NIGHT VISION

Night vision, also known as image intensification, allows humans to see in low to zero light conditions.  Night vision is made possible by a combination of of two approaches; sufficent spectral range, and sufficent intensity range.  A device(NVD) works by collecting photons through a lens(NVIS).  Then they pass through a photocathode which converts the photons into electrons.  These electrons then pass through a a microchannel plate, consisting of millions of closely spaced channels.  In here the electrons can hit the walls so they will release thousands of additional electrons.  Finally, the mutiplied electrons will hit a phosphor screen where they get turned back into light.  This light is thousands of times brighter than the original light and is in the visual spectrum of the human eye.  The intensified light is in the same orientation as it came in providing a clear view of objects in the dark.

Night vision has many uses.  The original use was for military operations.  NVDs are now used in security, police work and amateur work.  Night vision goggles are especially useful for rescue helicopter pilots because they work even when there are bright light behind them.  NVDs work in the near-infrared band at a wavelegnth of one micrometer.  Humans can see between 0.4 and 0.7 micrometers.  NVDs work by capturing ambient light usually from the moon and stars.  It only takes one star in the sky for an NVD to work.  

Night vision can work in one of two ways, active and passive.  Passive systems amplify the existing light and active systems rely on an infrared light source.  Early NVDs were designed to be used as active systems, as they did not have the sensitivity to operate on ambient light.  Active systems are commonly used today in closed-circut television security and on many home video cameras.  The military uses passive NVDs over active because infrared illumination is easily spotted and tracked by others equipped with it.  Modern NVDs are equipped with infrared illuminator for when there is no ambient light.

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