The military developed the first practical night vision scopes during the Vietnam war. This equipment, originally known as "starlight scopes," utilized image-intensifying tubes to amplify ambient light from the moon and stars and produce an electronic image of objects invisible to the naked eye. Designated Generation 1, this basic technology is still used today in the bottom tier of consumer-grade night vision scopes and binoculars. Since the 1970s, you have been able to purchase night vision devices up to the Generation 3 level. These systems evolve and greatly enhance the Generation 1 design to produce higher resolution in conditions of lower ambient light. The three essential elements of night vision technology are a photocathode, some form of amplification such as a micro-channel plate and a phosphor viewing screen. They remain consistent across generations. The visual trademark of all night vision equipment is the greenish hue of images in the viewer.
Photocathode
The large objective lens at the end of a night vision scope gathers available light, including the lower spectrum of invisible infrared. The lens focuses the light on the photocathode, the input surface of the image intensifier tube. The photons of this weak light energy convert into electrons, amplified and released from the cathode. An electronic field in the image intensifier circuit accelerates the electrons to boost their speed.
Micro-Channel Plate
The excited electrons flow into millions of holes in a micro-channel plate, each electron maintaining its alignment in relation to the original image. As they pass through the micro channels, the electrons rebound off a specially coated surface and multiply exponentially, forming dense clouds of electrons and intensifying the original image.
Phosphor Screen
Electrons cascade out of the micro-channels and collide with the back of the green phosphor screen. Energy from the electrons excites the phosphor, which converts the electrons into photons. These glowing photons form the greenish image on the screen inside the ocular viewing lens of night vision equipment.
Why Green?
The green tint of night vision images is entirely by design. Other colors have been studied, and red phosphor has been used in some equipment. However, experiments proved that gradations in the shadings that make up monochrome night vision images are most accurately perceived when displayed in green. The human eye perceives nearly twice as many subtle gradations in green wavelengths than in red, and nearly four times more than blue.
Tags: night vision, ambient light, have been, image intensifier, lens night, lens night vision, micro-channel plate