How do they work? Is the principle similar to what gives dogs, cats and other animals improved night vision?
I don’t know exactly how they work, and I don’t have time right now to do a search so I’ll – er, “take a shot in the dark”.
There is ambient light at night, even when it’s too dark for humans to see clearly. Night vision devices intensify the ambient light about 30,000 times and/or use an infrared spotlight (which I would think would not be used in certain tactical situations for obvious reasons, or when using them for flying). Generation I NVDs are fairly inexpensive – only a few hundred dollars – Generation II are more expensive, and Generation III NVDs can cost $5,000 or more for one from Litton.
Eyes have “rods” and “cones” in the retina. Cones allow vertibrates to see colours and rods allow animals to see faint light. The reason humans don’t see well at night is that we have more cones than rods. Dogs, which I believe are colour blind, have more rods and so they can see well at night.
Check here:
The underlying physics is the same for all animals’ eyes and most types of sensors. When a substance absorbs a photon, it often emits an electron. (It’s called the photoelectric effect.) Some substances are better than others, of course. In the human eye and photographic film, the resulting free electron causes chemical reactions. In electronic cameras, the electron is detected as an electrical signal. In night vision goggles, the electron is accelerated and amplified using electrical fields, then the electrons hit a fluorescent screen and cause a flash of light. One incoming photon creates one electron, which is then amplified into millions of electrons with the electric field, which then cause the fluorescent screen to emit millions of photons. Thus light is amplified.
Just a comment on that article in How Stuff Works.
Cryogencially cooled NV devices are usually cooled down to around -272 degrees F. They’re usually helium-cooled, and because of the design of the cooling system, I wouldn’t say they are more susceptible to damage. In a liquid cooled NVD, the coolant system would be one of the LAST things to go bad.
Also, voltages in the image intensifier can reach upwards of 17,000 volts, not just 5000. But that’s not usually in your hand-held devices.
Actually, there are a number of different kinds of ‘night vision’. One is simply a sensor that is sensitive to infra-red light, along with an infra-red light source. Most CCD devices in digital cameras and camcorders are sensitive in the infra-red. In fact, the manufacturers generally have to put an infra-red filter inside the camera to block it from the CCD. The Sony ‘Night Shot’ feature on their CCD cameras is nothing more than a switch that pulls the infra-red blocker away from the lens. A little IR LED on the camera acts as a light source.
Another way to see at night is to use a big honkin’ lens. The ability to see in low light has a lot to do with how much light gathering the optics get. That’s why nocturnal animals have huge eyes, and why ‘Starlight’ scopes have giant lenses.
The third type is what everyone is talking about here - photomultiplier tubes that take whatever light is collected and amplify it.