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How Do Night Vision Goggles Work

Narayani Karthik
Night vision goggles help enable vision in places where the light is too low. This equipment is normally used by military organizations, but it is also available for civilian use.
If you have flipped through channels like Nat Geo, you must have seen the images and videos of reptiles and animals, shot in the night. You may wonder as to how is it possible to view things in pitch black darkness present during the night? The devices that makes this possible are night vision goggles!

How Does Night Vision Work?

Energy contained in a light wave is inversely proportional to the wavelength of the light. Shorter wavelengths contain higher energies. For example, in the spectrum of visible light, violet contains maximum energy (having the shortest wavelength) and red contains the least amount of energy (pertaining to highest wavelength).
Infrared (IR) spectrum is just next to the visible light spectrum and it is further distinguished as near infrared (near visible spectrum, with wavelength of 0.7-1.3 microns), mid infrared (wavelength is 1.3-3 microns), and thermal infrared (3-30 microns). Of these, the thermal IR is an emitted energy rather than a reflected kind.
This thermal IR emission takes place due to the continual excitation of electrons at atomic level. In night vision, the thermal imaging technique uses this thermal infrared emission. A night vision equipment may contain cameras, goggles, binoculars, or night vision scopes as well. There are two techniques used in such devices.

Thermal Imaging

Every object emits some amount of thermal IR. This energy is gathered by a pair of special lens which focus this thermal IR. The focused light is scanned by an infrared detector element, which creates a detailed temperature pattern of the light, known as thermogram. The time taken to make a thermogram is about 1/30th of a second.
Thermogram is further converted into electric pulses, which are directed into a signal processing unit (SPU) (this contains a circuit board with a chip that translates the pulses into visual data). The SPU then sends the information to the display where the image is formed.
The quality of the picture depends upon the intensity of the infrared emission.

Image Enhancement

In this technique, a conventional (objective) lens is used to capture the infrared radiation. The gathered light is now sent to an image intensifier tube. An image intensifier tube receives its power supply from 2 N-Cells and 2 AA batteries (standard battery cells). The output voltage generated for image intensifier tube is about 5,000 volts.
This tube contains a photocathode which converts energy of photons into mobile electrons. These electrons are directed through the tube, where they loop in more electrons (released from atoms inside the tube by microchannel plate).
For people who are not aware of what this microchannel plate is, here you go: An MCP (microchannel plate) is a tiny glass disc inside the image intensifier tube, containing millions of microscopic holes. It multiplies the original number of electrons by a factor of thousands. MCP is made using the fiber optic technology.
So when the passing electrons hit the pair of electrodes of MCP, they are accelerated by the voltage bursts across the electrode pair. At the end of the image intensifier tube, the electrons hit a screen coated with phosphor.
As these electrons hit the phosphor screen, their alignment creates an image of the object, because these electrons are in the same alignment as the original photons. The phosphor screen creates a green image of the object when one views through the night vision goggles. The image can also be viewed through ocular lens which magnifies the image.
Be it in military and law enforcements or be it hunting expeditions, wildlife observation, and surveillance; be it navigation and security, or be it entertainment, night vision goggles, that are quite versatile, serve the purpose best!