In Generation 2 and 3 systems, the objective lens collects light that you cannot see with your naked eye and focuses it on the image intensifier. Inside the image intensifier a photocathode absorbs this energy and converts it to electrons. These electrons are then drawn toward a phosphor screen but first pass through a microchannel plate that intensifies them thousands of times. When this highly intensified electron image strikes the phosphor screen, it causes the screen to emit light that you can see. Since the phosphor screen emits this light in exactly the same pattern and degrees of intensity as the light collected by the objective lens, the bright nighttime image you see in the eyepiece corresponds precisely to the outside scene you are viewing.

Terminology:

Automatic Brightness Control (ABC)
An electronic feature that automatically reduced voltages to the microchannel plate to keep the image intensifiers brightness within optimal limits and protects the tube. The effect of this can be seen when rapidly changing from a low-light to high-light conditions; the image gets brighter and than after a momentary delay, suddenly dims to a constant level.

Black Spots
These are cosmetic blemishes in the image intensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the performance or reliability of a night vision device and some number of varying size are inherent in the manufacturing process. Spots due to dirt or debris between the lenses should be removed by careful cleaning if the system is designed for interchangeable optics.

Bright Source Protection (BSP)
An electronic function that reduces the voltage the voltage to the photocathode when the night vision device is exposed to bright light sources such as room lights or car lights. BSP protects the image tube from damage and enhances its life; however it also has the effect of lowering resolution when it is functioning.

Cycles per Milliradian (cy/mr)
Units used to measure resolution. A milliradian is the angel created by 1 yard at a distance of 1,000 yards. This means that a device that can detect two 1/2 yard objects separated by 1/2 yard at 1,000 yards has a resolution of 1.0 cy/Mr. Do not confuse cy/mr with line pair per millimeter. For example, a system resolution by a factor of 3, yet the image intensifier's resolution (measured in lp/mm) has not increased.

Diopter
The unit of measure used to define eye correction or the refractive power of a lens. Usually adjustments to an optical eyepiece accommodates for differences in individual eyesight. Many military systems provide +2 / -6 diopter range.

Distortion
Three types of distortion are most significant to night vision devices: geometric, "5", and shear. Geometric distortion is inherent in all Gen0 and Gen1 image intensifiers, and some Gen2 image intensifiers that use electrostatic rather than fiberoptic inversion for image inversion.

Generation 2
Usually an s-25 (extended red) photocathode (with photo sensitivity of 240+ a/lm and a microchannel plate to achieve gain.) Can be found with either electrostatic or fiberoptic inversion. Gen2 tubes provide satisfactory performance at low light levels and exhibit low distortion.

Generation 3
Uses galliu-arsenide for the photocathode and a microchannel plate for gain. The microchannel plate is also coated with an ion barrier film to increase tube life.

Line Pairs per Millimeter
Units used to measure image intensifier resolution. Usually determined from a 1951 Air Force Resolving Power Test Target. The target is a series of different sized patterns composed of three horizontal and three vertical lines. You must be able to distinguish all the horizontal and vertical lines and space them to qualify for the pattern.

Lumen
The unit denoting the photons perceivable by the human eye in one second.

Microamps per Lumen (A/lm)
The measure of electrical current (A) produced by a photocathode when it is exposed to a measured amount of light (lumens).

Microchannel Plate (MCP)
a metal-coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in Gen2 and Gen3 systems. These devices normally have anywhere from 2 to 6 million holes (or channels) in them. Electrons entering a channel strike the wall and knock off additional electrons which in turn knock off more electrons, producing a cascading effect. MCPs eliminate the distortion characteristic of Gen0 and Gen1 systems. The number of holes in an MCP is a major factor in determining resolution.

Milliamps per Watt( mA/W)
The measure of electrical current (m/A) produced by a photo cathode when exposed to to a specific wavelength of of light at a give radiant power (watt). As with microamps per lumen, usually the higher the value, the better the performance; however, it is important to know where in the spectrum the wavelength falls. Because more night light is present in the near-infrared region than in the visible or ultraviolet regions, a high value in the blue regions would not be as good as a moderate value in the near-infrared region.

Near-Infrared
The shortest wavelengths of the infrared region, normally 750 to 2,500 nanometers. Gen2 operates from around 440 to 950 nanometers.

Photocathode
The input surface of an image intensifier that absorbs light energy and in turn releases electrical energy in the form of an electron image. The type of material used is a distinguishing characteristic of the generations of image intensifiers.

Photoresponse(PR)
see Photosensitivity

Photosensitivity:
Also called photocathode sensitivity. The ability of the photocathode material to produce an electrical response when subjected to light waves (photons). Usually measured in microamps of current per lumen (A/lm). The higher the value, the better the ability to produce a visible image under darker conditions.

Resolution:
The ability of an image intensifier or night vision system to distinguish between objects close together. Image intensifier resolution is measured in line pairs per millimeter (lp/mm) while system resolution is measured in cycles per milliradian. For any particular night vision system, the image intensifier resolution will remain constant while the system resolution can be affected by altering the objective or eyepiece optics by adding magnification or relay lenses. Often the resolution in the same night vision device is very different when measured at the center of the image and at the periphery of the image. This is especially important for devices selected for photography or video where the entire image resolution is important.

Signal-to-Noise Ratio (SNR)
A measure of the light signal reaching the eye divided by the perceived noise as seen by the eye. A tube's SNR determines the low-light resolution of the image tube; therefore, the higher the SNR, the better the ability of the tube to resolve objects with good contrast under low-light conditions. Because SNR is directly related to phosphor efficiency and MCP operating voltage, it is the best single indicator of image intensifier performance.

Scintillation
A faint, random sparking effect throughout the image area. Scintillation is a normal characteristic of microchannel plate image intensifiers and more pronounced under low-light level conditions. Sometimes called "video noise." Do not confuse scintillation with emission points.

Spectrum
The range of electromagnetic energy from cosmic rays to extra-low frequency used in submersed submarine communication. Note: Electromagnetic frequency is measured in cycle per second and wavelength in microns or nanometers(nm). The ultraviolet regions extend from 100 to 400 NM with the near-ultraviolet normally 300 to 400 NM The visible portion of the spectrum extends from 400 to 750 NM The infrared region extends from 750 to 2x105nm with the near-infrared normally 750 to 2,500 NM

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