Attempts to model the thermal response of biological tissue to laser irradiation require an accurate description of the distribution of the light inside the tissue. This is difficult to describe theoretically because multiple scattering causes beam broadening. The Kubelka-Munk model is based on a model of two diffuse fluxes traveling in the forward and backward directions. This is unlike the directional laser source to be modeled. Multiple scattering theories are complex and add little intuitive insight to the light's behavior in the tissue. Our approach is to measure the growth of the beam as it is absorbed and scattered through our principal target, atheromatous plaque. We will describe an optical system that allows quantitative measurement of the 1/e2 spot size as laser light is scattered in arterial plaque samples. The input beams used were: a) collimated lmm direct output from a laser and b) the diverging output of an optical fiber. The measurement is made by imaging the exit face of the tissue onto a scanning slit detector with unit magnification. By compressing the sample to calibrated thicknesses, readjusting the imaging to retain unit magnification, we obtain plots of exit spot size versus tissue thickness. The data indicate that the spot expands linearly as the sample's thickness is increased. However, the rate of beam expansion varied with different tissue samples. The linear beam expansion seen is in disagreement with the exponential growth previously assumed.
""Measurement Of Laser Beam Spreading In Biological Tissue Scattering"", Proc. SPIE 0712, Lasers in Medicine, (10 March 1987); doi: 10.1117/12.937318; https://doi.org/10.1117/12.937318