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.

Two American soldiers pictured during the 2003 invasion of Iraq seen through an image intensifier .
Normalised absorption spectra of the three human photopsins and of human rhodopsin (dashed). Drawn after Bowmaker and Dartnall (1980). [ 5 ]
The pupil of the eye dilates in the dark to enhance night vision. Shown here is a pupil of an adult naturally dilated to 9 mm in diameter in mesopic light levels. The average human eye is not able to dilate to this extent without the use of mydriatics.
1974 US Army film about the development of military night vision technology
USMC M3 Sniperscope assembled on a M3 Carbine . Introduced during the Korean War , it was an early active infrared night vision equipment powered by a large 12 volt battery that was carried in a rubberized canvas backpack.
An M60 tank with an infrared searchlight mounted on the cannon.
Binocular night vision goggles on a flight helmet. The green color of the objective lenses is the reflection of the light interference filters, not a glow.
1:posterior segment 2:ora serrata 3:ciliary muscle 4:ciliary zonules 5:Schlemm's canal 6:pupil 7:anterior chamber 8:cornea 9:iris 10:lens cortex 11:lens nucleus 12:ciliary process 13:conjunctiva 14:inferior oblique muscule 15:inferior rectus muscule 16:medial rectus muscle 17:retinal arteries and veins 18:optic disc 19:dura mater 20:central retinal artery 21:central retinal vein 22:optic nerve 23:vorticose vein 24:bulbar sheath 25:macula 26:fovea 27:sclera 28:choroid 29:superior rectus muscle 30:retina 1: posterior segment 2: ora serrata 3: ciliary muscle 4: ciliary zonules 5: Schlemm's canal 6: pupil 7: anterior chamber 8: cornea 9: iris 10: lens cortex 11: lens nucleus 12: ciliary process 13: conjunctiva 14: inferior oblique muscule 15: inferior rectus muscule 16: medial rectus muscle 17: retinal arteries and veins 18: optic disc 19: dura mater 20: central retinal artery 21: central retinal vein 22: optic nerve 23: vorticose vein 24: bulbar sheath 25: macula 26: fovea 27: sclera 28: choroid 29: superior rectus muscle 30: retina
1:posterior segment 2:ora serrata 3:ciliary muscle 4:ciliary zonules 5:Schlemm's canal 6:pupil 7:anterior chamber 8:cornea 9:iris 10:lens cortex 11:lens nucleus 12:ciliary process 13:conjunctiva 14:inferior oblique muscule 15:inferior rectus muscule 16:medial rectus muscle 17:retinal arteries and veins 18:optic disc 19:dura mater 20:central retinal artery 21:central retinal vein 22:optic nerve 23:vorticose vein 24:bulbar sheath 25:macula 26:fovea 27:sclera 28:choroid 29:superior rectus muscle 30:retina