Modeling the local excitation fluence rate and fluorescence emission in absorbing and strongly scattering multilayered media

We present computationally efficient and accurate semiempirical models of light transfer for real-time analysis of multilayer fluorescing media exposed to normally incident excitation light. The model accounts for absorption and strong forward scattering as well as for internal reflection at the interface between the medium and the surrounding air. The absorption and scattering coefficients are assumed to be constant with depth; the fluorophore concentration is considered piecewise constant. The refractive index ranges from 1.0 to 2.0, and the transport single scattering albedo between 0.50 and 0.99. First, simple analytical expressions for local excitation fluence rate within the medium and surface fluorescence intensity emerging from its surface were derived from the two-flux approximation. Then, parameters appearing in the analytical expression previously derived were fitted to match results from more accurate Monte Carlo simulations. A single semiempirical parameter was sufficient to relate the diffuse reflectance of the medium at the excitation wavelength to the local excitation fluence rate within the medium and to the surface fluorescence emission intensity. The model predictions were compared with Monte Carlo simulations for local fluence rate and total surface fluorescence emission from (i) homogeneous semi-infinite fluorescing media, (ii) media with a semi-infinite fluorescing layer beneath a nonfluorescing layer, and (iii) media with a finite fluorescing layer embedded in a nonfluorescing semiinfinite layer. The model predictions of the local excitation fluence rate and of the total surface fluorescence emission fell to within 5% of predictions by Monte Carlo simulations for single scattering albedo greater than 0.90. The current model can be used for a wide range of applications, including noninvasive diagnosis of biological tissue. © 2010 Optical Society of America OCIS codes: 000.4430, 010.5620, 170.6280, 230.4170, 170.6510, 110.7050.