User:Jarcanist/NVT

III. NIGHT VISION TERMINOLOGY A. 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 rap-idly changing from low-light to high-light conditions; the image gets brighter and then, after a momentary delay, suddenly dims to a constant level. B. BLACK SPOTS -  These are cosmetic blemishes in the image in-tensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the per-formance or reliability of a night vision device and some number of varying size are inherent in the manufacturing process. Spots due to dirt or debris between the lenses should be removed by careful cleaning if the system is designed for interchanging optics.

C. BRIGHT-SOURCE PROTECTION (BSP) -  An electronic function that reduces the voltage of the photocathode when the night vi-sion 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 lower-ing resolution when functioning.

D. CYCLES per MILLIRADIAN (cy/mr) - Units used to measure sys-tem resolution. A milliradian is the angle created by 1 yard at a distance of 1,000 yards. This means that a device that can detect two 1/2 yard objects separated by 1/2 yard at 1,000 yards has a resolution of 1.0 cy/mr. Do not confuse cy/mr with line pair per millimeter. For example, a system can have a 3X lens attached and increase the system resolution by a factor of 3, yet the image intensifier's resolution (measured in Lp/mm) has not increased.

E. DIOPTER - The unit of measure used to define eye correction or the refractive power of a lens. Usually adjustments to an optical eyepiece accommodates for differences in individual eye-sight. Many military systems provide a +2 to -6 diopter range.

F. DISTORTION (next page)- Three types of distortion are most significant to night vision devices:  geometric, "S" and sheer. 1. Geometric distortion is inherent in all Gen 0 and 		Gen 1 im-age 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 mi-crochannel plate and fiber 		optics for image inversion; however, some S-distortion can 		occur in these tubes.

2. S-distortion results from the twisting operation in 	manufac-turing fiber-optic inverters. Usually S-distortion 		is very small and is difficult to detect with the unaided 		eye. Gen III image tubes manufactured to U.S. military 		standards since 1988 have nearly no perceptible 				S-distortion.

3. Sheer distortion can occur in any image tube that 		uses fi-ber-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 "sheered."

Figure 3. Distortion

G. EQUIVALENT BACKGROUND ILLUMINATION (EBI) -  This is the light you see in an image tube that is turned on but here is no light at all on the photocathode; it is affected by temperature where the warmer the night vision device, the brighter the back-ground illumination. EBI is measured in Lumens per square cen-timeter (lm/cm2) wherein the lower the value the better. The EBI level determines the lowest light level at which you can de-tect something at. Below this light level, objects will masked by the EBI.

H. 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 prob-lem. 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 light source in the scene being viewed.

I. EYE RELIEF -  The distance your eyes must be from the last element of an eyepiece in order to achieve the optimal image area.

J. 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 struc-ture of the microchannel plate and can be seen in virtually all Gen II and Gen III systems if the light level is high enough.

K. FOOTCANDLE -  A unit of illuminance equal to one lumen per square foot.

L. FOOTLAMBERT (fl) - A unit of brightness equal to one foot-candle at a distance of one foot.

M. GAIN -  Also called brightness gain or luminance gain. This is the number of times a night vision device amplifies light in-put. 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 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. U.S. military Gen II and Gen III image tubes operate at gains of between 20,000 and 37,000. 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 im-portant measurement to the user.

N. GALLIUM ARSENIDE (GaAs) - The semiconductor material used in manufacturing the Gen III photocathode. GaAs photocathodes have a very high sensitivity in the spectral region of about 450 to 950 nanometers (visible and near-infrared region).

O. LINE PAIRS per MILLIMETER (lp/mm) - Units used to measure image intensifier resolution. Usually determined from a 1951 Air Force Resolving Power Test Target. The target is a series of different sized patterns composed of three horizontal and three vertical lines. You must distinguish all the horizontal and vertical lines and the spaces between them to qualify for that pattern.

Figure 4. Line Pairs per Millimeter

P. LUMEN - The unit denoting the photons perceivable by the hu-man eye in one second.

Q. MICROAMPS per LUMEN - The measure of electrical current pro-duced by a photocathode when it is exposed to a measured amount of light (lumens).

R. 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 nor-mally 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 cascade effect. MCP's eliminate the distortion characteristic of Gen O and Gen I systems. The number of holes in an MCP is a major factor in determining resolution. Newer MCP's have 6.34 million holes or channels compared to the previ-ous standard of 3.14 million.

S. MILLIAMPS per WATT (Ma/W) - The measure of electrical cur-rent (MA) produced by a photocathode when exposed to a specified wavelength of light at a given radiant power (watt). As with microamps per lumen, usually, the higher the value, the better the performance; however, it is important to know where in the spectrum the wavelength falls. Because more night light is pre-sent in the near-infrared region than in the visible or ultra-violet regions, a high value in blue regions would not be as good as a moderate value in the near-infrared region.

T. NEAR-INFRARED - The shortest wavelengths of the infrared re-gion, nominally 750 to 2,500 nanometers. Gen III operates from around 450 to 950 nanometers.

U. PHOTOCATHODE - The input surface of 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 is a distinguishing characteristic of the different generations of image intensifiers.

V. PHOTORESPONSE - see photosensitivity.

W. PHOTOSENSITIVITY - Also called photocathode sensitivity. The ability of the photocathode material to produce an electri-cal response when subjected to light waves (photons). Usually measured in microamps of current per lumen of light. The higher the value, the better the ability to produce a visible image un-der darker conditions.

X. 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 mil-limeter (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 sys-tem resolution can be affected by altering the objective or eye-piece 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 photography or video where the entire image resolution is important.

Y. 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 abil-ity of the tube to resolve objects with good contrast under low-light conditions. Because SNR is directly related to the pho-tocathode's sensitivity and also accounts for phosphor effi-ciency and MCP operating voltage, it is the best single indica-tor of an image intensifier's performance.

Z. SCINTILLATION - A faint, random, sparkling effect throughout the image area. Scintillation is a normal characteristic of mi-crochannel plate image intensifier's and is more pronounced un-der low-light-level conditions. Sometimes called "video noise." Do not confuse scintillation with emission points.

AA. SPECTRUM - The range of electromagnetic energy from cosmic rays to extra-low frequency used in submersed submarine communi-cation. Electromagnetic frequency is measured in cycle per sec-ond and wavelength in microns or nanometers. The ultraviolet region extends from 100 to 400 nm with the near-ultraviolet nominally 300 to 400 nm. The visible portion of the spectrum extends from 400 to 750 nm. The infrared region extends from 750 to 2 X 10 (to the 5th power) nm with the near-infrared nomi-nally 750 to 2,500nm.