Study Of Spatial-resolved Diffuse Reflectance Based On P₃ Approximation Theory

The propagation of light in biological tissue has been focused extensively, as thebasis of Laser-life Science;it is one of the issues of tissue optics. Biological tissue is acomplex media, the complexity is described with scattering phase function of lightpropagating in biological tissue. P₃ approximation theory considering phase function,which is the high-order approximation of light transport theory, can describe theradiance in highly absorbing media or close to sources with accuracy superior to thatof diffusion theory. Phase function of biological tissue and its high-order parametershave been researched systemically, and the propagation of light in media close tosources based on P₃ approximation theory have been studied with theoretical,simulated and experimental methods in this dissertation. The main content of thedissertation involves:Firstly, light transport theory, P₃ approximation and diffusion theory are relatedwith the similar relations as the tache in the dissertation, and the whole theoreticalsystem is apprehended expediently. After that, the phase function considered in P₃approximation is discussed systemically, for the first time, the character of phasefunction and its parameters, and the influence of measuring the second-order opticalparameter γ with selecting phase function are studied. The results indicate that theerror of measuring parameterγ is big, if the experimental data of the tissue' phasefunction are fitted with single phase function, however the scattering properties of thetissue is character of combined phase function. On the basis of above analysis, theinfluence of phase function on P₃ approximation spatial-resolved diffuse-reflectanceis studied for the first time, the conclusion is that the influence of phase function ondiffuse-reflectance of ρ <1 mfp 'is more than that of the high-order moments of phasefunction. Moreover, the sensitivities of the parameters of phase function,the reducedscattering coefficient and absorption coefficient on the P₃ approximationspatial-resolved diffuse-reflectance are studied, the influence of these opticalparameters on P₃ approximation spatial-resolved diffuse-reflectance are analyzed anddiscussed accordingly. The optical parameters considered in the radiance distributionwith far from and close to sources are represented clearly, which are valuable fordetecting microstructure information from measuring macroscopical diffuse-reflectance light distribution in corresponding distances.To the question of theoretical researches above, the spatial-resolveddiffuse-reflectance close to sources are studied with simulation and experimentmethods. The influences of absorption and scattering on spatial-resolveddiffuse-reflectance close to sources are studied by measuring reflectance of simulativetissues employing double fibers and "two-point method". The experiments prove thatP3 approximation theory can describe the radiance close to sources. The influence ofHenyey-Greenstein phase function with different anisotropy factor on the radianceclose to sources is studied with Monte Carlo simulation. The simulation results provethat the diffuse reflectance close to sources about 1 transport mean free path areinfluenced from phase function, and it is necessary of P3 approximation consideringhigh-order parameters of phase function.After verifying the theoretical researches of P3 approximation, the application rangeand simplification of P3 approximation are studied and discussed. From the similarrelations of light transport theory, a parameter mfp ''' namely characteristic length isintroduced on the basis of P3 approximation satisfying the third-order similar relations.Characteristic length mfp ''' limits the appliance region of P3 approximation satisfyingr > mfp''' through studying the relations of spatial-resolved diffuse-reflectance closeto sources and mfp '''. After that, the relations among of the reduced albedo a 'charactering the absorption and characteristic lengths mfp 'and mfp ''' are established,the physical meanings of these relations are explained with the limit of a ' indiffusion theory, and a '>0.57 for P3 approximation appliance in highly absorption isconcluded by amplification. The superficial depth (or the litter volume)of tissueinvestigated are discussed by theoretical,experimental and Monte Carlo simulationmethods, and the specific ideas of solving are presented. In the thesis, thesimplification of P3 approximation diffuse reflectance theoretical expression is studiedfor the first time, the function JHP3 comprising( μ a, μ s', γ )based on P3approximation is obtained, and the influence of second-order parameterγ on theinversion of other optical parameters have been studied with JHP3. JHP3 makes upthe deficiency of the diffusion theory not describing the radiance close to sources andhighly absorption, P3 approximation theoretical expression too complex to obtainoptical parameters.