Night Vision Devices
The ability to wage battle at night is a great tactical advantage, but devices that let soldiers see well in the dark were not invented until the 1940s. Fighters illuminated land and sea battles with searchlights and flares at the beginning of the twentieth century, and star-shell artillery was used in the two world wars to cast light where it was needed. Then during World War II, both the Germans and the Allies developed more sophisticated night vision devices. This early night vision equipment consisted of an infrared filter placed over a searchlight. The observer looked through a binocular-like or telescope-like viewer that translated the infrared emissions into an electron flux displayed on a luminescent screen. Both the Germans and the Allies fitted their tanks with these viewers. But because both sides had infrared viewers, they could spot each otherŐs searchlights. Even after a searchlight was turned off, it continued to emit enough infrared to make it a target. So use of this equipment was risky.
American military scientists continued to develop night vision devices after World War II, but concentrated on systems that worked by intensifying available night light--starlight and moonlight. Most of the seminal research on these so-called image intensifiers was done at Fort Belvoir, Virginia, and the technology remained top secret for a long time. American soldiers used image intensifiers called Star-Tron scopes in the Vietnam War. The scopes were almost as heavy as the rifles they were mounted on, and were extremely expensive to manufacture. They worked by focusing the dim night scene onto a photo-cathode inside a tube. The photo-cathode accelerated and multiplied the electrons from the ambient light, and focused them into a visible image. The Star-Tron scope permitted a soldier to see in the dark about 400 yards ( 366 m), but it had several unfortunate drawbacks. The scope would shut down if it was exposed to a sudden bright light, and the device emitted a high-pitched whine that was often loud enough to give away its userŐs position, thus cancelling out the stealth effect.
Further research eliminated these problems in a second generation of image intensifiers. These worked by focusing ambient light on a microchannel plate (MCP), which was made up of a thin slice of a number of hollow glass tubes. The tubes each emitted many electrons when struck by a single electron. Because image intensifiers using MCPs were smaller and lighter than the older devices, they could be manufactured into binoculars affixed to a helmet. These are known as night vision goggles. Night vision goggles were widely worn by military pilots for night manuevers from the 1980s on.
Other night vision devices functioned with thermal imaging. In this system, sensors detect heat, and translate temperature differences into a visual image. Thermal systems have the advantage of being able to "see" even through fog, smoke, and dust.
In the 1980s, night vision technology ceased to be a military secret. Several companies in the United States and Europe worked to market night vision goggles for non-military use. Some night vision devices have found their way into ordinary automobiles, to make night driving safer. In the late 1990s, night vision goggles were increasingly purchased by municipal police departments.
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