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With the proper night-vision equipment, you can see a person standing over 183m away on a moonless, cloudy night! Night vision can work in two very different ways, depending on the technology used.
Thermal imaging - This technology operates by capturing the upper portion of the infrared light spectrum, which is emitted as heat by objects instead of simply reflected as light. Hotter objects, such as warm bodies, emit more of this light than cooler objects like trees or buildings.
Image enhancement - This works by collecting the tiny amounts of light, including the lower portion of the infrared light spectrum, that are present but may be imperceptible to our eyes, and amplifying it to the point that we can easily observe the image.
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 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 in the 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 colours depending on the intensity of the infrared emission. The combination of all the impulses from all of the elements creates the image.
Types of Thermal Imaging Devices
Most thermal-imaging devices scan at a rate of 30 times per second. They can sense temperatures ranging from -20°C to 2,000°C, and can normally detect changes in temperature of about 0.2°C.
There are two common types of thermal-imaging devices:
Un-cooled - This is the most common type of thermal-imaging device. The infrared-detector elements are contained in a unit that operates at room temperature. This type of system is completely quiet, activates immediately and has the battery built right in.
Cryogenically cooled - More expensive and more susceptible to damage from rugged use, these systems have the elements sealed inside a container that cools them to below 0°C. The advantage of such a system is the incredible resolution and sensitivity that result from cooling the elements. Cryogenically-cooled systems can \"see\" a difference as small as 0.1°C from more than 300m away. (Which is enough to tell if a person is holding a gun at that distance?)
Image Enhancement
While thermal imaging is great for detecting people or working in near-absolute darkness, most night-vision equipment uses image-enhancement technology.
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 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 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. 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 image-intensifier 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.
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