Night vision is the ability to see in reduced illumination. Whether by biological or technological means.

Development in night vision in past
NVDs have been around fore more than 40 years. Each substantial change in NVD technology establishes a new generation.

Active infrared:
The original night-vision system created by United States Army and used in World War ll and Korean War , these NVDs use active infrared. This means that a projection unit, called an IR illuminator, is attached to the NVD. The unit projects a beam of near-infrared light, similar to the beam of a normal flashlight. Invisible to the naked eye, this beam reflects off objects and bounces back to the lens of the NVD. These systems use an anode in conjunction with a cathode to accelerate the electrons. The problem with that approach is that the acceleration of the electrons distorts the image and greatly decreases the life of the tube. Another major problem with this technology in its original military use was that it was quickly duplicated by hostile nations, which allowed enemy soldiers to use their own NVDs to see the infrared beam projected by the device.

Passive infrared:
The next generation of NVDs moved away from active infrared, using passive infrared instead. Once dubbed starlight by the U.S Army, these NVDs use ambient light provided by the moon and stars to augment the normal amounts of reflected infrared in the environment. This means that they did not require a source of projected infrared light. This also means that they do not work very well on cloudy or moonless nights. Generation-1 NVDs use the same image-intensifier tube technology as Generation 0, with both cathode and anode, so image distortion and short tube life are still a problem.

Image-intensifier tubes:
Major improvements in image-intensifier tubes resulted in Generation-2 NVDs. They offer improved resolution and performance over Generation-1 devices, and are considerably more reliable. The biggest gain in Generation 2 is the ability to see in extremely low light conditions, such as moonless night. This increased sensitivity is due to the addition of the micro channel plate to the image-intensifier tube. Since the MCP actually increases the number of the electrons instead of just accelerating the original ones, the images are significantly less distorted and brighter than earlier-generation NVDs.

Improved image intensifying:
Generation 3 is currently used by the U.S military. While there are no substantial changes in the underlying technology from Generation 2, these NVDs have even better resolution and sensitivity. This is because the photo cathode is made using gallium arsenide, which is very efficient at converting photons to electrons.

Famous Approaches used for night vision

  • Spectral range:
    Night-useful spectral range techniques make the viewer sensitive to types of light that would be invisible to a human observer. Human vision is confined to a small portion of the electromagnetic spectrum called visible light. Enhanced spectral range allows the viewer to take advantage of non-visible sources of electromagnetic radiation (such as near-infrared or ultraviolet radiation). Some animals can see well into the infrared and/or ultraviolet compared to humans, enough to help them see in conditions humans cannot.
  • Intensity range:
    Sufficient intensity range is simply the ability to see with very small quantities of light. Although the human visual system cab, in theory, detect single photons under ideal conditions, the neurological noise filters limit sensitivity to a few tens of photons, even in ideal conditions. Some animals have evolved better night vision through the use of larger optical aperture, improved retina composition that can detect weaker light over a larger spectral range, more photon efficient optics in the eye, and improved neurological filtering.
  • Biological night vision:
    in biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as light is absorbed by them. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. Exposed to white light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully, but most of the adaptation occurs within the first five or ten minutes in the dark. Rhodopsin in the human rods is sensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye's rhodopsin stores in the rods and instead is viewed by the cones.
  • Thermal imaging:
    a special lens focuses the infrared light emitted by all of the objects in view. The focused light is scanned by a phased-array of infrared detector elements. The detector elements creates a very detailed temperature pattern called thermo-gram. It only takes about one-thirtieth of a second for the detector array to obtain the temperature information to make the thermo-gram. This info is obtained from several thousand points in the field of view of the detector array. The impulses are sent to a signal-processing unit, a circuit board with a dedicated chip that translates the info from the elements into the data for display.

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