Types of Night Vision Technology
Image Enhancement
Image-enhancement technology is what most people think of when you talk about night
vision. In fact, image-enhancement systems are normally called night-vision devices
(NVDs). NVDs rely on a special tube, called an image-intensifier tube, to collect and
amplify infrared and visible light.
The image-intensifier tube changes photons to electrons and back again.
Here's how image enhancement works:
• A conventional lens, called the objective lens, captures ambient light and some near-infrared light.
• The gathered light is sent to the image-intensifier tube. In most NVDs, the power supply for the image-intensifier tube receives power from two N-Cell or two "AA" batteries. The tube outputs a high voltage, about 5,000 volts, to the image-tube components
.
• The image-intensifier tube has a photocathode, which is used to convert the photons of light energy into electrons.
• As the electrons pass through the tube, similar electrons are released from atoms in the tube, multiplying the original number of electrons by a factor of thousands through the use of a microchannel plate (MCP) in the tube. An MCP is a tiny glass disc that has millions of microscopic holes (microchannels) in it, made using fiber-optic technology. The MCP is
contained in a vacuum and has metal electrodes on either side of the disc. Each channel is about 45 times longer than it is wide, and it works as an electron multiplier. When the electrons from the photo cathode hit the first electrode of the MCP,
they are accelerated into the glass microchannels by the 5,000-V bursts being sent between the electrode pair. As electrons pass through the microchannels, they cause thousands of other electrons to be released in each channel using a process called cascaded secondary emission. Basically, the original electrons collide with the side of the channel, exciting atoms and causing other electrons to be released. These new electrons also collide with other atoms, creating a
chain reaction that results in thousands of electrons leaving the channel where only a few entered. An interesting fact is that the microchannels in the MCP are created at a slight angle (about a 5-degree to 8-degree bias) to encourage electron collisions and reduce both ion and direct-light feedback from the phosphors on the output side.
• At the end of the imageintensifier tube, the electrons hit a screen coated with phosphors.
These electrons maintain their position in relation to the channel they passed through, which
provides a perfect image since the electrons stay in the same alignment as the original photons.
The energy of the electrons causes the phosphors to reach an excited state and release photons.
These phosphors create the green image on the screen that has come to characterize night vision.
Night-vision images are known for their eerie green tint.
• The green phosphor image is viewed through another lens, called the ocular lens, which allows you to magnify and focus the image. The NVD may be connected to an electronic display, such as a monitor, or the image may be viewed directly through the ocular lens.
Thermal Imaging
Here's how thermal imaging works:
• 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 create a very detailed temperature pattern called a thermogram. It only takes about one-thirtieth of a second for the detector array to obtain the temperature information to make the
thermogram. This information is obtained from several thousand points inthe field of view of the detector array.
• The thermogram created by the detector elements is translated into electric impulses.
• The impulses are sent to a signal-processing unit, a circuit board with a dedicated chip that translates the information from the elements into data for the display.
• The signal-processing unit sends the information to the display, where it appears as various colors depending on the intensity of the infrared emission. The combination of all the impulses from all of the elements creates the image.
The basic components of a thermal-imaging system
Types of Thermal Imaging Devices
Most thermal-imaging devices scan at a rate of 30 times per second. They can sense
Temperatures ranging from -4 degrees Fahrenheit (-20 degrees Celsius) to 3,600 F
(2,000C) and can normally detect changes in temperature of about 0.4 F (0.2 C).
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