Night vision

Night vision is the ability to see in (relative) darkness. All natural vision involves reception of some light by the eye. This is true even in the dark; vision is possible only when light is present. Often, what appears to be total darkness is not the absence of light but rather the poor reception of available light. To see well in the dark, humans either enhance reception of limited available light, or provide more light.

Light in the dark

Many techniques and technologies for improving night vision involve reception of certain frequency bands (colors) of light. These frequency bands include:

• infrared light (heat)
• red light
• soft ultraviolet (UV) light (also known as blacklight)

Infrared light

A deer's pelt is insulating, keeping the animal warm and limiting the amount of IR light emitted; the eyes and mouth, uninsulated, glow with IR light

Because the human eye does not detect IR light, our visual detection of this light requires some kind of electronic imaging device. Many military and civilian SAR helicopters are equipped with forward looking infra red (FLIR) devices.

Infrared (IR) imaging is used to search for clues that are expected to have a temperature far above ambient. Thus IR imaging is used most often at night (when ambient temperatures are lower) but also may be used in daylight, especially in cold weather. IR-detectable clues include hot engines, campfires, and live subjects. IR imaging cannot detect a clue that is not above ambient temperature, due to lack of contrast between clue and background. Thus, IR imaging may be ineffective during early night in hot deserts, and when the subject is under cover, dressed in heavy winter clothes, or deceased.

Red light

Spectra of the three human photopsins (red, green, and blue) and of human rhodopsin (dashed)

In humans, the rod cells that provide normal night vision are not sensitive to red light. Thus, a source of red light (such as a flashlight with a red filter) can be used to provide sufficient light for the cone cells without "blinding" the rod cells.

Black light

Scorpion under black light

Scorpions appear black in daylight but under black light they glow blue. Black light flashlights are used to find scorpions in the wild.

Airman Battle Uniform

Humans normally do not glow under black light, but often our clothing does. Most laundry detergents contain optical brighteners, fluorescent dyes that absorb black light and emit blue light. These dyes make clothing appear brighter under daylight; under black light, they glow (fluoresce) blue. Black light may be used at night to search for a subject's clothing. Because these dyes are in most laundry detergents, even in those labeled "no dyes", it may be not helpful to ask the subject's household members what brand of laundry detergent they use. Shining a black light on a sample of the subject's clothing may be helpful.

In 2007, the United States Air Force distributed laundry instructions for camouflage uniforms that included the instruction to use no detergents containing these dyes. The Air Force later rescinded this instruction on the ground that research by the US Army found the effect was not important in the context of military combat.

Because horses' eyes are highly receptive for blue light, the combination of black light flashlights and mounted SAR field teams may be particularly effective.

Seeing in the dark

Reception of limited available light can be enhanced by using advanced technology and/or ancient behaviors most widely used by trackers and hunters.

• electronics

Scene viewed through an image intensifier

• larger lenses: animals with very large eyes (eyeballs) tend to have very large lenses. Thus, horses have far better night vision than humans do. Humans can enhance their own night vision by using binoculars or scopes with lenses of very large diameter. All else being equal, due to the high cost of manufacturing large high quality lenses, these devices are far more expensive than others.

A night scope mounted on a military weapon

• rods instead of cones

• peripheral instead of central vision

Left eye scene on retina (simulation)

Human vs dog

Human dog horse

• 
  Human pupils are round
• 
  Dogs pupils are round
• 
  Horse pupils are horizontal ovals

Left eye field of view

External links

• Wikipedia: eye

Questions

1. Why is the US military not concerned about dyes on camouflage uniforms? Because the effect of these dyes occurs only when illuminated with black light, and for combat use artificial illumination has limited tactical advantage and greater tactical disadvantage.

1. Does this lack of concern mean black light is not useful for SAR? Why? No, because for SAR black light illumination has more tactical advantages than disadvantages.