![]() The methods based on the diffusion approximation limit the size of the ratio of the absorption coefficient μ a to the reduced scattering coefficient μ s′. ![]() However, in each of these methods, the difference from the MCS depends on the type of approximation, and often the applicable range of parameters is limited. For example, various studies have proposed diffusion approximation, hybrid diffusion and two-flux approximation, the path-integral method, and an empirical method aided by MCS. Therefore, to shorten the calculation time, several methods have been considered for approximating the results of an MCS. A sufficiently short calculation time is crucial for those applications, since it is necessary to calculate the reflectance many times. Increasing the number of photons increases the precision of the MCS, but it also increases the calculation time, which may become too long to be used for the inverse problem, imaging, or interactive tools. In a MCS, a large number of photons are generated, and each photon numerically propagates through the skin model following designated probabilities of scattering and absorption from a statistical analysis of these results, the reflectance and other physical quantities are then calculated. Because of its high reliability, a MCS is often used to evaluate approximation methods, although it has a high computation time. ![]() A multilayered structure is unavoidable when evaluating the reflectance of skin, which consists of an epidermis that contains melanin and an underlying dermis that contains hemoglobin. In addition, a MCS can be applied easily to arbitrary multilayered structures. When deriving this information, a Monte Carlo simulation (MCS) has often been used as the standard, since a MCS precisely follows the behavior of each photon that is scattered from or absorbed in a medium, and the range of scattering and absorption coefficients to which it can be applied is not limited. The reflectance of skin noninvasively provides information about the inner conditions, such as the scattering coefficients, absorption coefficients, and chromophore concentrations.
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