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.

How done

Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range.

Human eyes

Humans have poor night vision compared to many animals, in part because the human eye does not have a tapetum lucidum.

Properties

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).

Animal eyes

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

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.

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 or in earlier days, for viewing the sighting of planes at Air ports, in India.