Glossary: Night Vision Systems
This electronic feature automatically reduces voltages to the microchannel plate to keep the image intensifier’s brightness within optimal limits and protects the tube. This is most apparent 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-Gating reduces the duty cycle of the cathode voltage by rapidly switching the voltage on and off. It is a proprietary means to preserve the high MTF (Modulation Transfer Function) and resolution that is obtained at the full operating voltage while reducing the strain of high illumination levels on the image intensifier tube. This is done by reducing the average current coming from the photocathode. Auto-Gating constantly operates to improve the quality of the image, not only during day-night-day transition, but also under dynamic lighting conditions such as military/law enforcement operations in urban environment which define many of today's missions.
These are common blemishes in the image intensifier of the night vision device, or can be dirt/debris between its 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. Every night vision image intensifier tube is different.
These are signal-induced blemishes in the image area caused by a flaw in the film on the microchannel plate. A bright spot is a small, non-uniform, bright area that may flicker or appear constant. Bright spots generally go away when the light is blocked out. A test can be performed as follows: Place your hand over the lens to block out the 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.
Viewing a single image source with both eyes. For example, watching a TV.
Viewing a scene through two channels, meaning one channel per eye. This allows for depth perception (stereoscopic vision) which is often beneficial for overall situational awareness.
Impairing the whole night vision image, parts of it, or small parts of it, due to intensifier tube overloading by a bright light source. Also, known as a “halo” effect, when the viewer sees this halo effect around visible light sources. When such a bright light source comes into the night vision device’s view, the entire night vision scene, or parts of it, becomes much brighter, which whites out objects within the field of view. Blooming is common in Generation 0 and 1 devices.
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 longevity. However, BSP may have the effect of lowering resolution when it is functioning.
The alignment of a weapon-mounted/aiming device to the bore of the weapon. See also: Zeroing.
The process of aligning the various internal optical axes of a system with each other.
The shifting of an observer's eyes inward to view a nearby object, much like crossing the eyes.
An irregular pattern of dark thin lines in the field of view either throughout the image area or in parts of the image area. Occasionally, these lines will form hexagonal or square wave-shape lines.
Generally made of a soft plastic or rubber cap 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. Near field focus only usable in this fashion.
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.
A glass filter assembly that snaps onto the eye piece of night vision / thermal device to prevent condensation from forming on the optics during rapid temperature changes, heavy and/or long use.
A unit of measure used to define eye correction or the refractive power of a lens. Generally, adjustments to an optical eyepiece accommodate for differences in individual eyesight. Most night vision/ thermal systems provide a +2 to -6 Diopter range.
This is the amount you see in an image intensifier 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.
There is a defect in the image area of the NVG. Edge glow is a bright area (sometimes sparkling) in the outer portion of the viewing area.
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.
A steady or fluctuating point of bright light visible through the image produced by the night vision device which does not go away 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 night-time conditions, it is not indicative of a problem. If the emission point remains bright under all lighting conditions, the system needs to be repaired. It should be noted that an emission point may be incorrectly confused with a point of light source in the scene being viewed.
The distance a person’s eyes must be from the last element of an eyepiece in order to achieve the optimal image area.
The width or spatial angle of the outside scene that can be viewed through the intensifier tubes measured laterally and vertically.
FOM is 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. It is difficult to have an objective measure of quality for a night vision device since it is intrinsically reliant on external factors such as available light and weather conditions. Therefore the FOM is universally recognized as a measurable value to adequately determine the performance of the image intensifier tube.
Also referred to as brightness gain or luminance gain; this is the number of times an image intensifier tube 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.
The semiconductor material used in manufacturing the Gen 3 photocathode. GaAs photocathodes have a very high photosensitivity in the spectral region of about 450 to 950 nanometers (visible and near-infrared region).
Image intensifier tubes are classed by generation (GEN) numbers assigned by the US Army's Night Vision Laboratory. Each GEN denotes a new contract or manufacturing process. Note: Generations do not define the performance specs of the image intensifier.
- Generation I: It started with electrostatically focused Generation I tubes featuring high image resolution, a wide dynamic range and low noise.
- Generation II: Introduced the microchannel plate for much higher gain in the 1980’s. The original image resolution was less than that of the first generation intensifiers but the gain was much higher.
- Generation II+: Later advancements in GEN II technology brought the tactical characteristics of “GEN II+” devices (equipped with better optics, SUPERGEN tubes, improved resolution and better signal-to-noise ratios) into the range of GEN III devices, which has complicated comparisons.
- Generation III: In the late 1980’s an Image Intensifier with a GaAs photocathode was developed showing an enhanced sensitivity in the Near-Infrared. In the late 1990’s GEN-III tubes with greatly improved performance appeared on the market.
- Generation IV: U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) is part of the US governing body that dictates the name of the generation of night vision technologies. Although the recent increased performance associated with the GEN-III OMNI-VI (and above) components is impressive, the U.S. Army has not yet authorized the use of the name GEN-IVWeaver Mounting System for these components. This means the term GEN-IV is used more as a marketing tool than it is valid terminology, often used to describe high-end GEN-III tubes with an autogating power supply.
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 which subsequently then bounce off and down another hole. Halo’s 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’s generally used as an indication that you’re looking at something that is too bright.
The distance between the user’s pupils (eyeball centers). Most of military personnel falls within the 55 to 72mm range of IPD.
