Journal of Biomedical Optics • Open AccessRelationships of skin depths and temperatures when varying pulse repetition frequencies from 2.0-µm laser light incident on pig skin
Human perception of 2.0-µm infrared laser irradiation has become significant in such disparate fields as law enforcement, neuroscience, and pain research. Several recent studies have found damage thresholds for single-pulse and continuous wave irradiations at this wavelength. However, the only publication using multiple-pulse irradiations was investigating the cornea rather than skin. Literature has claimed that the 2.0-µm light characteristic thermal diffusion time was as long as 300-ms. Irradiating the skin with 2.0-µm lasers to produce sensation should follow published recommendations to use pulses on the order of 10 to 100 ms, which approach the theoretical thermal diffusion time. Therefore, investigation of the heating of skin for a variety of laser pulse combinations was undertaken. Temperatures of ex vivo pig skin were measured at the surface and at three depths from pulse sequences of six different duty factors. Differences were found in temperature rise per unit exposure that did not follow a linear relation to duty factor. The differences can be explained by significant heat conduction during the pulses. Therefore, the common heat modeling assumption of thermal confinement during a pulse may need to be experimentally verified if the pulse approaches the theoretical thermal confinement time.