Research On Methods And Theory For Non-line-of-sight Imaging Based On Laser Range-gated Imaging

In modern warfare, anti-terrorist, social security and disaster relief events, urban streets aresome of the most common scenes. It is difficult to effectively observe the suspicious targets andscenes within the buildings or behind the street corners by direct vision because of obscurationby buildings and streets walls. Non-line-of-sight optical imaging technology is a novelapplication of imaging technology developed in recent years to detect suspicious objects bybypassing or penetrating obstacles. Laser range-gated imaging technology can be utilized torealize non-line-of-sight imaging through the intermediate reflective surfaces with certainreflection properties. Although effective progress has been made in abroad aboutnon-line-of-sight imaging based on laser range gated imaging has been, domestic research inthis field is still in the blank.This paper studied the methods and theory for non-line-of-sight imaging based onlaser range-gated imaging. The novel achievements are as follows:1) To establish appropriate assessment method for non-line-of-sight imaging based onlaser range gated imaging, multi reflection process of illumination laser has been analyzed.Image contrast model of non-line-of-sight imaging based on laser range gated imaging isestablished. The established contrast model is the function about reflective properties ofintermediate reflective surface, parameters of laser range gated imaging system, andimaging distance. Utilizing typical parameter setting of laser range gated imaging system,simulation of image contrast has been carried out. The simulation results indicate thatreflective properties of intermediate reflective surface is one key factor affectingnon-line-of-sight imaging. Although conventional active image can realizenon-line-of-sight imaging with intermediary reflective surface of obvious specularreflection properties, laser range gated imaging technique is indispensable fornon-line-of-sight imaging of most practical intermediary reflective surface. The imagecontrast model of non-line-of-sight imaging based on laser range gated imaging establishedin this paper is of instructive importance for the theoretical research, system design andapplication of laser range gated non-line-of-sight imaging technique. 2) Semi-empirical BRDF (Bidirectional Reflectance Distribution Function)model hasbeen proposed to solve the problem of lack of suitable BRDF model for describing thepractical intermediary reflective surface reflection properties. Hemisphere directionalReflectance data of specified kinds of ceramic tile with visible light illumination has beensampled using two different bidirectional reflectometers. Data analysis indicates that BRDFfor ceramic tile of different manufacturing craft can have a uniform expression, that thecombination of a narrow peak specular reflection, a Lambert-like diffuse reflection, and aforward diffuser scattering. Surface fitting to measured data is used to simplifyHarvey-Shack surface scattering theory, and then the semi-empirical BRDF model forpractical intermediary reflective surface is established. The established semi-empiricalBRDF model is then used to improve the image contrast model of laser range gatednon-line-of-sight imaging. Semi-empirical BRDF data of white ceramic tile was used tosimulate image contrast of laser range gated non-line-of-sight imaging. The simulationresults indicate that, through the entire laser range gated non-line-of-sight imaging, theLambert-like diffuse reflection properties leads to the attenuation of laser energy, and theforward diffuser scattering increases the illuminance of the target on one hand anddecreases the image contrast on the other hand.3) To solve the problem of lacking assessment model for laser range gated non-line-of-sight imaging, the assessment model of minimum resolvable dimension (minimumresolution) based on MRC(Minimum Resolvable Contrast) was proposed, which can providetheoretical basis for system design and performance assessment of laser range gatednon-line-of-sight imaging. The minimum resolvable dimension model was a function aboutdistance among optoelectronic detector, intermediate reflective surface and object, andabout multi propagation of illumination laser. Since the equivalent exposure theory in MRCis lack of experiment support, an equivalent exposure experiment was carried out using aICMOS. The ICMOS was composed of a gated type intensifier, a coupling micro lens, anda CMOS from Basler. Images were obtained under different gating signal frequencies anddifferent gating signal duty ration. Image data analysis showed that output signal of ICMOSis positive related to gating signal duty ration, and that output signal of ICMOS is nothingto do with the gating signal frequencies, that is, the equivalent exposure theory was verified.4) Laser range gated non-line-of-sight imaging experiment system was set upaccording to the possible application situation. Non-line-of-sight images of different targetson different distance were obtained utilizing clean glass, glass with dust, and ceramic tile asintermediate reflective surfaces respectively. Image data analysis indicated that clean glassand glass with dusk can be favorable intermediate reflective surfaces to obtain goodnon-line-of-sight image for a large distance, and that ceramic tile can only be used to obtainnon-line-of-sight image for a short distance since it has strong attenuation of the laserenergy and thus degrades image contrast. The experiment results is consistent with themodels established above.