Night Vision Glossary and Abbreviations

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.

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.

Viewing a scene through two channels, i.e., one channel per eye. A good example of binocular night vision system is Armasight® BNVD.

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

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.

See Halo.

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.

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.

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.

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.

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.

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

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.

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.

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.”

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.

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.

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.

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.

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.

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.

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.

A faint hexagonal (honeycomb) pattern throughout the image area which often occurs under high-light conditions. This pattern is inherent in the structure of the micro channel plate and can be seen in virtually all Gen 2 and Gen 3 systems if the light level is high enough.

A unit of brightness equal to one footcandle at a distance of one foot.

The range within which an optical device can be adjusted or focused on a target.

Also called “Brightness Gain” or “Luminance Gain.” This is the number of times a night vision device amplifies light input. The gain of a tube is measured in one of two possible ways. The most common way is cd/m2/lx or candles per meter squared per lux. The other way to measure gain is fL/fc (foot-lamberts over foot-candles). This creates issues with comparative gain measurements since neither is a pure ratio, although both are measured as a value of input intensity over out- put intensity. This creates ambiguity in the marketing of night vision devices as the difference between the two measurements is effectively pi or approximately 3.14159 times. This means that a gain of 10,000 cd/m²/lx is the same as 31.4159 fL/fc. With a lack of convention on this item, if the units for gain are not specified, fL/fc should typically be assumed. Gain is usually measured as tube gain and system gain. Tube gain is measured as the light output (in fL) divided by the light input (in fc). This figure is usually expressed in values of tens of thousands. If tube gain is pushed too high, the tube will be “noisier”, and the signal-to-noise ratio may go down. U.S. military Gen 3 image tubes operate at gains of between 20,000 and 120,000. On the other hand, system gain is measured as the light output (fL) divided by the light input (fc) and is what the user sees. System gain is usually seen in the thousands. U.S. military systems operate at 2,000 to 3,000. In any night vision system, the tube gain is reduced by the system’s lenses and is affected by the quality of the optics or any filters. Therefore, system gain is a more important measurement to the user.

The semiconductor material used in manufacturing the Gen 3 photo cathode.

In the night vision world, the word “Generation” (Gen) refers to major advancements in technology. The higher the generation, the more sophisticated the night vision technology. The generation gap is the change in technology that drives the change in nomenclature.

Gen 0: 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: 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: 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: 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: 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.

Similar to blooming, a halo is the circular region around a bright light that appears “brighter” – it is caused by elastic collisions of electrons with the microchannel plate surface, (also known as electron scattering) which subsequently then bounce off and down another hole. Halos are the same size all over the screen and the size is dictated by the distance between the photocathode and the MCP. Basically, it is the round circle around lights when you look at them with night vision and it is generally used as an indication that you are looking at something that is too bright. The lower the value the better. Objects can be masked or hidden behind bright blooms of light in tubes that have higher halo values. Lasers can also have more apparent bloom off certain objects in some tubes as opposed to others due to this attribute. A high halo value can contribute to a lowering of tube resolution (high-light res) thus affecting what can be seen/detected by the user.

An image intensifier protection feature incorporating a sensor, microprocessor, and circuit breaker. This feature will turn the system off during periods of extremely bright light conditions.

Collects and intensifies the available light in the visible and near-infrared spectrum. Offers a clear, distinguishable image under low-light condition.

The distance between the user’s eyes (pupils) and the adjustment of binocular optics to adjust for differences in individuals. Improperly adjusted binoculars will display a scene that appears egg-shaped.

The distance between the user’s pupils (eyeball centers). The 95th percentile of U.S. military personnel falls within the 55 to 72mm range of IPD.

The area outside the visible spectrum that cannot be seen by the human eye (between 700 nanometers and 1 millimeter). The visible spectrum is between 400 and 700 nanometers.

Many night vision devices incorporate a built-in infrared (IR) diode that emits invisible light, or the illuminator can be mounted on to it as a separate component. The unaided eye cannot see IR light; therefore, a night vision device is necessary to see this light. IR Illuminators provide supplemental infrared illumination of an appropriate wavelength, typically in a range of wavelengths (e.g., 730nm, 830nm, 920nm). They eliminate the variability of available ambient light, but also allow the observer to illuminate only specific areas of interest while eliminating shadows and enhancing image contrast.

High-power devices provide long-range illumination capability. Ranges of several thousand meters are common. Most are not eye-safe and are restricted in use. Each IR laser should be marked with a warning label. Consult FDA CFR Title 21 for specific details and restrictions.

A high-vacuum device, which collects photons and amplifies these as electrons. This amplification of photons allows the viewer to see more light than without a night vision device.

A warning device in a night vision system that signals low battery power.

Denotes the photons perceptible by the human eye in one second.

The magnifying power of the lens. Four power (4X) indicates that the image will appear four times larger (or closer) than if viewed with the naked eye.

The measure of electrical current (mA) produced by a photocathode when exposed to a specified wavelength of light at a given radiant power (watt).

As the image intensifier is the heart of the night vision goggle, the micro channel plate is the heart of the image intensifier. The MCP receives the electrons that are from the photocathode and multiplies those electrons. The MCP is a thin piece of glass with millions of holes in it called channels. The glass has high secondary emission coefficient properties which provides a multiplication effect for each electron that enters a channel. Each electron that enters a channel and hits the wall, results in hundreds of electrons coming off the wall. This is where the key operation of electron multiplication occurs.

The minimum acceptable requirement for products procured by the Department of Defense. Use of the term MIL-SPEC indicates that the product meets applicable military specifications.

