Depth Penetration of Light into Skin as a Function of Wavelength from 200 to 1000 nm

Louise Finlayson,Kenneth Wood,Ewan Eadie,Sally H Ibbotson,Lewis Mcmillan,C Tom A Brown,Isla R M Barnard

doi:10.1111/php.13550

Abstract

An increase in the use of light-based technology and medical devices has created a demand for informative and accessible data showing the depth that light penetrates into skin and how this varies with wavelength. These data would be particularly beneficial in many areas of medical research and would support the use and development of disease-targeted light-based therapies for specific skin diseases, based on increased understanding of wavelength-dependency of cutaneous penetration effects. We have used Monte Carlo radiative transport (MCRT) to simulate light propagation through a multi-layered skin model for the wavelength range of 200-1000 nm. We further adapted the simulation to compare the effect of direct and diffuse light sources, varying incident angles and stratum corneum thickness. The lateral spread of light in skin was also investigated. As anticipated, we found that the penetration depth of light into skin varies with wavelength in accordance with the optical properties of skin. Penetration depth of ultraviolet radiation was also increased when the stratum corneum was thinner. These observations enhance understanding of the wavelength-dependency and characteristics of light penetration of skin, which has potential for clinical impact regarding optimizing light-based diagnostic and therapeutic approaches for skin disease.

Highlights

With the markedly increased incidence of skin cancer [1,2], and the increasing demand for noninvasive, light-based therapies and technologies [3,4,5,6], it is more important than ever to understand the intricate interactions that occur between light and skin
One of the challenges faced by the clinicians who prescribe UV-phototherapies, psoralen-UVA (PUVA) photochemotherapy, photodynamic therapy (PDT) or laser therapy is that of gaining an accurate quantification of the most effective dose
The penetration depth increases with increasing wavelength until water absorption starts to take effect in the infrared (IR) region where it decreases

Summary

Introduction

With the markedly increased incidence of skin cancer [1,2], and the increasing demand for noninvasive, light-based therapies and technologies [3,4,5,6], it is more important than ever to understand the intricate interactions that occur between light and skin. A detailed knowledge of the penetration depth that light can reach within skin is fundamental to this understanding. One of the challenges faced by the clinicians who prescribe UV-phototherapies, psoralen-UVA (PUVA) photochemotherapy, photodynamic therapy (PDT) or laser therapy is that of gaining an accurate quantification of the most effective dose. This challenge stems from the difficulty in gaining a reliable picture of the fluence rate distribution that the phototherapeutic source will produce within an individual’s skin as well as an accurate idea of where the light will be absorbed [7,8,9]. On the other hand, measuring the level of protection skin has against ultraviolet (UV) radiation is vital in determining the risk from devices such as sunbeds and germicidal UV lamps [10,11]

Methods

Results

Discussion

Conclusion