Night Vision Terminology

Autogating 
When the power supply is “auto-gated,” it means the system is turning itself on and off at a very rapid rate. This is done to reduce the amount of light reaching the photocathode and thus maintains a higher-quality image during high light and variable lighting environments. Without autogating, there will be image degradation and the photocathode will become overwhelmed with light, resulting in what users refer to as "blooming" where subjects and targets around the periphery of the light source becomes difficult to see. 
 
Automatic Brightness Control (Auto-Gain)
An electronic feature that automatically reduces voltages to the microchannel plate to keep the image intensifier’s brightness within optimal limits and protect 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.

Biocular 
A system where there is one objective lens and 2 ocular lenses, in which the optical components are shared by both of the viewer's eyes. PVS-7s are an example of a biocular device. This is not to be mistaken for a BINOcular device (see below)
 
Binocular
A system where there is 2 objective and 2 ocular lenses in which each eye observes a separate optical components. RNVGs are an example of a Binocular device. 

Black Spots/Dark Spots (AKA: "Blems")  
There are typically two types of dark spots - factory dark spots and user-induced dark spots (aka "Blems"). Due to the sensitive nature of the manufacturing process, sometimes debris and other particles can become trapped inside the intensifier and present themselves as factory dark spots. Typically factory dark spots are more "fuzzy" in nature and sometimes has a slightly lighter ring around them. User-induced blemishes are typically not circular in nature and are often induced by high light sources such as high power lasers or lights. Finally other blemishes can be caused by debris inside the housing sitting on top of the output screen that is caused by improper assembly or a previously-stuck piece of debris that has fallen onto the output screen due to vibration or shock to the night vision device. For more details on factory dark spots and blemishes, see our article HERE.

Blooming
Loss of the entire 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 a “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, become much brighter, “whiting out” objects within the field of view. Blooming is common in Generation 0 and 1 devices. The lights in the image to the right would be considered to be “blooming”.

Bright Spots
These can be defects in the image area produced by the NVG. This condition is 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 usually go away when the light is blocked out and are cosmetic blemishes that are signal induced.

Bright Spot Protection (BSP)
High-Light Cut-Off 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, it also has the effect of lowering resolution when functioning. On NNVT intensifiers, in lieu of autogating, BSP is employed to maintain uniform contrast through the image but at the expense of system resolution. 

Chicken Wire or "fixed pattern noise"
An irregular pattern of dark thin lines in the field of view either throughout the image area or in parts of the image area. Under the worst-case condition, these lines will form hexagonal or square wave-shape lines but is nothing to be concerned about as they are most often present in high light conditions and often are not present in low light conditions.

Daylight Cover/Day Cap
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 when the unit is on.
 
Diopter
The unit of measure used to define eye correction or the refractive power of a lens. Usually, adjustments to an optical eyepiece accomodate for differences in individual eyesight. Most systems provide a +2 to -6 diopter range with most users finding -0.5 being the most comfortable/natural for use.

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 design of the optics or image intensifier tube causes straight lines at the edge of the field of view to curve inward or outward. This curving of straight lines at the edge will cause a square grid pattern to start to look like a pincushion or barrel. This distortion is the same for all systems with the same model number. Good optical design normally makes this distortion so low that the typical 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 use 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
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. This can be reduced by using a rubber eyecup on the eyepiece assembly but this comes at the expense of removing any situation awareness and ambient light awareness.

Emission Point
A steady or fluctuating pinpoint of bright light in the image area that 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 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 point of light source 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. 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 in order to achieve the optimal image area. Milspec eye relief is 25mm but most users will find that distance to be quite close and 35mm is a more acceptable eye relief for comfortable use with common ballistic eye protection (eg: Oakley M-Frames)

Field of View (FOV)
The diameter of the imaged area when viewed through an optic

Figure of Merit (FOM)
A general measurement of tube performance, calculated on resolution (lp/mm) multiplied by the signal to noise ratio. For more information see our short article HERE.

Fixed Pattern Noise  
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 2 and Gen 3 systems if the light level is high enough.

Foot-lambert
A unit of brightness equal to one foot-candle at a distance of one foot. This is typically a unit of measure used to denote how "bright" a light source is. For example - on the Hoffman test set, high light scenarios simulates 1.5fL while low light is 0.003fL

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. Tube gain is measured as teh 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 “noiser” U.S. military Gen 3 image tubes operate at gains of between 20,000 and 45,000. On the other hand, system gain is measured as teh light output (fL) divided by the light input (also fL) and is what the user actually sees. System gain is usually seen in the thousands. U.S. military systems operate at 3,000 to 13,000. 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 with the primary contributing factor of system gain being the intensifier. Higher system gain caused by substituting lenses may not always mean better performance as it does not measure amount of refraction. As an example, some lightweight lens systems will give a higher system gain number but this is caused by less anti-glare coatings. For a discussion on how lightweight lens systems affect gain, see our article HERE. 

