detect what can not be detected by the human eye. Thermal imaging cameras are widely used to complement new or existing security networks. See Thermographic camera.

Image intensifier

Main article: Image intensifier

The image intensifier is a vacuum-tube based device that converts visible light from an image so that a dimly lit scene can be viewed by a camera or the naked eye. While many believe the light is "amplified," it is not. When IR light strikes a charged photocathode plate, electrons are emitted through a vacuum tube that strike the microchannel plate that cause the image screen to illuminate with a picture in the same pattern as the IR light that strikes the photocathode, and is on a frequency that the human eye can see. This is much like a CRT television, but instead of color guns the photocathode does the emitting. The image is said to become "intensified" because the output visible light is brighter than the incoming IR light, and this effect directly relates to the difference in passive and active night vision goggles. Currently, the most popular image intensifier is the drop-in ANVIS module.

Night vision devices

Main article: Night vision device

A night vision device (NVD) is a device comprising an IR image intensifier tube in a rigid casing, commonly used by military forces. A specific type of NVD, the night vision goggle (or NVG) is a night vision device with dual eyepieces; the device can utilize either one intensifier tube with the same image sent to both eyes, or a separate image intensifier tube for each eye. Other types include monocular night vision devices with only one eyepiece which may be mounted to firearms as night sights.

See also

• Thermographic camera

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

• U.S. patent 7,173,237

• US 6158879 - Infra-red reflector and illumination system

World Manufacturers of Night Vision Devices

• US Night Vision USA
• American Technologies Network Corporation USA
• ITT USA

External links

• Night Vision & Electronic Sensors Directorate - Fort Belvoir, Virginia