A measurement of the ability of an optical system to reproduce (transfer) various levels of detail from the object to the image, as shown by the degree of contrast (modulation) in the image.

A single channel optical device. A good example of monocular night vision system is Armasight® MNVD.

The shortest wavelengths of the infrared region, nominally 750 to 2,500 nanometers. Also, see IR (infrared).

The photocathode is on the inside (the vacuum side) of the faceplate. The photocathode is a layer of material that upon absorption of light, emits electrons. Another way to explain this operation is that the photocathode converts light energy (photons) to electrons.

Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The units of photocathode sensitivity are micro-amps/lumen (μA/lm) and are always measured in isolation with no amplification stage or ion barrier (film). It is the ability of the photocathode to produce an electrical response when subjected to light waves (photons). The higher the value, the better the ability to produce a visible image under darker conditions. A lumen is a scientific unit that measures light at wavelengths the human eye can see (violet through red). Since image intensifier tubes see light that the eye does not, it is important to know the spectral (color) content of the light used in testing photocathode sensitivity. Photocathode sensitivity is measured using a light source with a color spectrum similar to a theoretical black body operating at 2856°K (2856 degrees Kelvin). This light source was chosen because it has a color spectrum similar to the color of a night sky illuminated only by stars. Photocathode sensitivity measured with a different color spectrum light source will yield different readings.

The ability of an image intensifier or night vision system to distinguish between objects is measured in line pairs per millimeter (lp/mm). There is a difference between system resolution and image intensifier resolution. System resolution can be affected by altering the objective or eyepiece optics, or by adding magnification lenses. Image intensifier resolution remains constant. System resolution is very important in determining the quality of a system.

Limiting Resolution: This is measured as tube resolution (lp/mm) This is a measure of how many lines of varying intensity (light to dark) can be resolved within a millimeter of screen area. However, the limiting resolution itself is a measure of the Modulation Transfer Function. For most tubes, the limiting resolution is defined as the point at which the modulation transfer function becomes three percent or less. The higher the value, the higher the resolution of the tube.

System resolution: This is measured in cycles per milliradian (cy/mr). The more significant measurement is system resolution because this is what is visible to the user. Most systems produce an optimal resolution at some point between very high-light and very low-light conditions. As long as resolution is measured the same way using the same magnification and the same conditions (i.e., per U.S. military specs), then the higher the value and the better the ability to present a sharp picture. However, be aware that many devices will produce an image that is sharp in the center of the viewing area, but less sharp (or less defined) toward the periphery. The inability to obtain a clear, uniformly sharp image throughout the viewing area may be due to older technology (Gen 0, Gen 1 tube) or to the system’s optics. .

An adjustable aiming point or pattern (i.e., crosshair) located within an optical weapon sight.

Also known as electronic noise. A faint, random, sparkling effect throughout the image area. Scintillation is a standard characteristic of microchannel plate image intensifiers and is more pronounced under low-light-level conditions.

The screen refers to the phosphor layer that is applied to the inside (the vacuum side) of the output optic. The phosphor is a material that when electrons are absorbed, emits photons (light). As the cathode “converts” photons to electrons, the screen converts electrons to photons (allowing us to see the image). The phosphor that is applied on most high manufacturing processes has traditionally been green phosphor (P-43), however white phosphor (P-45) is quickly becoming the preferred phosphor color. One of the advances in night vision is white phosphor technology. This technology results in a grayscale display. Images produced by a white phosphor device may be more familiar to the human eye, which naturally perceives poorly lit scenery in shades of gray. White phosphor is less affected by blooming (i.e., the loss of an image due to additional light sources), which makes it useful for urban environments. The P45 screen provides similar decay times to P43, with excellent recognition capability. The image presented to the user of a P45 screen is black and white, which results in less eye fatigue, faster recognition (especially in sandy/rocky terrain), and a slightly better discrimination of shades of intensity than the traditional green.

A ratio of the magnitude of the signal to the magnitude of the noise. If the noise in the scene (see “scintillation” definition) is as bright and as large as the intensified image, you cannot see the image. SNR changes with light level because the noise remains constant but the signal increases (higher light levels). The higher the SNR, the darker the scene can be and the device still performs. The effect of SNR in I2 devices is like that of a television far away from the TV station. At long distances from the station, the TV picture becomes noisy, and the “snow” blocks the picture. This criterion is tied with EBI (Equivalent Background Illumination) and illustrates the amount of interference (also known as “hazing” or “snow”) visible in extremely low-light conditions. The higher the signal-to-noise ratio, the better the ability of the tube to display objects with good contrast under low-light conditions. It is the single best indicator of an image intensifier’s performance. A good example in digital domain is, if you for example have a ratio of 30 to 1, then for every 30 pixels of signal you get one pixel of noise.

When two views or photographs are taken through one device. One view/photograph represents the left eye, and the other, the right eye. When the two photographs are viewed in a stereoscopic apparatus, they combine to create a single image with depth and relief. Sometimes this gives two perspectives. However, it is usually not an issue because the object of focus is far enough away for the perspectives to blend into one.

Equal to tube gain minus losses induced by system components such as lenses, beam splitters, and filters.

Senses radiation and temperature differentiation from the 7.5 to 13.5-micron range and creates a thermal picture (image of emitted heat energy). Better for detection than recognition.

A U.S. weapon mounting system used for attaching sighting devices to weapons. A Weaver Rail is a weapon-unique notched metal rail designed to receive a mating throw-lever or Weaver Squeezer attached to the sighting device.

Zeroing is a method of boresighting an aiming device to a weapon and adjusting to compensate for projectile characteristics at known distances.