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The engineers and scientists at the US Armys Communications-Electronics Command (CECOM) Night Vision & Electronic Sensors Directorate (NVESD) have discovered ways to capture available electro-magnetic radiation outside that portion of the spectrum visible to the human eye and have developed equipment to enable the American soldier to fight as well at night as during the day and to "Own the Night". Night vision devices provide night fighters with the ability to see, maneuver and shoot at night or during periods of reduced visibility. The Army used two different types of Night vision devices image intensifiers and thermals. Image-Intensifying Devices are based upon light amplification and must have some light available. These devices can amplify the available light from 2,000 to 5,000 times. Thermal Forward-Looking Infrared (FLIR) detectors sometimes called "sensors" work by sensing the temperature difference between an object and its environment. FLIR systems are installed on certain combat vehicles and helicopters.
Night Vision Goggles are electro-optical devices that intensify (or amplify) existing light instead of relying on a light source of their own. Image intensifiers capture ambient light and amplify it thousands of times by electronic means to display the battlefield to a soldier via a phosphor display such as night vision goggles. This ambient light comes from the stars, moon or sky glow from distant manmade sources, such as cities. The devices are sensitive to a broad spectrum of light, from visible to infrared (invisible). Users do not look through Night Vision Goggles, you look at the the amplified electronic image on a phosphor screen.
Light enters the Night Vision Goggles through an objective lens and strikes a photo cathode powered by a high energy charge from the power supply. The energy charge accelerates across a vacuum inside the intensifier and strikes a phosphor screen where the image is focused. The eyepiece magnifies the image.
A soldier can conduct his combat missions without any active illumination sources using only image intensifiers. The main advantages of image intensifiers as night vision devices are their small size, light weight, low power requirements and low cost. These attributes have enabled image intensifier goggles for head-worn, individual soldier applications and resulted in hundreds of thousands of night vision goggles to be procured by the US Army. Research and development continues today on image intensifiers in the areas of longer wavelength spectral response, higher sensitivity, larger fields of view and increased resolution.
The view through Night vision devices can be a lot like looking down a tunnel. Your normal field of view is almost 190 degrees but that is cut down to 40 degrees with Night vision devices. That side -- or "peripheral" -- vision youre accustomed to, and from which you often see dangers, is just not there. To adjust for that you must constantly turn your head to scan for the dangers on either side of you that you cant see in your narrow field of view. Normally you use both eyes (binocular vision) to pick up cues to help estimate the distance and depth of an abject. However, with Night vision devices you are essentially using one eye (monocular) vision, which can pose real problems. For example, when you are wearing NVDs and you view two objects of different sizes that are side-by-side, the larger object appears to be nearer. When you view overlapping objects through a Night vision device, the one that is in front "appears" to be nearer maybe much more so than is true. In addition, some objects viewed through Night Vision Goggles may appear to be farther away than they actually are. The reason for that is that we tend to associate the loss of detail sharpness with distance. On the other hand, a light source that is not part of a terrain feature may look closer than it actually is. Its important to be aware of these potential problems and that Night Vision Goggle users tend to overestimate distance and underestimate depth .
Your eye needs time to adjust from day to night vision. Thats why you can barely see when you first enter a dark movie theater during the daytime your eyes need time to adjust to the darkness. So it is with Night Vision Goggles. You are basically getting a dim-day view, so when you remove your Night Vision Goggles, your eyes need time to adapt to the darkness. The amount of time you need depends on how long you have been wearing the Night Vision Goggles. Most people achieve about a 75 percent dark-adaptation within 30 seconds of removing the goggles. This is especially important to keep in mind if you are using your Night Vision Goggles as binoculars basically lifting them to your eyes and then lowering them.
