Automatic Brightness Control (ABC) – An electrinc feature that automatically reduces voltages to the microchannel plate (2nd & 2rd Gen) of the intensifier tube 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 constant level.
Black Spots – These are cosmetic blemishes in the image intensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the performance or reliability of a night vision device and some number of varying size are inherent in the manufacturing processes. Spots due to dirt or debris between the lenses should be removed by careful cleaning if the system is designed for interchanging optics.
Bright-Source Protection (BSP) – An electronic function that reduces the voltage to the photocathode (usually 1st Gen) 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.
Distortion – Three types of distortion are most significant to night vision devices: geometric, “S” and sheer.
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 fiber-optic inversion of the image. Geometric distortion is eliminated in image tubes that use a microchannel plate and fiber-optics for image inversion, however, some S-distortion can occur in these tubes.
S-Distortion – results from the twisting operation in manufacturing fiber-optic inserters (fiber optic twister – Russian tubes do not use a fiber 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 fiber-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.
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 centimeter (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.
Eye Relief – The distance your eyes must be from the last element of the eyepiece in order to achieve the optimal image area.
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 or system gain. Tube gain is measured as the light output divided by the light input. This figure is usually seen in values of tens of thousands. If tube gain is pushed too high, the tube will be “noisier” and the signal-to-noise ratio may go down. On the other hand system gain is measured as light output divided by the light input and is what the user actually sees. System gain is usually seen in the thousands. In any night vision device, 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).
Generation 0 – Typically uses an S-1 photocathode with peak response in the blue-green region, electrostatic inversion, and electron acceleration to achieve gain. Consequently, Gen 0 tubes are characterized by the presence of geometric distortion and the need for active infrared illuminator.
Generation I – Typically uses an S-20 photocathode (with photosensitivity of 180-200mA/lm), electrostatic inversion and electron acceleration to achieve gain. Because of higher photosensitivity, Gen I was the first throughly passive image intensifier. Gen I is characterized by geometric distortion, performance at low light levels, and blooming.
Generation II – Usually an S-20 (extended red) photocathode (with photosensitivity of 240+mA/lm) and microchannel plate to achieve gain. Can be found with either electrostatic or fiber-optic inversion. Gen II tubes provide satisfaction, performance at low light levels and exhibit low distortion.
Generation III – Uses gallium-arsenide for th ephotocathode and a microchannel plate for gain. The microchannel plate is also coated with an ion barrier film to increase tube life. Produces more then 800mA/lm in the 450 to 950 nanometer (near-infrared) region of the spectrum. Gen III provides very good to excellent low-light-level performance, long tube life. Recent mil-spec quality tubes have no perceptible distortion.
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 a channel strike the wall and knock off additional electrons which in turn knock off more electrons producing a cascading effect. MCP’s eliminate the distortion characteristic of Gen 0 and Gen I systems. The number of holes in an MCP is a major factor in determining resolution.
Photocathode – The input surface to 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 in a distinguishing characteristic of the different generations of image intensifiers.
Photosensitivity – Also called photocathode sensitivity. The ability of the photocathode material to produce an electrical response when subjected to light waves (photons). Usually measured in miroamps of current per lumen of light. The higher the value the better the ability to produce a visible image under darker conditions.
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 millimiter (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 center 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.
Infra-Red Illuminators – All Starlight scopes need some light to amplify. This means that if you were in complete darkness you could not see. Due to this AMT corporation built in infra-red illuminator (IRI) on all of their scopes. Basically what an IRI does is throw out a beam of infra-red light that is near invisible to the naked eye but your NVD can see it. Allowing you to use your scope even in total darkness. The IRI works like a flashlight and the distance you can see with it will be limited. AMT corporation uses the most powerful eye-safe illuminator on the market. This allows their IRI to extend to 100 yards. However, because of the power at a short distance the IRI may cover only 40-60% of the viewing area.
How Far Can You See
There many different variables that can affect the distance that you can see with a Night Vision Device. First, what are you trying to see? Are you looking for another boat on the water or are you looking for a rabbit in the woods? The larger the object the easier it is to see. Plus, are you trying to see details (what we call recognition range) or are you just trying to see if something is there or maybe youwill just see movement but won’t be able to 100% determine who or what it is. This is called detection range. Second, another variable is lightning conditions. The more ambient light you have (starlight, moonlight, infrared light) the better and further you will be able to see. You can always see further on a night where the moon and stars are out then if it is cloudy and overcast. Typically you can tell the difference between a male and a female or a dog and a deer at about 75 to 100 yards. However, if you were looking across an open field and there was a half moon out you could see a barn or a house 500 yards away. Remember that the purpose of an NVD is to see in the dark not necessarily a long ways like a binocular.
Performance is the most important factor in night vision capabilities. Will a particular device allow you to see objects in complete darkness? Night vision equipment offered by Excalibur will provide high quality images under extremely low light conditions such as cloud-covered starlight. There are several terms used to compare the qualities of one tube to another. These parameters will determine how well a system will perform.
Photocathode Response (photosensitivity) – the ability of the image tube to detect light under very dark conditions and convert that low light level into an image that you can see. The higher the numerical value, the better the ability to produce a visible image under darker conditions.
Luminance Gain – the ability of a tube or a system to amplify the light it detects. The higher number will be the better the luminance gain.
Resolution – the ability of the tube to distinguish between objects. The higher the numerical value, the better the tube will distinguish between objects. An image tube’s resolution is measured in line pairs per millimeter (lp/mm).
Signal-to-Noise Ratio – the computed ratio of measured data from photosensitivity, gain, and resolution. The higher the ratio, the better the ability of the tube to produce a clear image under very dark conditions.
Cosmetic Quality (spots) – During the manufacturing process, microscopic spots may develop in the tube. This is caused by minute defects in the fiber optics or mixture on the photocathode. These cosmetic defects in no way hinder the reliability or function of the system. All tubes have some of these spots to one degree or another. The best tube will have the fewest spots, and therefore the best image quality.