Study Of Free-space Photon Propagation In Non-contact Optical Imaging

Optical imaging has attracted much attention in recent years due to its reasonable spatial and temporal resolution and affordable cost, In particular, its non-contact measurement has become a valuable tool for the noninvasive detection because of its significant advantages in detection sensitivity, image quality and system simplicity. However, there are still some challenging problems to be solved. Theoretically, photon transport simulation in free-space is an awkward problem, which has been less researched and is different from the photon transport simulation in tissues. In free-space, the complexity of the optical imaging system induces difficulties to photon transport simulation. Specially, the presence of camera lens further complicates the photon transport process in free-space. Therefore, to develop an accurate and effective free-space photon transport model is urgently needed.For this purpose, this paper concentrates on the problem of photon transport in free-space and proposes two free-space photon transport models, that is, Monte Carlo method based free-space photon transport model and hybrid radiosity-radiance theorem based free-space photon transport model. As a gold standard algorithm for simulating photon transport process in tissues, Monte Carlo method is widely used to verify other numerical or analytical algorithms. However, it has not been extended to the simulation of photon transport process in free-space effectively, In this paper, a uniform framework for non-contact optical imaging is proposed based on Monte Carlo method, which consists of the simulation of photon transport both in tissues and in free-space. Specifically, the simplification theory of the optical imaging system is utilized and Monte Carlo method is adopted to describe the energy transformation in free-space. However, large numbers of photons should be simulated to obtain an accurate result because of the statistical characteristic of Monte Carlo method. Furthermore, the detection result of Monte Carlo simulation is so sparse that it is hard to meet the requirement of practical applications. To handle this problem, an analytical model is also developed in this paper based on hybrid radiosity-radiance theorem. It combines the radiosity theorem based Lambertian source theory with the thin-lens model based radiance theorem to describe energy transformation in free-space. In addition, the simplification model of the complicated optical imaging system is also employed in this model. This model can obtain a smooth simulation result because of the Lambertian source characteristic of the outgoing photons.Finally, the two models proposed in this paper are both validated with numerical simulations and physical phantom experiments. The verification results show the accuracy and effectiveness of the proposed models. In particular, quantitative error analysis demonstrates the feasibility and potential of the proposed models for simulating photon transport process in free-space.