Night vision Totally Explained

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.

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 UV radiation). Some animals can see well into the infrared and/or ultraviolet, compared to humans; enough to help them see in conditions humans cannot.
Artificial night vision commonly works in two very different ways, depending on the technology used.

Image enhancement - This works by collecting the tiny amounts of light, including the lower portion of the infrared light spectrum, that are present but may be imperceptible to our eyes, and amplifying it to the point that we can easily observe the image.
Thermal imaging - This technology operates by capturing the upper portion of the infrared light spectrum, which is emitted as heat by objects instead of simply reflected as light. Hotter objects, such as warm bodies, emit more of this light than cooler objects like trees or buildings.

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 (External Link

). 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. 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 won't 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 tapetum lucidum in the back of the eye that reflects light back towards the retina, increasing the amount of light it captures. In humans, only 10% of the light that enters the 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.

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 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. The unique makeup of the plastic allows for maximum durability and heat resistance. IR filters provide a more cost effective and time efficient solution over the standard bulb replacement alternative. All Generations of night vision devices are greatly enhanced with the use of IR Filters.

Thermal vision

For infrared, or thermal, sensing is generally not considered night vision because it's 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's reflected or radiated by objects and can be focused and sensed, but these are also not generally considered night vision.
Some animals 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 isn't 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's processed as an image by the snake's brain. Since these sense structures lack imaging optics, the spatial sensory perception resolution is considered very poor.
See Thermographic camera.

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's 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

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.

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