Gallium Arsenide (GaAs)  
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).

Generations
To date, there have been four generations of l² devices, identified as Gen 0, Gen 1, Gen 2, and Gen 3. Developmental laboratory work is on-going, and the U.S. military may designate the resulting as Gen 4. However, no definition for Gen 4 presently exists.

• Generation 0 – The first night vision aids (also called Generation Zero or Gen 0) were sniper scopes that came into use during World War II and the Korean conflict. These were not true image intensifiers, but rather image converters, which required a source of invisible infrared (IR) light mounted on or near the device to illuminate the target area.
• Generation 1 – The “starlight scopes” developed during the early 1960’s for use in Vietnam were the first Generation (Gen 1) of image intensifier devices. In Gen 1 night vision units, three image intensifiers were connected in a series, making the units longer and heavier than future night vision units would be. Gen 1 equipment produced an image that was clear in the center of the field of view but suffered from large optical distortion around the periphery. Gen 1 equipment was also subject to “blooming”. Most low-cost imported night vision units use Gen 1 technology, though often under the guise of a higher “generation”.
• Generation 2 – The development of the microchannel plate, or MCP, in the late 1960s brought on the second generation (Gen 2) in l² night vision. The MCP accelerated and multiplied electrons which provided the gain previously supplied by coupling three image intensifiers together (Gen 1). The introduction of the MCP significantly reduced size and weight for image intensifier tubes, enabling design of smaller night vision goggles and hand-held devices. The MCP also provided much more robust operation when bright lights entered the field of view. The Gen 2 tubes used the same tri-alkali photocathode as the Gen 1 devices. This generation was implemented to reflect the change in how the light was amplified (MCP versus three-stage coupling). Current Gen2+ technology such as Photonis Echo and 4G systems rival and in some cases surpass performance of Gen3 systems. 
• Generation 3 – Third-generation (Gen 3) image intensifiers were developed in the mid-1970s and became available during the early 1980s. Gen 3 introduced two major technological improvements: the gallium arsenide (GaAs) photocathode and the ion barrier coating to the microchannel plate. The GaAs photocathode increases the tube’s sensitivity to light from the near-infrared range of the spectrum, enables it to function at greater detection distances, and improves system performance under low-light conditions. Application of a metal-oxide ion barrier to the MCP increases the life of the image tube. This generation was implemented to reflect the change in the photocathode (tri-alkali replaced with GaAs).

 
Myth vs. Fact
Generation 4 Some say that generation (Gen) 4 is the most advanced night vision you can buy. This is not the case. To dispel this myth, let’s start with the basics. There are four Generations of night vision; however, they are Gen 0-3, not Gen 1-4. Historically, the U.S. Army has defined each Generation of night vision. In the late 90’s the Army did define Gen 4 as the removal of the ion barrier film creating a “filmless” tube. This new advancement was to reduce halos while increasing sensitivity, signal-to-noise ratio (SNR) and resolution, for overall improved performance. While performance was improved, the lack of an ion barrier in Gen 4 tubes led to high failure rates, ultimately leading the U.S. Army to recant the existence of Gen 4 definition. Recognizing the high failure rates of Gen 4 tubes, ITT chose to improve upon the existing Gen 3 technology and create a “thin-filmed” tube. By keeping the protective ion barrier, but greatly reducing its thickness, ITT was able to maintain the reliability of Gen 3 while—at the same time—delivering on the Army’s performance requirements intended for Gen 4. This innovation resulted in the production of the Gen 3 thin-filmed tube.

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.

I2 (Image intensification) 
Collects and intensifies the available light in the visible and near-infrared spectrum. Offers a clear, distinguishable image under low-light conditions.

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)
The distance between the user’s pupils (eyeball centres). The 95th percentile of people fall within the 55 to 72mm range of IPD.

Infrared Light
Area outside the visible spectrum that cannot be seen by the human eye (between 700 nanometers and 1 millimetre). The visible spectrum is between 400 and 700 nanometers.

IR Illuminator 
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. IR light cannot be seen by the unaided eye; 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. 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.

Lp/mm (Line pair per millimetre) 
Units used to measure image intensifier resolution. Usually determined from a 1951 U.S. 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. 

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

MCP (Microchannel Plate)
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. 

Monocular
A single channel optical device. Such as a PVS-14

Near Infrared (NIR)
The shortest wavelengths of the infrared region, nominally 750 to 2,500 nanometers.

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 
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. 

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).

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.

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.

Stereoscopic Night Vision
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. 

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

Variable Gain Control
Allows the user to manually adjust the gain control ( basically like a dimming a light) in varying light conditions. PVS-14s and some binoculars such as the Nocturn Industries Manticore-R include this as a feature.