Monte Carlo simulation of light transport in tissue: unscattered absorption events

A source of error in the Monte Carlo simulation of the fluence rate in turbid media is the inaccurate recording of unscattered absorption events. The form and magnitude of the error have been studied for Gaussian and uniform beam profiles simulated in cylindrical and Cartesian coordinates. In each case the error decreases as the lateral sampling lattice spacing decreases and is less than 2% of the incident peak irradiance when the beam radius is greater than five lattice spacings. To avoid the error, one may calculate analytically the fluence rate caused by unscattered absorption events.