Reflective tomography using a TCSPC system - a study of current limitations and possible improvements

Abstract: Time-correlated single photon counting (TCSPC) systems are used for range profiling. The systems offer cm precision at kilometer ranges. This opens up for long range imaging with high resolution, for example by reflective tomography. With range profiles from various aspect angles around a target reflective tomography can be used to create an image. The tomographic image is a reconstruction of the boundary of the cross-section of the target. Images can be used for various purposes, e.g. identification of satellites. The quality of the tomographic reconstruction depends on the accuracy of the TCSPC system. Range profiles with a cm precision allows studies and reconstruction of complex objects. With this work we investigated the current limitations when reconstructing complex targets with reflective tomography and present possible solutions to existing problems. The limitations were investigated by studying parameters such as the intensity of the laser beam, SNR, center of rotation, angular resolution, and the angular sector. We also present methods that can improve the tomographic image. A new pre-processing method that adjusts range profiles after estimating responses with RJMCMC was introduced. We also studied different types of filters in the reconstruction process. Lastly we introduced two new post-processing methods. One that removes artifacts by considering the convex hull and one that sharpens edges in the tomographic image. The performance study showed that reflective tomography using a TCSPC system is robust in a controlled environment. Details in the low cm-range of an object can be reconstructed with high precision. However, for some target types issues appear. Of the tested performance parameters a high angle resolution was deemed to be the most important. When considering moving targets the importance of the center of rotation and integration time will also increase. The study of improvement methods showed that choosing the generalized ramp filter in the FBP more then doubled the SNR. Adjusting the range profiles, considering the convex hull, and sharpening edges are methods that work well for specific signal types. We showed that many issues that arise when measuring on complex objects can be solved with signal processing. Therefore we believe that reflective tomography can be used in various applications in the future.