Glossary of Terminology
Acuity - The human eye has a nominal resolution of 1 minute of arc. The common measure of visual acuity is based on reading letters with 1 minute line width, Snellen
letters, or patterns with similar detail, suc h as Landolt rings. Visual acuity is reported as a fraction. The denominator is the test distance (usually 20 feet). The numerator is the relative size of line that can be resolved. That is, 20/40 indicates that the resolution was 2 minutes of arc, twice the nominal value. In other words, that individual can resolve at 20 feet what a "normal" person can at 40 feet
Automatic Brightness Control (ABC) - An electronic feature that automatically reduces voltages to the microchannel plate to keep the image intensifier's brightness within optimal limits, and protects the tube. The effect of 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
Black Spots - Blemishes - Cosmetic blemishes in the image intensifier or dirt/debris between the lenses. Black spots in the image intensifier do not affect the performance or reliability of the device and are inherent in the manufacturing processes
Brightness Gain - When referring to an image intensification tube, brightness gain is the ratio of the brightness of the output in units of foot-Lambert, compared to the 10
illumination of the input in foot-candles. A typical value for a GEN III tube is 25,000 to 30,000 Fl/fc. A tube gain of 30,000 Fl/fc provides an approximate system gain of 3,000. This means that the intensified NVG image is 3,000 times brighter to the aided eye than to the unaided eye
Bright Source Protection (BSP) - An electronic function 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, BSP may have the effect of lowering resolution when it is functioning
Bright Spots. These are signal- induced blemishes in the image area caused by a flaw in the film on the MCP. A bright spot is small, non- uniform, bright area that may flicker or appear constant. Bright spots usually go away when the light is blocked out. Not all bright spots make the ANVIS unserviceable. A test can be performed as follows: Place a cupped hand over the lens to block out all light. Make sure any bright spot is not simply a bright area in the viewed scene. If the bright spot remains, an emission point exists and needs to be checked.
Collimation - The act of making rays of light travel in parallel lines. Also the process of aligning the various internal optical axes of a system with each other.
Convergence. The shifting of an observer's eyes inward to view a nearby object i.e. crossing the eyes.
Chicken Wire - An irregular pattern of dark lines in the Field-of-View (FOV) throughout the image area or in parts of the image area. Under the worse condition, these
lines will form hexagonal or square-wave shaped lines.
Counterweight System - Counterweight systems are used to adjust the center of gravity of the pilot’s flight helmet with goggles installed. Without counterweights, there can be a fatiguing forward and downward force on the pilot’s neck. The counterweight system may consist of a weight bag and counterweights. The Army’s recommended initial weight is 12 ounces for one of its systems. Pilots are instructed to add or remove weight to achieve the best balance and comfort, not to exceed 22 ounces. Attachment of the weight bag is below the back of the helmet with the battery pack mounted vertically above it. The adjustment of the weight is to be made with the binoculars attached and flipped down.
Cycles per Milliradian (cy/mr) - Units used to measure resolution. A milliradian is the angle created by one 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.
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 for training purposes only, and is not recommended for an extended period of time.
Daylight Training Filter- A glass filter assembly designed to fit over the objective lens of a night vision device. The filter reduces light input to a safe (night-time) level, allowing safe extended daytime use of the night vision device.
Diopter - The unit of measure used to define eye correction or the refractive power of a lens. Adjustments to an optical eyepiece accommodate for differences in individual eyesight. Most systems provide a +2 to -6 diopter range.
Dipvergence - The shifting of an observer's eyes vertically, one up and one down.
- Geometric Distortion - is inherent in all Gen 0 (infrared Tubes ie. B-20) and Gen I image intensifiers and in some Gen II image intensifiers that use electrostatic rather than fibre-optic inversion of the image. Geometric distortion is eliminated in image tubes that use a microchannel plate and fibre-optics for image inversion, however, some S-distortion can occur in these tubes.
- S-Distortion - results from the twisting operation in manufacturing fibre-optic inserters (fibber optic twister - Russian tubes do not use a fibre optic twister). Usually S-Distortion is very small and is difficult to detect with the unaided eye. Gen III tubes manufactured to U.S. military standards since 1988 have nearly no perceptible S-Distortion.
- Sheer Distortion - can occur in any image tube that uses fibre-optic bundles for the phosphor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area as through the line were sheered.
*NOTE: Non-inverting image intensifiers that use microchannel plates and clear glass for the optics are free of distortion. The 2nd Gen. image intensifiers that Russians make are distortion free, since they do not have fiberoptics twister.
