Automatic Brightness Control (ABC)

An electronic feature of image intensifier power supplies which automatically reduces voltages to the microchannel plate to keep the image intensifier's brightness within optimal limits, protecting the tube. This can be seen when rapidly changing from low-light to high-light conditions; the image gets brighter and then, after a momentary delay, suddenly dims to a constant level.

Auto-Gated Power Supply (Autogate)

A technology commonly present in contemporary Night Vision Image Intensifier Tubes. Auto-gating is activated in the night vision system in dynamic light conditions to always keep the best possible resolution and contrast. When the image intensifier tube, the heart of night vision systems, is exposed to sudden, high intensity, bright light (from car lamps, intensive wildland fires, flares, or gun flashes) the auto-gating feature automatically controls the power supply to the photocathode by very rapidly switching the voltage on and off. There are two benefits. First, the tube's performance is maintained at an optimal level and the system is not momentarily blinded or shut down, which can be the case with a non-autogated tubes or some legacy technologies. Thanks to this feature, the user always has proper visual acuity and can continue carrying out his nighttime mission in a non-disrupted manner. The second effect is that the tube itself is additionally protected against the negative influence of bright light. Automatic gain control is a great feature to have in dynamic light environments like urban settings or any other area with background light sources.

Binocular

Viewing a scene through two channels, i.e., one channel per eye. A good example of binocular night vision system is the LLI MH-1.

Biocular

Viewing a single image source with both eyes. The most commonly deployed bi-ocular system in use is AN/PVS-7.

Black Spots / Blems

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 are inherent in the manufacturing processes. Most manufacturers will measure the size of spots and record the location in tube zones during the quality assurance acceptance process. These zones are concentric circles called zone 1, 2, and 3. Zone 1 is the center, zone 2 is in between the center and outside, and zone 3 is the outside.

Black Boxing

A method of stimulating the peculiar 'self-healing' process that some image intensifiers have been found to have, where the image intensifier is left running for extended periods of time in a completely dark enclosure.

Blooming

See Halo.

Bore Sighting

These can be defects in the image area produced by the NVG. A flaw causes this condition in the film on the microchannel plate. A bright spot in a small, non-uniform, bright space that may flicker or appear constant. Bright spots usually go away when the light is blocked out and are cosmetic blemishes that are signal induced.

Bright Spots

These can be defects in the image area produced by the NVG. A flaw causes this condition in the film on the microchannel plate. A bright spot in a small, non-uniform, bright space that may flicker or appear constant. Bright spots usually go away when the light is blocked out and are cosmetic blemishes that are signal induced.

Bright-Source Protection (BSP)

An electronic function of night vision systems that reduces 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 lowers resolution when functioning.

Burn-in

Similar to blemishes, except that the areas aren't completely dark, but instead just less bright than the rest of the image, usually due to exposure to very bright light. Burn-ins are a precursor to blemishes, usually due to shorter or less intense exposure to the light source. Burn-ins can sometimes be partially or fully reverted by using Blackboxing or Whiteboxing.

Chicken Wire

An irregular pattern of thin dark lines in the field of view either throughout the image area or in parts. Under worst-case conditions, these lines will form hexagonal or square wave-shaped lines.

C-Mount

A standard still and video camera lens thread-size for mounting to the body of a camera. Usually 1/2″ or 3/4″ in diameter.

COMSPEC (Commercial Specification)

A term used to describe image tube quality, testing, and inspection done by the original equipment manufacturer (OEM).

Date of Manufacture (DOM)

The date the device was manufactured on.

Daylight Lens Cover

Usually made of soft plastic or rubber with a pinhole that allows a small amount of light to enter the objective lens of a night vision device. This should be used only for training and is not recommended for an extended period.

Diopter

The unit of measure used to define eye correction or the refractive power of a lens. Usually, adjustments to an optical eyepiece accommodate for differences in individual eyesight. Most night vision and thermal imaging systems provide a +2 to -6 diopter range.

Distortion

There are two types of distortion found in night vision systems. One type is caused by the design of the optics, or image intensifier tube, and is classical optical distortion. The other type is associated with manufacturing flaws in the fiber optics used in the image intensifier tube.

