For understanding the mechanisms of low-level laser/light therapy (LLLT), accurate knowledge of light interaction with tissue is necessary. We present a three-dimensional, multilayer reduced-variance Monte Carlo simulation tool for studying light penetration and absorption in human skin. Local profiles of light penetration and volumetric absorption were calculated for uniform as well as Gaussian profile beams with different spreads over the spectral range from 1000 to 1900 nm. The results showed that lasers within this wavelength range could be used to effectively and safely deliver energy to specific skin layers as well as achieve large penetration depths for treating deep tissues, without causing skin damage. In addition, by changing the beam profile from uniform to Gaussian, the local volumetric dosage could increase as much as three times for otherwise similar lasers. We expect that this tool along with the results presented will aid researchers in selecting wavelength and laser power in LLLT.
For understanding the mechanisms of low level laser/light therapy (LLLT), accurate knowledge of light interaction with
tissue is necessary. In this paper, we present a three dimensional, multi-layer Monte Carlo simulation tool for studying
light penetration and absorption in human skin. The skin is modeled as a three-layer participating medium, namely
epidermis, dermis, and subcutaneous, where its geometrical and optical properties are obtained from the literature. Both
refraction and reflection are taken into account at the boundaries according to Snell’s law and Fresnel relations. A
forward Monte Carlo method was implemented and validated for accurately simulating light penetration and absorption
in absorbing and anisotropically scattering media. Local profiles of light penetration and volumetric absorption densities
were simulated for uniform as well as Gaussian profile beams with different spreads at 155 mW average power over the
spectral range from 1000 nm to 1900 nm. The results show the effects of beam profiles and wavelength on the local
fluence within each skin layer. Particularly, the results identify different wavelength bands for targeted deposition of
power in different skin layers. Finally, we show that light penetration scales well with the transport optical thickness of
skin. We expect that this tool along with the results presented will aid researchers resolve issues related to dose and
targeted delivery of energy in tissues for LLLT.