The problem of pencil optical beam reflectance by semi-infinite biological media is considered using microscopic hybrid
approach to the radiative transfer equation (RTE). This approach is based on the reciprocity of the Green function for the
radiative transfer theory and the iteration procedure of the solution to the integral equation for Green function written in
reciprocal form. As a result the solution to the RTE is represented as a sum of two terms. The first one is diffusely
reflected by biological media radiation with a finite-order scattering. The second term represents a convolution of the
RTE Green function with an effective source function. The microscopic hybrid method enabled improving accuracy of
the standard diffusion approximation in the problem of pencil optical beam reflectance by biological media. The relative
error of the microscopic hybrid method with respect to Monte Carlo simulation is within 6% and included also the media
with peak forwarded phase function of single scattering event. The main advantage of our method is that it does not use
heuristic assumptions the well known in the literature hybrid models are based on. However, it is desirable to use a
simple renormalization of standard diffusion asymptotics for practical applications, for example, to determine a blood
oxygenation. It can significantly reduce processing time of experimental measurements. Such a simple renormalization
of diffusion asymptotics is given in this work. The method lies in the selection of a factor for diffusion asymptotics at
given interval distances from incident point along the surface. It is shown that acceptable accuracy for the measurements
of relative values of reflectance by semi-infinite biological media is provided.
The reflection of light by a semi-infinite biological medium is modeled by using a combination of finite multiplicity scattering and the diffusion approximation employing the Monte Carlo simulation. The solution to the radiative transfer equation (RTE) is represented as a sum of two terms. The first one is backscattered radiation with a scattering order of no greater than <i>N</i>. The second term represents a convolution of the RTE Green function and an effective source function of an order of (<i>N</i> + 1). The first term and the effective source are calculated using the Monte Carlo method, and the RTE Green function is obtained in the diffusion approximation. The solution to the problem of light reflection obtained by using the hybrid approach is compared to the results of the Monte Carlo simulation. The finite scattering order <i>N</i> , which provides a relatively high accuracy of the above hybrid method in the optical study of biological media, is estimated with respect to anisotropy factor and albedo of a single scattering event.