Electronic Noise - Also known as Scintillation. A faint, random, sparkling effect throughout the image area. Scintillation is a normal characteristic of microchannel plate image intensifiers and is more pronounced under low-light-level conditions
Equivalent Background Illumination (EBI) - This is the amount you see in an image tube that is turned on but there is no light at all on the photocathode; it is affected by temperature where the warmer night-vision device, the brighter the background illumination. EBI is measured in lumens per square centimetre (lm/cm2) wherein the lower the value the better. The EBI level determines the lowest light level at which you can detect something. Below this light level, objects will be masked by the EBI.
Eyepiece Lens - The eyepiece lens focuses the image from the fiber optic inverter on to the eye by adjusting for individual eye acuity. There are two eyepiece lens assemblies in current systems; the 15 mm and the 25 mm eyepiece lens assembly. Tests show the larger eyepiece is more effective. This lens assembly is designed to provide some adjustment for the user to compensate for minor vision deficiencies (i.e. diopter adjustment). However, the assembly does not correct for all eye deficiencies and does not replace the need for wearing prescribed spectacles or contact lenses.
Eye Relief - The focus-range distance between the user's eye and the eyepiece lens of an optical device at which the viewed scene can clearly be seen.
Exit Pupil - In an optical system, the rays of light passing through the system will be limited by either the edges of one of the components such as the eyepiece lens, or by an internal aperture. The image passing through the entrance side of the optical system is the entrance pupil. The image passing out the exit side is the exit pupil. This image forms a small disk containing all of the light collected by the optics from the entire field-of-view.
Field-of-View (FOV) - The width or spatial angle of the outside scene that can be viewed through the intensifier tubes measured laterally and vertically. Typical NVGs have a 40° FOV. There are NVGs in development that attempt to increase this FOV significantly in an effort to enhance pilot performance. An example of a wide FOV NVG would be the Panoramic NVGs'.
Fixed Pattern Noise (FPN) - A faint hexagonal (honeycomb) pattern throughout the image area that most often occurs under high-light conditions. This pattern is inherent in the structure of the microchannel plate and can be seen in virtually all Gen II and Gen III systems if the light level is high enough.
Gain - Also called brightness gain or luminance gain. This is the number of times a night vision device amplifies light input. It is usually measured as tube gain and system gain. 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.
Gallium Arsenide (GaAs) - The semiconductor material used in manufacturing of the Gen III photocathode. GaAs photocathodes have a very high photosensitivity in the spectral region of about 450 to 950 nanometers (visible and near-infrared region).
Generations of Image Intensifiers - Image intensifier tubes are classed by generation (GEN) numbers assigned by the US Army's Night Vision Laboratory. Each GEN denotes a new technology or manufacturing process that improved since the previous generation.
- GEN I - Vietnam Era technology. GEN I has the most distortion and the least tube life. GEN I image tubes are most commonly found in imported night vision devices.
- GEN II - 1960s development of microchannel plate (MCP) amplifier component provided the needed breakthrough for high gain and small size.
- GEN II+ - 1970s development increased image tube bias voltage to improve gain. Additionally, a glass faceplate was added to improve resolution.
- GEN III - 1980s development provided substantially improved gain and a bandwidth that reached farther into the near IR region. Improvements in the microchannel plate (MCP) and a gallium-arsenide photocathode greatly enhanced tube life and performance.
- GEN III+ - Generally referred to as GEN III Enhanced, 1990's improvements in the photocathode and MCP resulted in increased gain and resolution. Not a generally accepted term, but used herein for brevity.
Highlight Shutoff - An image intensifier protection feature incorporating a sensor, microprocessor and circuit breaker. This feature will turn the system off during periods of extreme bright light conditions.
Image Distortion - This problem is more easily detected in high- light conditions. Image distortion is evidenced by vertical objects, such as trees or poles appearing to wave or bend when the user moves his head vertically or horizontally when looking through the goggles. Ground surfaces in the direction of hover may appear to swell or sink. Distortion does not change during life of an image intensifier. Limits on allowable distortion are an important part of performance specifications since excess distortion can interfere with viewing the image and thus with the operator’s ability to perform necessary flight maneuvers.
Image Intensifier Tube (IIT - I2) - A high-vacuum electro-optical device that collects photons and amplifies these as electrons. This amplification of photons allows the viewer to see more light than without a night vision device. Being an (tube) which detects & amplifies light to produce a visual image.
