Night vision

Humans have poor night vision compared to many animals such as cats, dogs, foxes and rabbits, in part because the human eye lacks a tapetum lucidum,[1] tissue behind the retina that reflects light back through the retina thus increasing the light available to the photoreceptors.

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 arrays of photodetectors.

The net effect of this anatomical change is to multiply the light sensitivity of the retina by a factor of eight to ten with no loss of focus.

Digital night vision is also emerging, which instead uses high sensitivity CMOS image sensors with a passthrough system.

In the military context, Image Intensifiers are often called "Low Light TV" since the video signal is often transmitted to a display within a control center.

These are usually integrated into a sensor containing both visible and IR detectors and the streams are used independently or in fused mode, depending on the mission at hand's requirements.

[11] The image intensifier is a vacuum-tube based device (photomultiplier tube) that can generate an image from a very small number of photons (such as the light from stars in the sky) so that a dimly lit scene can be viewed in real-time by the naked eye via visual output, or stored as data for later analysis.

This causes the image screen to illuminate with a picture in the same pattern as the light that strikes the photocathode and on a wavelength the human eye can see.

Recently, the US Navy announced intentions to procure a dual-color variant of the ANVIS for use in the cockpit of airborne platforms.

[13] These sensors can be head-mounted in night vision goggles and rifle scopes, but are also used in security camera systems, astronomy, and microscopy.

The resulting scene, which is apparently dark to a human observer, appears as a monochrome image on a normal display device.

However, since active infrared light can be detected by night-vision goggles, there can be a risk of giving away position in tactical military operations.

Range gating is a technique which controls the laser pulses in conjunction with the shutter speed of the camera's detectors.

One of the key advantages of this technique is the ability to perform target recognition rather than mere detection, as is the case with thermal imaging.

They are widely used to complement new or existing security networks, and for night vision on aircraft, where they are commonly referred to as "FLIR" (for "forward-looking infrared").

For example, enhanced vision systems (EVS) have become available for aircraft, to augment the situational awareness of pilots to prevent accidents.

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.

Some higher end devices including the PVS-31 binocular and GPNVG-18 quad-tube night vision are used by special forces groups, but are costly.

An automotive night vision system is used to improve a vehicle driver's perception and seeing distance in darkness or poor weather.

Such systems typically use infrared cameras, sometimes combined with active illumination techniques, to collect information that is then displayed to the driver.