Classical Optical Distortion: Classical optical distortion occurs when the optics or image intensifier tube design causes straight lines at the edge of the field of view to curve inward or outward. This curving of straight lines at the border will generate a square grid pattern to look like a pincushion or barrel. This distortion is the same for all systems with the same model number. The superior optical design typically makes this distortion so low that the average user will not see the curving of the lines.

Fiber Optics Manufacturing Distortions: Two types of fiber optics distortions are most significant to night vision devices: S-distortion and shear distortion.
S-Distortion: Results from the twisting operation in manufacturing fiber-optic inverters. Usually, S-Distortion is very small and is difficult to detect with the unaided eye.
Shear Distortion: Can occur in any image tube that uses fiber-optic bundles for the phosphor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area, as though the line were 'sheared.'

Edge Glow

This is a defect that can appear in the image area of the image intensifier tube. The edge glow is a bright area (sometimes sparkling) in the outer portion of the viewing area.

Emission Point

A steady or fluctuating pinpoint of bright light in the image area that does not disappear when all light is blocked from the objective lens. The position of an emission point within the field of view will not move. If an emission point disappears or is only faintly visible when viewing under brighter nighttime conditions, it is not indicative of a problem. If the emission point remains bright under all lighting conditions, the system needs to be repaired. Do not confuse an emission point with a light source point in the scene being viewed.

Equivalent Background Illumination (EBI)

This is the amount of light you see through a night vision device when an image tube is turned on but no light is on the photocathode. In short, EBI measures how well the tube can form an image in low-light levels. The lower the number the better. EBI is affected by temperature, the warmer the night vision device, the brighter the background illumination. EBI is measured in lumens per square centimeter (lm/cm2). The lower the value the better. The EBI level determines the lowest light level at which an image can be detected. Below this light level, objects will be masked by the EBI.

Eye Relief

The distance a person's eyes must be from the last element of an eyepiece to achieve the optimal image area (exit pupil). Eye relief is usually measured in mm.

Field-of-View (FOV)

The angular extent of what can be seen, either with the eye or with an optical instrument, such as a camera, telescope, or a night vision device. The wider the FOV, the more one can see of the observable world. It is measured horizontally, vertically, and diagonally. The optical system lens, its focal length, and the sensor size all play a part in determining the FOV.

Figure of Merit (FOM)

An abstract measure of image intensifier tube performance, derived from the number of line pairs per millimeter multiplied by the tube's signal-to-noise ratio (Resolution x SNR). Therefore, the higher the FOM, the better the image. FOM is recognized as a measurable value to adequately determine the performance of the image intensifier tube. It is a quick way to determine the performance of a tube, mostly for export purposes set forth by the U.S. State Department. Because FOM measures only two data points, it is not the only indication of a tube's performance. In other words, you could have a low to average FOM tube that can, in certain lighting conditions, outperform a tube with a higher FOM, or at least match its real-world performance. EBI, photocathode sensitivity, gain, and halo are also very important data points to take note of when looking at a tube.

Filmed or Unfilmed / Filmless

A type of categorization, which refers to the tube component called the Micro Channel Plate (MCP) that is within the image intensifier. All MCPs start out filmless (unfilmed) – they are manufactured without a film on the MCP. There is an extra process step that a manufacturer can take to place a film on the MCP. The benefit of a filmed MCP is that the film on the MCP provides protection to the internal photocathode, allowing for long life and more importantly, high performance over that long life. The filmed tube has been shown to easily meet U.S. military life requirements and has a relatively flat performance over that life cycle. The con of a filmed MCP is that the film is an added process, increasing complexities in manufacturing. The benefit of an unfilmed MCP is that it makes it easier to reach relatively high signal-to-noise values. The con of an unfilmed MCP is that it does not provide protection to the internal photocathode, which in turn reduces the performance significantly as the tube gains hours of operation. The unfilmed tube has been shown to meet the U.S. requirement for life, however the performance over that life is significantly reduced as compared to the first couple hundred hours of operation.

Gen 0: Historical Foundation

The Gen 0 image converter used an S-1 photocathode, an IR-sensor with a high-voltage electron acceleration electrostatic field, and a phosphor screen. The S-1 cathode (AgOCs) did not have as much quantum efficiency as the cathodes used today, but it was able to provide images with the help of the IR illuminator. The process by which the image was intensified was quite simple in this generation. The reflected IR illuminator light entered the tube, and the photocathode converted the light to electrons. Electronic elements focused these electrons through a cone-shaped component (anode) and accelerated them using very high voltage, so they hit the phosphor screen with greater energy, recreating a visible image. Accelerating the electrons in this manner did not produce much gain and caused distortion in the image. Also, tube life was not very good by today's standards.