Interpupillary Adjustment - 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 or as a reclining figure-8.
Interpupillary Distance - (IPD). Interpupillary distance is the distance between the centers of the pupils of the eyes when the eyes are parallel. Adjustment provisions for
variable IPD should be a feature of the NVG to allow the full image to be seen by the NVG user. The recommended range of adjustment should be at least 57-70mm to
accommodate an estimated 90% of the potential user population. If no adjustment is provided, then the exit pupil must be large enough for the user to get a full field of view.
IR Illuminator- Many night vision devices incorporate an infrared (IR) diode that emits invisible light. IR light cannot be seen by the unaided eye; therefore, a night vision device is necessary to see this light.
IR Laser - High-power devices providing 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 like the one shown here. Consult FDA CFR Title 21 for specific details and restrictions.
Light Interface Filter (LIF). An optical filter that protects the NVG device and its user from some laser hazards by the LIF. The LIF’s, if installed, are mounted on an adapter attached to the end of the objective lens.
Light-Secure Eyeguard (Shuttered Eyeguard) - A rubber eyecup fitted to an eyepiece of a night vision device that incorporates a shutter that closes when not depressed by the user's eye. Prevents face glow that can compromise the user's location
Line Pairs per Millimetre (lp/mm) - 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. The lines and spacing between lines in each of the different patterns differ in width; the narrower the width, the greater the resolution is needed to distinguish the lines in a given pattern. Human test subjects must be able to clearly distinguish all the horizontal and vertical lines of a particular pattern in order for an image intensifier to achieve the resolution represented by that pattern.
Low Voltage Indicator - A warning device in a night vision system that signals low power.
Lumen - The unit denoting the photons (light) perceivable by the human eye in one second.
Lux - A unit measurement of illumination. The illuminance produced on a surface that is on meter square, from a uniform point source of one-candela intensity, or one lumen per
Microchannel Plate (MCP) - A metal coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in GEN II and GEN III systems.
These devices normally have anywhere from 2 to 6 million holes (or channels) in them. Electrons entering the channel strike a wall and knock off additional electrons which in turn knock off more electrons, producing a cascading effect. MCPs eliminate the distortion characteristic of GEN I systems. The number of holes in an MCP is a major
factor in determining resolution.
MILSPEC - Term for Military Specification. It is the minimum acceptable requirements for products procured by the US Department of Defence. Use of the term MILSPEC indicates that the product meets applicable military specifications
NATO-STANAG- Term for the North Atlantic Treaty Organization STANdard AGreement. This can be described as an international MILSPEC.
Near-Infrared - The shortest wavelengths of the infrared region, normally 750 to 2,500 nanometers (nm). GEN II operates from around 440 to 950 nanometers.
Objective Lens - The objective lens assembly collects the available light energy and focuses it on the photocathode (front end of the image intensifier tube). It is housed in
an assembly that is used for distance focusing. A coating is placed on the inside portion of the lens that filters out specific wavelengths, thus allowing the use of properly
modified interior lighting.
Output Brightness Variation - This condition is evidenced by areas of varying brightness in or across the image area. The lower contrasts do not exhibit distinct lines of
demarcation nor do they degrade image quality. This condition should not be confused with shading.
Phosphor Screen -The phosphor screen converts electrons into photons. A very thin layer of phosphor is applied to the output fiber optic system, and emits light when struck by electrons. See also Photocathode.
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.
Photosensitivity (Photoresponse) - Also called photocathode sensitivity or photoresponse. The ability of the photocathode material 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. Usually measured in microamps of current per lumen (A/lm).
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 millimetre (lp/mm) while system resolution is measured in cycles per miliradian. 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 centre of the image and at the periphery of the image. This is especially important for devices selected for photograph or video where the entire image resolution is important. Measured in line pairs per millimetre (lp/mm).
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 the photocathode's sensitivity and also accounts for phosphor efficiency and MCP operating voltage, it is the best single indicator of an image intensifier's performance
Scintillation - Also known as electronic noise. A faint, random, sparkling effect throughout the image area. Scintillation is a normal characteristic of microchannel plate image intensifiers and is more pronounced under low-light-level conditions
Screen- The image tube output that produces the viewable image. Phosphor (P) is used on the inside surface of the screen to produce the glow, thus producing the picture. Different phosphors are used in image intensifier tubes, depending on manufacturer and tube generation. P-20 phosphor is used in the systems offered in this catalogue
Weaver Mounting System - A US 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