| The theory and methods improving the detection performance of low-light-level (L3) imaging system are researched. Analyzing the operation process of the L3 imaging system, those interior and exterior factors are observed that affect the detection process of the system at night. The atmospheric scatter characteristic and its attenuation to the object contrast in long distance are considered principally. A night vision experimentation at heavy fog has been taken. Based on the detection equation of the ideal photoelectric imaging system, a practical detection model of L3 system is deduced in which the imaging theory related to atmospheric scatter is consulted. According to the model, those factors that are propitious to improve the detection performance of the L3 system are analyzed.After the discussion based the atmospheric scatter effect and the detection model, a design principle is put forward in which the Modulation Transference Function (MTF) is preferential for the design of a L3 imaging system. According to the need on "detection" and "recognition" for the night vision application, a measure is brought forward in the design in which the MTF of the system in the lower spatial frequency region should be ensured chiefly. The MTF of the image intensifiers are researched and measured accordingly. Both the optics design for objective lens, eyepiece and relay lens, and the choice of image intensifier, CCD, CRT are done based the MTF first principle.By the theoretic analyzing and test, there important topics are chosen on the extending detection ability of L3 night vision system. The topics consist of the validity of the MTF first design principle, the dynamic characteristic of image tube, the aid-vision technology and its detection ability. There kinds of prototypes are built correspondingly, some direct observable type and television type L3 systems for long distance night observation, a 3rd ICCD system could operated at high speed gated, an active imaging system consist of a semiconductor diode laser used for illuminance and a gated L3 imaging device.The laboratory test and outdoor experimentation are completed. In the long distance observation experimentation, the detection region is 13.7 km to 20.0 km and the recognition region is 8.0 km to 10.0 km to an air object, whose wingspan is 9.3 m, flight speed is about 700 km/h and its flight height is 2 km. In the lab test about the dynamic resolution, the result is 400 TVL at 10-5 lx and 600 TVL at 10-3lx for the 3rd generation ICCD gated imaging system. In the active imaging experimentation, under the illuminance using a laser device whose peak value power is 100 W, wavelength is 850 nm and pulse width is 200 ns, some spatial section images of trees at interval distances 30m are obtained, and the image of a house's window behind a hurst is discovered by the spatial gated image system.The research results are presented. The atmospheric scatter effect to the optical radiant should be considered in the design of the L3 imaging system for far distance observation. The modified L3 imaging system detection model based on the scatter effect is more correct than the ideal photoelectric imaging system detection equation to reflect the relationship between the detection ability and those correlative factors. The MTF first design principle is effective to extend the detection performance of L3 imaging system. The 3rd generation image intensifier has a good dynamic imaging characteristic and is suitable for the high speed gated application. By using the gated L3 device and the laser illuminance device, the active imaging system can detect the hided object piercing some substance, such as a tree, and get the image in a specified spatial section. Active imaging method is effective also for extending the detection ability of L3 imaging system.The farther work in the extension of the detection performance of L3 imaging system is to research the scatter imaging theory and the long distance active imaging system technology.
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