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Image intensifier system IIS are the imaging systems built using an image intensifier tube consisting of a photocathode, an anode in form of a phosphor screen, and other optional components. The tube intensifies a low-luminance image of the observed objects created on the photocatode into a brighter image created on the anode. In other words, the photocathode due to the radiation impinging onto its surface emits electrons that are focused onto a phosphor screen that emits brighter light. Spectral sensitivity of the IIS depends on design but generally IIS used for surveillance applications are sensitive to radiation from about 0.4 m m to 0.8 m m and sometimes up to 0.9 m m. Typical image intensifier system IIS consists of an optical objective, an image intensifier tube, an ocular, a chassis, and an optional illuminator. The image intensifier tube is a heart of the image intensifier system. Therefore the term "image intensifier" is often used as synonym of the image intensifier system. Next, the image intensifier systems IIS are commonly called night vision devices (NVD) although also other infrared imaging systems like thermal imagers or LLLTV cameras enable imaging at night conditions. The technology of image intensifier tubes has progressed steadily over the years. There are so far five generations of image intensifier tubes: 0, 1, 2, 3 and possibly 4. From design/ application point of view night vision devices can be divided onto four distinct groups:
Practically almost all night vision devices can be classified to one of the mentioned above groups. Over 90% of night vision devices offered on world market can be treated as equivalents of the models listed above and shown in Fig. 1. a)
c)
Fig. 1 Photos of four types of night vision devices (courtesy of ITT Night Vision – www. nightvision.com): a) AN/PVS-7 Night Vision Goggle Cyclop type, b) Monocular night vision devices AN/PVS-14, c) Night Vision Dual-channel Goggle AN/AVS-9, d) Night vision sight F7001.
From the application point night vision devices can be divided into two basic groups:
Resolution is typically defined in military standards as maximal spatial frequency of a standard USAF 1951 target that can be revolved by an observer at a certain illuminance level and the target contrast Spot defects in form of dark spots are the most common type of blemishes present in images generated by I2 tubes, and also in I2 systems. The defects are defined as the opaque or dark spots which exceed contrast of 30 percent of their surrounding area. It is typically required that the dark spots shall not exceed the size and quantities specified in special tables presented in MIL standards. Attention: It is recommended to measure dark spots during testing image intensifier tubes (see ITS-I test station) because then it is possible to carry out objective measurement with higher accuracy. During testing image intensifier systems only subjective evaluation is possible. System gain (luminance gain) is a parameter defined as the ratio of the output screen luminance to the input illuminance at the target plane. This parameter gives information how much brighter is the tubes screen seen by the ocular in comparison to the average illumination at the target plane. Minimum Resolvable Contrast function is a function of a minimum contrast difference between the bars of the standard target and the background required to resolve the image of the bars by an observer versus spatial frequency of the target at different levels of target luminance. The USAF 1951 target is used as the standard target during MRC measurement. In other words MRC is at a resolution measured at different illumination levels for different contrast levels; and the resolution is a point at the MRC characteristic. Because it is much more difficult to measure MRC than the resolution, therefore resolution is more frequently measured than MRC. Minimal resolvable contrast (MRC) function is considered as the most important characteristic of visible imaging systems because having known MRC function it is possible to calculate the ranges of detection, recognition and identification of the targets of interest. Distortion is a parameter that gives information about distortion of image of different geometrical figures. There are many definitions of distortion in literature. In case of distortion of I2 systems we recommend to use definition of so the called gross distortion presented in MIL standards that define parameters of I2 tubes. In this way we can defined distortion as a phenomenon that causes the image of a straight line to curve. Attention: It is recommended to measure dark spots during testing image intensifier tubes (see ITS-I test station) because then it is possible to carry out objective measurement with higher accuracy. Magnification is a parameters defined as a ratio of the angular size of the image of the target seen by the observer using the I2 system (via the ocular) to the angular size of the true target seen using only a naked eye. Field Of View (FOV) is a parameter defined as an angular size of the area of the observed scenery that can be seen using the I2 system. FOV is determined by the useful size of the photocathode of the image intensifier tube and the focal length of the input optics of the I2 system. Collimation is an angle between optical axis of dual channel goggles. When this angle is too high then the observer can have problems to use both eyes for stereoscopic observation.
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