I Can See In The Dark!

Some Basic Science

The basic definition of night vision, which probably everyone knows, is the ability to see in a dark environment.  Night vision can be made possible by either biological or technological methods using a combination of two approaches, which are specific intensity range and sufficient spectral range.  Human beings have poorer night visibility than many animals, probably the result of differences in the morphology and anatomy of their eyes.  There are several differences including the fact that an animal has a tapetum lucidem - a carpet of light - which is located behind the retina, and it reflects light back onto the retina from the rear, increasing the amount of light available.  That's why the dog's eyes glow when a flash picture is taken of him or when headlights hit an animal's eyes.  That also might be why the cat wonders why you tripped over her in the dark.  Okay, so much for the biology lesson.

Night Vision Devices

Night vision devices (NVDs) allow for images to be seen in minimal lighting, approaching total darkness.  While the majority of use is in the military and law enforcement fields, there are those in the civilian population who use them as well.  Night vision devices were initially used in WW11 and became more widely used in Vietnam.  Technology has advanced the performance of such equipment and has lead to several generations of night vision equipment being produced.  Most night vision devices do not detect color information, hence a single color phosphor screen.  Since the human eye is most sensitive to the color green, which is in the middle of the color spectrum, most NVD screens are in that color.

Binoculars and Telescopes

Night glasses, including night vision binoculars and telescopes have a large diameter objective and a fairly large exit pupil of 7mm or more in order to allow all of the light gathered to enter the user's eye.  Because many people have limited dilation of the pupil it is difficult to take advantage of the larger exit pupil, consequently soldiers are generally issued atroping eye drops to dilate the pupil.

Infrared and Active Infrared

Infrared filters block over 99% of the visible light spectrum from any "white" light source and allow for maximum output and, at the same time, extreme covertness.  Active infrared is used today in many security applications enabling brighter view of night time, low light imagine.  Night vision goggles can detect active infrared light and as a result, the military does not generally use this system of lighting.

Thermal Vision

Thermal vision does not require light to produce an image and can give visibility through fog, rain and smoke. They are able to detect what the human eye cannot.  Today they are used widely to enhance and complement security systems.  Thermal vision forms an image using infrared radiation which operates in wavelengths.

Night vision devices are also constructed in a monocular fashion in order to be mounted on firearms.  The military and law enforcement agencies have been using them for years, but now they are being marketed to the general public.