Multiple-pulse laser damage thresholds in porcine skin at 1645 and 2090 nm

Infrared lasers with high repetition rates and pulse energies represent an emerging concern for laser safety given limited available exposure data. This study explored porcine skin response to 1645 and 2090 nm nanosecond-scale pulsed laser exposures at pulse repetition frequencies of 100, 300, and 1000 Hz. Numbers of pulses ranged from 60 to 1000, resulting in exposure durations between 0.06 and 10 s. The laser beam featured an average diameter of 0.45 and 0.77 cm (1/e2) at 1645 and 2090 nm, respectively. Skin minimum visible lesion (MVL) thresholds are reported 24 h postexposure and compared to the maximum permissible exposure (MPE) defined by the ANSI Z136.1-2022 standard. Safety factors range from 7.2 to 25.7 for 1645 nm and from 10.8 to 16.1 for 2090 nm, demonstrating adequacy of the MPE for these wavelengths. Data analysis at 1645 nm implies an intersection of the MVL threshold trend and the MPE at a total exposure duration of 300 μs, as the definition of the MPE between 1500 and 1800 nm is a static 1 J/cm2 with no dependence on exposure duration, unlike the 1800–2600 nm band. These results support consideration of a time-dependent MPE in the 1500–1800 nm range. Additionally, skin thermal response was lower at threshold levels compared to continuous-wave infrared laser skin studies and established critical temperature levels from Arrhenius assessments of skin damage. This could indicate induced thermoelastic stress lowering the temperature required for protein denaturation, implying the need for a more comprehensive rate process model for multiple-pulse laser skin damage.