terminology
Auto-Gated Power SupplyWhen the power supply is “auto-gated,” it means the system is turning itself on and off at a very rapid rate. This, combined with a thin film attached to the microchannel plate (an ion barrier) reduces blooming. While “blooming” can be noticeably less on systems with a thin film layer, systems with thicker film layers can be perfectly acceptable depending on the end user’s application. Deciding which night vision goggle is better should not be based solely on blooming. 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 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. Black SpotsThese 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. BloomingLoss of the entire night vision image, parts of it, or small parts of it, due to intensifer 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 SpotsThese 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-Source 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. Boresighting The alignment of a weapon aiming device to the bore of the weapon. Chicken WireAn 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. 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 teh 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 distortionS-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”. Emission PointA 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. The lower the EBI number the better. 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 2 and Gen 3 systems if the light level is high enough. Footlambert(fL) A unit of brightness equal to one footcandle at a distance of one foot. GainAlso 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” 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 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 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. Lp/mm (Line Pairs per Millimeter)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. MCP (Microchannel Plate)A metal-coated glass disk that mulitplies 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. ITT Industries’ MCPs have 10.6 million holes or channels compared to the previous standard of 3.14 million. Photocathode SensitivityPhotocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The measureing 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. ResolutionThe 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. Objective LensThe objective lens is the lens on the front of the night vision system. This lens is used to adjust distance focus for the system. Distance focus will range from close focus to infinity. Ocular LensThe ocular lens, also known as the eyepiece, is the rear lens closest to the users eye. This lens is used to adjust the focus to the correct diopter setting for the users eye. Since this focus is adjusted to the prescription of the users eye, once focused it does not have to be refocused unless moved or the users eye sight changes. Figure of Merit (FOM)Figure of Merit, also known as FOM, is the result of multiplying a tubes center resolution and signal to noise ratio values together. FOM is used as a primary benchmark for tube performance and export control. It is important to note that there are several performance requirements other than FOM to be considered when choosing a tube, however, FOM is a great starting point for gauging tube performance. HaloHalo is rings of light that form around concentrated bright light sources when looking through night vision. Halo is typically prevalent with street lights and other similar concentrated light sources. SHADINGThe viewed image should be a full circle. If shading is present, you will not see a fully circular image. Shading is indicative of a dying photocathode and is caused by a defective vacuum seal of the image intensifier. Shading is very dark and you cannot see an image through it. INFINITY FOCUSAdjustment of the night vision device’s objective lens so that a distant object, such as a star or the point light on a distant tower, forms the sharpest image. SCINTILLATIONA faint, random, sparkling effect throughout the night vision device’s image area. Scintillation is a normal characteristic of the night vision image intensifier and should not be confused with emission points. Scintillation is more pronounced under low light conditions.