Skin Color and Tissue Thickness Effects on Transmittance, Reflectance, and Skin Temperature When Using 635 and 808nm Lasers in Low Intensity Therapeutics

This is a human study that examined the role of skin color and tissue thickness on transmittance, reflectance, and skin heating using red and infrared laser light. Forty volunteers were measured for skin color and skin-fold thickness at a standardized site. Transmittance, reflectance and skin temperature were recorded for various energy doses using 635 nm (36mW) and 808nm (40mW) wavelength laser diodes with irradiances within American National Standards Institute safety guidelines (4.88mm diameter, 0.192 W/cm2 and 4.88mm diameter, 0.214W/cm2, respectively).

Skin color and skin thickness affect transmittance and reflectance of laser light:

• 635nm red light had less penetration than 808nm infrared light regardless of tissue thickness, though transmittance decreased as tissue thickness increased for both wavelengths (with most light losses occurring in the first 4mm of tissue)
• Even though 808 nm had better penetration, this was also affected by melanin (decreased in darker skin)
• There was less reflectance with darker skin with a greater decrease in reflectance for red light than near infrared light
• As energy dosage increased, absorption did not change (prolonged PBMT irradiation does not modify the optical properties of the skin)

The authors concluded that skin color and skin thickness must be accounted for when selecting energy dose to ensure therapeutic effectiveness at the target tissue.