Assessment of the effective attenuation coefficient of scattering media illuminated by a LED array : Effect of the beam size

Abstract

The knowledge of the light fluence rate distribution inside a biological tissue irradiated by a Laser (or LED) is fundamental to achieve medical treatments. In this paper, we present a semi-analytical model based on the 2-D Fourier Transform of the diffusion equation. This method can be applied to any irradiation source (cylindrically symmetric or not) at the surface of the tissue. Two particular beam shapes are studied: planar irradiation and flat beam with finite radius. The total fluence rate along the depth in tissues was computed by adding the collimated and the diffuse components. The analytical solution was also used to study the effect of the beam radius on the light attenuation. Measurements were performed using a tank filled with a liquid-simulating medium (Milk), illuminated with a LED array (660 nm, 100mm×100mm). Several circular diaphragms were used to obtain uniform circular beams with well defined radii. An optical fibre (with an isotropic tip) was used to measure the fluence rate inside the medium. Preliminary experimental results are in agreement with theoretical predictions.