Effect of wavelength and beam width on penetration in light-tissue interaction using computational methods

Caerwyn Ash,Kelvin Donne,Michael Dubec,Tim Bashford

doi:10.1007/s10103-017-2317-4

Abstract

Penetration depth of ultraviolet, visible light and infrared radiation in biological tissue has not previously been adequately measured. Risk assessment of typical intense pulsed light and laser intensities, spectral characteristics and the subsequent chemical, physiological and psychological effects of such outputs on vital organs as consequence of inappropriate output use are examined. This technical note focuses on wavelength, illumination geometry and skin tone and their effect on the energy density (fluence) distribution within tissue. Monte Carlo modelling is one of the most widely used stochastic methods for the modelling of light transport in turbid biological media such as human skin. Using custom Monte Carlo simulation software of a multi-layered skin model, fluence distributions are produced for various non-ionising radiation combinations. Fluence distributions were analysed using Matlab mathematical software. Penetration depth increases with increasing wavelength with a maximum penetration depth of 5378 μm calculated. The calculations show that a 10-mm beam width produces a fluence level at target depths of 1–3 mm equal to 73–88% (depending on depth) of the fluence level at the same depths produced by an infinitely wide beam of equal incident fluence. Meaning little additional penetration is achieved with larger spot sizes. Fluence distribution within tissue and thus the treatment efficacy depends upon the illumination geometry and wavelength. To optimise therapeutic techniques, light-tissue interactions must be thoroughly understood and can be greatly supported by the use of mathematical modelling techniques.

Highlights

Human skin is the interface between man and his environment
It can be suggested that if a maximum penetration depth of 5 mm is achieved using the Intense pulsed light (IPL) with a reduction in the intensity to the extreme of 1% maximum intensity, it is safe to assume that the IPL does not have much of an effect at depth greater than 5 mm and so there would be remote risk of damage to tissues beneath the skin and no damage to organs or a foetus which rests beneath the skin, adipose and muscular tissue
This work addresses the risk assessment of laser and IPL devices that at these wavelengths and radiant exposures, photons are unable to reach vital organs as these results show a maximum penetration depth of 5 mm of 1% incident light

Summary

Introduction

Human skin is the interface between man and his environment. One of the environmental factors it has to deal with is ultraviolet (UV) radiation. Ultraviolet radiation may cause negative effects such as erythema, skin ageing and skin cancer. The upper layer of skin, the epidermis, acts as a natural UV filter and may thicken or develop a stronger pigmentation as an adaption to excess UV exposure. UV radiation induces photo production of previtamin D3 in the skin and provides the major natural source for vitamin D3 for humans. It seems reasonable that skin has not evolved as a mere sun block but as a sun screen

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