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.
In this paper we present preliminary results obtained from fluorescence lifetime measurements on human skin using time-correlated single photon counting (TCSPC) techniques. Human skin was exposed to light from a pulsed LED of 700 ps pulse width at a wavelength of 375 nm and fluorescence decays were recorded at four
different emission wavelengths (442, 460, 478 and 496 nm) using a photomultiplier tube (PMT) coupled to a monochromator. Measurements were carried out on the left and right palms of subjects recruited for the study after obtaining consent using a UCLA IRB approved consent form. The subjects recruited consisted of 18 males and 17 females with different skin complexions and ages ranging from 10 to 70 years. In addition, a set of experiments were also performed on various locations including the palm, the arm and the cheek of a Caucasian subject. The fluorescence decays thus obtained were fit to a three-exponential decay model in all cases and were approximately 0.4, 2.7 and 9.4 ns, respectively. The variations in these lifetimes with location, gender, skin complexion and age are studied. It is speculated that the shorter lifetimes correspond to free and bound NADH while the longer lifetime is due to AGE crosslinks.
We discuss laser fabrication of microstructures in photoetchable glass ceramics called Foturan (Schott Company, Elmsford, NY). A KrF excimer laser (= 248 nm, = 25 ns) is used for surface micromachining, and a femtosecond laser (= 800 nm, = 80 fs) is used for fabricating 3-D structures. Important aspects of the machining, such as depth of machining resulting from different laser processing parameters and threshold laser fluences, are presented. A detailed analysis of the absorption process of both lasers in photoetchable glass ceramics is provided.
We discuss laser fabrication of microstructures in photoetchable glass-ceramics, FOTURAN. A KrF excimer laser (λ = 248 nm, τ = 25 ns) is used for surface micromachining, and a femtosecond laser (λ = 800 nm, τ = 80 fs) is used for fabricating three-dimensional structures. Other aspects of the machining, such as the fluence and crystallization depth resulting from these two methods are presented. A detailed analysis of the absorption process of both lasers in FOTURAN is discussed.