Source of error in calculation of optical diffuse reflectance from turbid media using diffusion theory

Diffusion theory and similarity relations were used to calculate the optical diffuse reflectance of an infinitely narrow laser beam incident upon a semi-infinite turbid medium. The results were analyzed by comparison with the accurate results from Monte Carlo simulations. Because a large number of photon packets were traced, the variance of the results from Monte Carlo simulations was small enough to reveal the detailed defects of the diffusion theory and the similarity relations, which are broadly used in photomedicine. We demonstrated that both diffusion theory and similarity relations provide very accurate results when the photon sources are isotropic and buried more deeply than one transport mean free path in turbid media. We found that the key factor affecting the accuracy of the diffusion theory application was the conversion from the infinitely narrow laser beam to an isotropic point source in turbid media.