Gen 1: Vietnam Era Technology

The Starlight Scope, developed during the early 1960s and used during the Vietnam War, was made using Gen 1 image intensifier tubes. In this scope, three image intensifier tubes were connected in series, making the unit larger and heavier than today's night vision goggles. This early generation produced a clear center image with a distorted periphery. The use of multiple tubes connected in series allowed for much greater overall light gain as the output of the first tube was amplified by the second and the second by the third. Due to the simple power supply design, the image was subject to instances of blooming — momentary image washout due to an overload in the intensifier tube caused by bright light sources.

Gen 2: Microchannel Plate Revolution

Developed in the late 1960s, Gen 2 technology brought a major breakthrough in night vision with the development of the microchannel plate. Additionally, the photocathode process used for Gen 1 was further refined to the S-25 cathode and produced a much higher photo response. Nevertheless, it was the introduction of the MCP that made Gen 2 unique. The MCP begins with two dissimilar pieces of glass. A large tube of solid glass (core) is placed within a tubular sleeve of glass (clad). The two glasses are then heated together and stretched to form a very small diameter glass fiber. The fibers are ultimately compressed together to form a bundle of glass fibers called a boule. The boule is then sliced at an angle to obtain thin discs. Further chemical processing removes only the core glass, thus creating the channels within the MCP. During the tube operation, the electrons travel into the channels and as they strike the channel walls, they produce secondary electron emissions which create several hundred electrons. The close spacing of the channels within the MCP, along with the close spacing of the MCP to both the photocathode and the phosphor screen, allow an image to be created without the distortion characteristic of the Gen 0 and Gen 1 tubes. However, the channels within early MCPs were quite large compared with today's MCPs. As such, the resolution within early Gen 2 tubes was not as good as that of Gen 0, Gen 1 or today's Gen 2 and Gen 3 tubes. The other advancement with Gen 2 was the reduction in overall size and weight of both the tube module and the power supply. This reduction allowed Gen 2 tubes to be the first image intensifiers used within user-mounted devices such as head and helmet-mounted goggles.

Gen 3: Gallium Arsenide Advancement

Developed in the mid-1970s and placed into production during the 1980s, Gen 3 was mainly an advance in photocathode technology. The overall appearance between Gen 2 and Gen 3 tubes is quite similar. Gen 3 tubes use gallium arsenide (GaAs) for the photocathode. This increases the tube's sensitivity dramatically and particularly in the near-IR. The increased sensitivity improved system performance under low-light conditions, or, to put it another way, enabled the tube to detect light at far greater distances. However, the highly reactive GaAs photocathode could be easily degraded by the inherent chemical interactions that take place within a tube under normal operation. Most of the chemical reactions take place within the MCP due to the electron interactions with the walls of the MCP channels. Thus, to overcome the degrading effects of the photocathode, a thin metal-oxide coating was added to the input side of the MCP. This coating, more commonly known as an ion barrier film, not only prevented premature degradation of the photocathode but also enhanced the tube life by many times that of the Gen 2 tubes. Both Gen 2 and Gen 3 tube manufacturers have made continuous improvements through the years to increase the signal-to-noise ratio within each respective technology. Additionally, continuous improvements have been made within MCP manufacturing so as to improve the overall resolution. There has been considerable effort expended in developing a Gen 3 tube without the ion barrier film. The effort proved successful, but the manufacturing costs were excessive compared to the performance improvements. There are a few countries that manufacture Gen 3 image intensifiers. Currently, none come close to the overall performance of the image intensifiers manufactured in the U.S.

Gen 4: Clarification

Important Note: For a brief period, the Gen 3 tube without the ion barrier film or with thin film was termed Gen 4. This terminology, however, was rescinded shortly after it was announced, though some resellers of night vision tubes still use the nomenclature. In short, there is no Gen 4.

Zone Classification System

Zone 1: Central viewing area (inner third) - most critical for performance
Zone 2: Intermediate area between center and edge
Zone 3: Outer edge of the viewing area

Manufacturers measure and document the size and location of any cosmetic blemishes within these zones during quality assurance processes. Zone 1 imperfections have the greatest impact on user experience.

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