Study On The Technologies For Detecting Small Dark Target With Laser Active Imaging

The imaging systems are divided into active imaging systems and passive imaging systems according illumination sources and no illumination sources. The main feature of passive imaging systems is that they haven't illumination sources and depend on lights from targets or surrounding conditions. However, the passive imaging systems are easily affected by environment sources.The active imaging systems adopt an artificial optical radiation source, generally laser, and a receiver. The receiver is used to collect and detect the part of direct or reflective radiation from target scene. The active imaging systems have the advantage of high definition and contrast and are immune to environment sources.The photoelectric imaging detection systems, such as surveillance, search, capture and tracking, feature the passive imaging detection. They have good covert ability and aren't easily found or recognized because they work in passive mode. However, passive imaging systems are very difficult to detect the small dark target even adopting the most sensitive receiver in very low light level conditions such as overcast nights or in degraded visibility conditions such as aerosols.The infrared imaging systems don't work properly in scenes where there is little or no thermal contrast. The printed numbers or other registration signs such as license plates are good examples of objects that have reached thermal equilibrium with their background. The infrared imaging systems can't identify printed numbers from backgrounds.The main reason of the passive imaging systems don't work well is that the reflected signal energy is very weak. The addition of the assistant illumination is the effective method to overcome this kind of shortcomings. Using an artificial optical radiation source, generally laser, to illuminate the small dark target can increase the energy of reflected signal. However, laser propagation in the atmosphere will be absorbed, scattered, backscattered by gas molecule and aerosols such as rain, fog, mist and dust and be affected by background radiation.Laser beam is propagated in the long range and diffused by the target in the solid angle n. So the real reflected optical energy is very weak. In order to obtain high resolution image of the small dark target in the poor visibility conditions needto increase the laser power. The backscattering lights will increase if the laser power increase. The useful signal will be merged in the powerful backscattering lights so that the targer can't be detected and recognized. So how to overcome the backscattering lights is the key problem that must be solved for the laser active illumination imaging systems.Some of the shortcomings of passive imaging systems can be overcomed by using a pulsed laser to illuminate the target in the long range and adopting the range gating to overcome the backscattering lights. The active, range gated imaging system can detect and recognize the target in all weather conditions and has the advantage of high resolution, adequate contrast and without the affection of evironment sources.The laser attenuation character in the atmosphere is analyzed. The affection of the backscattering lights to the laser active illlumination imaging systems is analyzed. The range gating is the effective method to overcome the backscattering lights. The principle of the laser range gated (LRG) imaging and the component of the LRG imaging system are illustrated in detail. The key techniques of the LRG imaging system are analyzed. The synchronization control technique for the range gating is presented. The advantage and disadvantage of two kinds of synchronization modes between the pulsed laser and the gated receiver, which is the inner trigger mode and the external trigger mode, is analyzed. The trigger signal generating circuit for the LRG imaging system is designed according to the experiment demands. On the basis of the monostabtle trigger based on the complex programmable logic device (CPLD), the synchronization control circuit for the LRG imaging system is designed. The two models for the LRG imaging system which are a simple illumination model and the pixel related laser radar model are presented. The relation between the range and the laser power is simulated adopting the simple illumination model. The relation between the maximum range and the laser pulse energy for the LRG imaging system are simulated adopting the pixel related laser radar model. The simulated relations show the potential of the LRG imaging system and the ability of detecting the the small dark target in poor visibility conditions and lay the foundation for designing the LRG imaging system. The gating function is realized by using the synchronization control circuit based on CPLD to control the homemade WGS46-2 super generation II gated low light observer.