NIGHT VISION TERMINOLOGY
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.
Auto-Gating is the electronic solution that 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 operations in urban terrain which define many of today's missions.
These are common blemishes in the image intensifier of the NVD or can be dirt or debris between the lenses of the NVG. 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. They are like diamonds.
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.
Viewing a single image source with both eyes (example: watching a television set).
Viewing a scene through two channels; i.e. one channel per eye.
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".
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.
The alignment of a weapon aiming device to the bore of the weapon. See also Zeroing.
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).
An irregular pattern of dark lines in the image area or in parts of the image area. Under the worse condition, these lines will form hexagonal or square-wave shaped lines.
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.
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.
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.
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 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.
Two types of fiber optics distortions are most significant to night vision devices: S-distortion and shear 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 phospor 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 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.
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 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. 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.
The diameter of the imaged area when viewed through an optic
Image Intensification tube specification designation, calculated on line pair per mm x signal to 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.
A unit of brightness equal to one footcandle at a distance of one foot.
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 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 "noiser" and the signal-to-noise ration many go down. 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 the 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 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 photocathode. GaAs photocathodes have a very high photosensitivity in the spectral region of about 450 to 950 nanometers (visible and near-infrared region).
Two technologies are referenced as night vision; image intensification and thermal imaging (see definitions). Because of cost and the fact that image intensifier scenes are easier to interpret than thermal (thermal images show targets as black or white - depending upon temperature - making it more difficult to recognize objects), the most widely used night vision aid in law enforcement is image intensification (l²) equipment. 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.
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.
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".
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).
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. The operational life of Gen 3 tubes is in excess of 10,000 hours, compared to that of Gen 2 tubes which is about 2,000 to 4,000 hours. This generation was implemented to reflect the change in the photocathode (tri-alkali replaced with GaAs).
Gated filmless technology was created in 1998, but without the reliability required for military delivery. By removing the ion barrier film and "gating" the system power supply, the technology demonstrated substantial increases in target detection range and resolution. In the process, however, it was discovered by ITT, that the same performance results could be achieved using a Generation 3 tube, but with a thinner ion barrier film and a auto-gated power supply, without sacrificing reliability and life-span of the intensifier tube. See also: Myth vs. Fact: Generation 4 directly below.
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.
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.
The distance between the user's pupils (eyeball center). The 95th percentile of US military personnel falls within the 55 to 72mm range of IPD.
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.
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.
Collects and intensifies the available light in the visible and near-infrared spectrum. Offers a clear, distinguishable image under low-light conditions.
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.
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. Generation 3 tubes generally have a range of 64 - 72 lp/mm, although line pair measurement does not indicate the generation of the tube. Some Generation 2+ tubes measure 28-38 lp/mm, while a Generation 1+ tube may have measure at 40 lp/mm.
Lumen: Denotes the photons perceptible by the human eye in one second.
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).
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.
The shortest wavelengths of the infrared region, nominally 750 to 2,500 nanometers.
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. While for most latest 3rd generation image intensifiers the photoresponse is in the 1800 µA/lm (2000 µA/lm for the latest Omni VI Pinnacle tubes), the actual number is more like 900 µA/lm.
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 millimeter (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 millimeter (lp/mm).
An adjustable aiming point or pattern (i.e. crosshair) located within an optical weapon sight
This is probably the single most significant factor in determining a system's ability to see when it gets dark. Be aware that SNR can be computed many ways to get desired results. Be sure to find out how SNR was computed. When measured according to U.S. mil specs, the SNR takes into account the photosensitivity, as well as the efficiency of the phosphor screen in reconverting the electron image to visible light and the "noise" contribution of the microchannel plate. Because the, SNR determines an image intensifier's low-light-resolution, the higher the ratio, the clearer will be the signal compared to the background noise, hence, the better the ability to see under increasingly darker conditions.
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
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.
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.
Allows the user to manually adjust the gain control ( basically like a dim control ) in varying light conditions.
A method of boresighting an aiming device to a weapon and adjusting to compensate for projectile characteristics at known distances.