Pigmentation-dependent skin injury thresholds with nanosecond single-pulse laser exposure.

Abstract

Significance: Maximum permissible exposure limits defined in ANSI Z136.1 are broadly protective, but they do not explicitly account for pigmentation differences in skin. As a result, the lack of experimental data quantifying the effect of pigmentation on injury thresholds constrains accurate interpretation when melanin is a primary optical absorber, which has direct implications for the performance of optics-based imaging methods.

Aim: We experimentally determined the pigmentation-dependent laser exposure dose that produces a visible injury in 50% of exposed sites (i.e., ${\mathrm{ED}}_{50}$ injury thresholds) with nanosecond, 750-nm-wavelength pulses, and we provide the first known quantification of relationships between melanin index (MI) and individual typology angle (ITA) or erythema index (EI) through colorimetric measurements.

Approach: Fifty total abdominal skin sites per swine were identified on three swine with different pigmentation phenotypes (median ITA values per swine ranged $-18°$ to 87°). Single-laser pulses with 5-ns pulse duration, 750-nm-wavelength, and energies of 35, 40, 50, 60, and 65 mJ per pulse were delivered to 10 sites per energy level per swine. Laser-exposed sites were visually assessed at early post-exposure intervals, followed by colorimeter measurements of MI, ITA, and EI. Probit regression analysis was used to compute the ${\mathrm{ED}}_{50}$ thresholds, followed by a polynomial regression analysis conducted to relate ITA to MI and EI to MI.

Results: Dark-skinned swine ( $\text{median}\mathrm{ITA}=-18°$ ) exhibited a 27.7% lower ${\mathrm{ED}}_{50}$ ( $323.94\mathrm{mJ}/{\mathrm{cm}}^2$ ) than light-skinned swine ( $\text{median}\mathrm{ITA}=69.5°$ , ${\mathrm{ED}}_{50}=448.36\mathrm{mJ}/{\mathrm{cm}}^2$ ), which is consistent with increased melanin-mediated absorption in the darker-skinned swine. The lightest-skin swine ( $\text{median}\mathrm{ITA}=87°$ ) did not develop visible erythema within the tested exposure range, preventing ${\mathrm{ED}}_{50}$ estimation. Mathematical MI-ITA and MI-EI relationships revealed pigmentation-dependent optical and inflammatory responses across the three swine.

Conclusions: The measured differences in ${\mathrm{ED}}_{50}$ values quantify the impact of skin pigmentation on laser-induced injury risk and demonstrate that darker skin has greater susceptibility to damage with the investigated laser parameters, relative to lighter skin. This foundational work addresses a previously uncharacterized source of biological variability that is relevant for the interpretation of laser safety margins and the design and evaluation of optics-based systems.