|Night vision goggles|
|Tuesday, 05 December 2006|
The central component of a night vision system is an image intensifier tube which functions to multiply incident light received by the system to produce a display that is sufficiently bright for presentation to a viewer's eyes. Image intensifiers multiply the amount of incident light received by the image intensifier to provide a visible image. These devices typically require some low-level residual light, such as moon or star light, in which to operate. Image intensifiers are constructed for a variety of applications and therefore vary in both shape and size, with proximity focused image intensifiers comprising a particular type of image intensifier having the smallest size and weight of all categories of image intensifiers. The image intensifier is a vacuum tube device similar to a miniature video camera and screen packaged together. Typically, the intensifier includes a photocathode receptor which receives visible light and infrared light energy and converts the visible light and infrared light energy into electrons, a microchannel plate which multiplies the number of electrons emitted by the photocathode (thus serving as an amplifying device), and a green phosphor screen which converts the electrons into a visible image. Most of these night vision imaging systems also include lens assemblies, battery packs and adjustable mountings, and are manufactured as monocular or binocular assemblies depending upon the specific application. There are several types of Image intensifiers currently manufactured, commonly referred to as generation II (GEN 2), generation III (GEN 3) and generation IV (GEN 4) type image intensifier tubes. The primary difference between eneration II (GEN 2), generation III (GEN 3) types of image intensifier tubes is in the type of photocathode employed in each. Image intensifier tubes of the GEN 2 type have a multi-alkali photocathode with a spectral sensitivity in the range of 400-900 nanometers (nm). This spectral range can be extended to the blue or red by modification of the multi-alkali composition and/or thickness. GEN 3 image intensifier tubes have a p-doped gallium arsenide (GaAs) photocathode that has been activated to negative electron affinity by the adsorption of cesium and oxygen on the surface. An extension of the spectral response to the near infrared can be accomplished by alloying indium with gallium arsenide. A generation IV (GEN 4) image intensifier is similar to the GEN 3 except improvements have been made to the microchannel plate. Modern image intensifiers typically use phosphor outputs that present bright green images to the viewer. These images can be emerald green to various shades of lime green in color, depending upon the phosphor that is used. Image intensifiers typically require sophisticated power supplies and circuitry to control the operation of the image intensifier tube and sophisticated optical arrangements that direct the infrared energy into the image intensifier tube and visible fight away from the image intensifier tube. In military applications, the various military personnel are trained in how to use and adjust the night vision devices they are issued.
Night vision goggles are binocular viewers that operate at very low light levels to allow the user to see, move, and work in the dark. Night vision goggles are used to facilitate viewing of nocturnal activities as well as activities that occur in places with minimal light. These night vision goggles are operable at very low light levels and capable of amplifying light as it reflects off objects rendering such objects sufficiently observable in the dark. The vast majority of known night vision goggles have an objective lens focusing mechanism including a manually operable ring rotatable for axially displacing the objective lens or lens assembly linearly in and out to focus on far and near objects. Basically, night vision goggles have a phosphor screen display on which is formed an intensified image of a scene being viewed by the wearer. The intensified image of the scene is created by means of an image intensifier device which drives the phosphor elements of the phosphor screen. The image intensifier generally amplifies infrared radiation from the scene being viewed, thereby dramatically improving the brightness of the scene being viewed by the wearer of the night vision goggles, and thus making possible activities at night time which were impossible or impractical prior to the advent of night vision goggles. The image intensifier tube includes a photocathode which is composed of a photosensitive material. The photosensitive material of the photocathode releases electrons in an amount proportional to the intensity of the incident radiant energy. The image intensifier device also includes a microchannel plate provided with a multiplicity of microchannels through which pass the electrons released by the photosensitive material of the photocathode of the image intensifier tube. The microchannels of the microchannel plate act as an electron gain medium, so that the electrons which pass therethrough stimulate the release of a cascade of additional electrons. The amplified streams of electrons which are produced by the microchannels of the microchannel plate of the image intensifier device form an intensified/amplified electron image of the low-luminance scene being viewed by the wearer of the night vision goggles. Night vision goggles have an adjustment for the focus of the intensified image at the viewing screen. The focus adjustment is made by adjusting the eyepiece lens which allows individuals with different eye focus characteristics to see the intensified image clearly, much like the adjustment for the eye found on microscopes and telescopes. Night vision goggles are generally adapted for amplifying light as it reflects off objects so that objects are observable essentially in the dark. Starlight or light from the moon is sufficient to allow an object to be distinguished from its surroundings.
Night vision goggles are frequently worn attached to the user's head by headgear of some type. Night vision goggles are frequently worn in addition to a helmet or other similar equipment. The night vision goggles are generally mounted on a head gear, facemask, or other similar equipment. Attaching the night vision goggles to a headgear allows the user to operate the goggles essentially hands free. Having both hands free assists the user or wearer to carry on other activities while using the goggles. Military night vision goggles are typically "passive" in the sense that it relies upon infrared radiation naturally present in the environment and/or low levels of light emitted by objects being observed. Infrared passive apparatus is desirable for military use because it gives an indication of the temperature of the object being viewed, warmer objects appearing brighter than colder ones. Night vision goggles display monochrome light with limited resolution and limited contrast, producing a limited depth of field and a narrow field of view. They take white light and input and amplify all colors and produce a monochrome display which is usually green due to the phosphors used. Several approaches to produce a color night vision system have been attempted. One technique uses two synchronized spinning color filter wheels in front and back of the image intensifier tube assembly. The most prevalent type of night vision goggles is binocular-style NVG, which have a pair of parallel optical channels, one for each eye. Night vision devices are widely used in the military to provide soldiers, aviators and sailors with the ability to passively view objects at night or during other low light conditions. Night vision goggles have proven to be essential to pilots, police, and the military, during nighttime maneuvers. Consumer oriented night vision goggles are also developed for use in nighttime marine piloting, nighttime security, surveillance, hunting, fishing, backpacking, navigation, underwater vision, search and rescue and law enforcement.