Many night vision devices incorporate a built-in (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), and 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.
Area outside the visible spectrum that cannot be seen by the human eye (between 700 nanometers and 1 micrometer). The visible spectrum is between 400 and 700 nanometers.
High-power devices providing long-range illumination capability. Ranges of several thousand meters are common.
ITAR represents a set of US Government regulations that control the export of defense-related materials, articles, and services on the United States Munitions List. These regulations implement the provisions of the Arms Export Control Act. The Department of State interprets and enforces ITAR. Basically, ITAR dictates that any defense related items (including Night Vision Equipment and IR Equipment) cannot be exported from the United States in any way, without express permission from the US Department of State. Failing to follow ITAR will result in felony charges which can lead to heavy fines and/or prison sentences.
Units used to measure image intensifier resolution. Usually determined from a 1951 US Air Force Resolving Power Test Target. The target is a series of different-sized patterns composed of three horizontal and three vertical lines. A user must be able to distinguish all the horizontal and vertical lines and the spaces between them. Typically, the higher the line pair, the better the image resolution.
A single-channel optical device.
Term for the North Atlantic Treaty Organization STANdard AGreement. This can be described as an international MILSPEC.
The measure of electrical current (mA) produced by a photocathode when exposed to a specified wavelength of light at a given radiant power (watt).
The magnifying power of the lens. Four power (4X) indicates that the image will appear four times larger than if viewed with a 1X lens.
A metal-coated glass disk that multiplies the electrons produced by the photocathode. An MCP is found only in Gen 2 or Gen 3 systems. MCPs eliminate the distortion characteristic of Gen 0 and Gen 1 systems. The number of holes (channels) in an MCP is a major factor in determining resolution (typical range of channels is between 3 to 10 million).
The Mil-Dot reticle was designed around the unit of measurement called the milliradian. The dots of a mil-dot reticle allow the shooter to estimate range to a target of a known size, hold over targets with the mil-dots as a reference, and give a recognizable lead for moving targets. The practical application has become one mil = 3.5 MOA (minutes of angle). The Mil-Dot reticle is employed by placing the reticle over the target and aligning one end of the target to the flat of the reticle. The number of Mil-Dots are counted to provide an accurate reading.
Term for Military Specification in the US, but has been adopted internationally as well. 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.
Commonly used in the shooting world as a point of reference, a Minute Of Angle is 1.047 inches at 100 yards. Normal shooter application uses the simplified 1” at 100 yards. For perspective, the difference between using the exact 1.047 and 1” is 0,47” at 1,000 yards. So, if a reticle has a 1 MOA dot, the dot would cover 1” at 100 yards. MOA is also relative to the click adjustments on scopes and iron sights. Scopes with 1/4MOA clicks would require 4 clicks to shift the bullet impact 1MOA (1”) at 100 yards. The same adjustment would move the bullet impact 1MOA at 200 yards, which is 2 inches. A simple way to calculate MOA at ranges less than 100 yards is to use the 100 yard adjustment and doubling the number of clicks for 50 yards and doubling again for 25 yards. For example, if a scope has ½ MOA click adjustments, it will require 2 clicks to move the bullet 1” at 100 yards, 4 clicks to move the impact 1” at 50 yards and 8 clicks to move it 1” at 25 yards.
The shortest wavelengths of the infrared region, normally 750 to 2,500 nanometers.
13-digit code identifying all ‘standardized material items of supply’ as they have been recognized by the United States Department of Defense. National Stock Numbers have come to be used in all NATO countries pursuant to the NATO Standardization Agreements (STANAGs).
A very thin layer of phosphor is applied to the output fiber optic system, and emits light when struck by electrons. The purpose of the phosphor screen is to convert electrons into photons. See also Photocathode.
The input surface of an image intensifier tube that absorbs light energy (photons) and in turn releases electrical energy (electrons) in the form of an image. The type of material used is a distinguishing characteristic of the different generations.
Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The measuring units of photocathode sensitivity are micro-amps/lumen (µA/lm) or microamperes per lumen. This criterion specifies the number of electrons released by the Photocathode (PC). PC response is always measured in isolation with no amplification stage or ion barrier (film). Therefore, tube data sheets (which always carry this “raw” figure) do not reflect the fact that over 50% of those electrons are lost in the ion barrier.
Named after the Picatinny Arsenal in New Jersey where it was developed, the rail is comprised of a series of ridges with a T-shaped cross-section interspersed with flat “spacing slots”. Scopes and similar products are mounted either by sliding them on from one end or the other; by means of a “rail-grabber” which is clamped to the rail with bolts, thumbscrews or levers; or onto the slots between the raised sections. Picatinny rail sections are the current standard attachment methods for weapon accessories among NATO forces.
The ability of a 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 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.
An adjustable aiming point or pattern (i.e. crosshair) located within an optical weapon sight.
A clear glass filter assembly designed to fit over the objective lens of a night vision device and designed to protect the optics against physical damage (chips, scratches, etc).
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 microchannel plate's operating voltage, it is the best single indicator of an image intensifier’s performance.
Allows the user to manually adjust the gain control (basically like a dim control) in varying light conditions. This feature sets the PVS-14 apart from other popular monoculars that do not offer this feature.
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 matching throw-lever or Weaver Squeezer attached to the sighting device
A method of boresighting an aiming device to a weapon and adjusting to compensate for projectile characteristics at known distances.
