The Science Behind Night Vision

Two American soldiers pictured during the 2003 Iraq War seen through an image intensifier.

Humans are made for staying up and moving during the daytime and resting and sleeping in the night (dark). The retina (the light-sensitive part inside our eyes) has cells called rods (for detecting movement and dim light) and cones (for seeing colored light). We have around 20 times more rods than cones (only 6 million cones and over 120 million rods), yet we’re still unable to see things correctly in the dark.

How to make a Night Vision?

Picture this: your job is to create a pair of eyeglasses to help others see at night when it is dark. It’s clear what you have to do. Light rays will sail into the glasses at the front, so you must catch them somehow, raise them in strength, and then fire them into the viewer’s eyes. But how can you catch and increase light? Telescopes, binoculars, and even eyeglasses will carry light to a focus, but they don’t make it brighter. It’s easy to create a pair of glasses that make things dimmer: you coat the lenses with something that swallows some of the light—and that’s how sunglasses actually work. But glasses that make things more brighter are a piece of art.

On the other hand, electricity is very easy to boost in strength. People have created all sorts of electrical devices that take in a tiny electric current at one end and produce an immense flow at the other. Something that does has a familiar name: amplifier. For instance, a hearing aid uses a small electronic component called a transistor to increase sounds (amplify their volume) caught by a microphone so that someone hard of hearing can hear them more easily. An electric guitar uses a much more strong amplifier to turn the strings’ plucking sounds into vibrations that can fill a stadium.

So here’s a method to create goggles that enhance light. What if we transform the light into electricity, increase the electricity, and then turn the grown electricity back into the light? That should make the incoming light much brighter so we can see even at night. This unlikely-sounding trick really does work—and this is the exact science behind night vision goggles

Five steps mentioned below will help you understand the precise way night vision goggles work.

  1. The dim light from a night scene enters the lens at the front. The light is made of photons of all colors. A photon is a bunch of electromagnetic energy. It is the fundamental unit that makes up all light. The photon is also referred to as a “quantum” of electromagnetic energy.
  2. As the photons penetrate the goggles, they hit a light-sensitive surface, which is called a photocathode. It’s a bit like an exact solar panel: its job is to convert photons into electrons (the small, subatomic particles that carry electricity around a circuit).
  3. The electrons are amplified by a photomultiplier, a type of photoelectric cell. Each electron entering the photomultiplier results in many more electrons coming out of it.
  4. The electrons dropping out of the photomultiplier hit a phosphor screen, similar to the screen in an old-fashioned TV set. As the electrons hit the phosphor, they produce tiny flashes of light.
  5. Since there are many more photons than initially entered the goggles, the screen makes a much brighter variant of the original scene.

Why does everything look green through night vision goggles?

The photons, even during nighttime, that penetrate the lens at the front of night vision goggles carry the light of all colors. But when they are transformed to electrons, there’s no way to save that information. So, the incoming, colored light is transformed into white and black. Why, then, don’t night vision goggles look white and black? The phosphors on their screens are carefully chosen to make green pictures because our eyes are more susceptible and sensitive to green color. It’s also more comfortable to look at green screens for long periods than to look at white and black ones (that’s why initial computer screens were green). Hence, night vision goggles have their unique, eerie green glow.

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