Influence of Numerical Aperture on Focal Spot in Turbid Media Microscopy

The transportation and focusing in turbid media by means of Monte Carlo method is presented; by changing the numerical aperture (NA) of lens, the distributions of the intensity of focal spots were obtained. It shows: the NA is larger, the radius of focal spot is larger; all that answers the result of the experiments. Whereas in theory of scalar diffraction: the focal spot is getting smaller when the NA of lens increases. In the process of Monte Carlo simulation, every steps of photons motion are pursued and the scattering and absorption are taken into account; but we have no way to consider the scalar diffraction in it with the limit of the method itself. In fact, the scalar diffraction plays a role in light propagation otherwise it couldn't focus in turbid media. The figure of the NA versus focal spot is plotted using two methods with Monte Carlo simulation and with the scalar diffraction theory. With two methods, we suppose there is a balance position, which we can choose the lens of appropriate NA and get the optimal focal spot.