Defining Safe Light Intensity Limits of Near-Infrared Illumination Avoiding Skin Heating in Medical Optical Diagnostic Methods

Abstract

This study examined the thermal response of human skin in vivo to near-infrared illumination (764 nm) delivered either as a free laser beam or through direct contact using a dermal diffusive probe. The aim was to quantify skin heating under increasing illumination intensities and to identify safe exposure thresholds for different skin tones and illumination methods. Research participants with skin tones 1–5 (ranging from light to brown) were included. The results show that skin tone significantly influenced heating under free-beam illumination, whereas temperature increases with dermal probe illumination showed no significant dependence on skin tone. The maximum safe irradiance was determined to be 7.6 mW/mm² for free-beam illumination and 2.3 mW/mm² for dermal probe illumination, corresponding to a maximum skin temperature of 43°C. These limits apply to skin tones 1–5, and further research is required to establish safety thresholds for skin tone 6.