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What is night vision?
Night vision is the ability to see in low light environments through the use of a night vision device (NVD) that is equipped with an image intensification tube (IIT) or a camera sensor (digital NV). NVD’s can be a monocular, binocular or riflescope. NVD’s are used for a variety of applications such as hunting, security, wildlife observation, stargazing, and camping.
How does it work?
Time to whip out your junior high physics textbook. We're going to be talking about the electromagnetic spectrum, specifically visible and infrared light. Visible light, light we can detect with our eyes, ranges from 380nm to 750nm and represents a small portion of the entire electromagnetic spectrum. Infrared is just next to visible on the spectrum, at a lower energy level.
Image Intensifier Tubes
An image intensifier tube is composed of three main parts, a photocathode, a microchannel plate (MCP) and a phosphor screen. IIT’s collect existing ambient light through the objective lens of the NVD (that's the front lens). This light may be starlight, moonlight or artificial light, which all consist of photons (light particles). When photons pass through the photocathode they are converted into electrons. Once through the photocathode, the electrons are then released into a vacuum and accelerated towards the MCP.
The MCP is a thin glass disc, less than .5mm thick, with millions of small channels. When electrons strike the inner walls of these channels, they kick off a chain reaction, generating secondary electrons. Money for nothing. For each electron that enters the MCP, approximately 1,000 are generated. The newly-generated flood of electrons accelerate towards the phosphor screen. The phosphor screen is a thin light-emitting layer deposited on the inside of the output window of the intensifier tube.
This screen converts the electrons back into photons to create a bright image - that's what you actually see when you look through. It's important to note IIT’s are sensitive to light just into the near-IR spectrum, approximately 930nm. IIT’s cannot be used in daylight conditions without risking permanently burning them out.
Digital Night Vision
Instead of using an IIT, digital night vision uses a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) and a micro display. Light that is projected onto the CCD or CMOS array from the objective lens is converted to an electronic signal. That means they're completely safe to use during daytime.
This signal is then processed and sent to the micro display to be viewed by the user. Digital night vision generally produces a black and white image, but displays can show images in any color.
Unlike IIT’s, digital night vision units require the addition of artificial light to create bright images, but digital night vision can be used in daylight conditions. Digital night vision can also record images directly to an internal memory card or be sent through a video output to a DVR. Digital night vision has become a viable replacement for Gen 2 IIT’s as digital offers similar performance and resolution but at a comparable or lesser cost than Gen 2.