Dynamic Measurements of Laser Light Attenuation by Cryogen Film and Frost Formation

The purpose of this study was to investigate the dynamics of laser light attenuation during cryogen spray cooling (CSC). Two detection schemes were used to approximate collimated and diffuse light transmittance measurements of continuous-wave (λ = 594 nm) and pulsed (λ = 585 nm) laser light during application of short (20-100 ms duration) cryogen spurts on a glass substrate. High-speed video images were also obtained during CSC. Collimated light transmittance varied considerably during CSC. Comparison of collimated and total transmitted light detection indicated that the diffuse component was substantial. Light attenuation occurred despite transparency of the liquid cryogen layer. Light scattering by cryogen results in a diverging laser beam incident on the skin surface. Since specular reflectance at the cryogen-skin interface may differ for diffuse light, further study of light scattering during CSC is warranted. Due to the differences in optical properties of glass and skin, experiments on skin need to be performed to extrapolate our results to the clinical scenario. For dermatologic procedures such as laser port wine stain and vascular lesion removal, hair removal, and nonablative skin rejuvenation, recommended τd are 10-80 ms. This range of τd appears to be appropriate, although more studies are required to arrive at a definite conclusion.