Successful treatment of unwanted hair is dependent on an understanding of the optical properties of the skin. It is these properties that determine the behavior of light within the hair shaft and bulb, including the relative amount of absorption of incoming photons.
For optimal laser hair removal, one needs to use an optimal set of laser parameters based on anatomic and physical principles. This is determined by a time-temperature combination with the ultimate effect being transfollicular denaturation.
A detailed understanding of laser-tissue interaction emerged in 1983 as the theory of selective photothermolysis was conceived for the laser treatment of pediatric port wine stains (Anderson et al. 1983)
The theory of selective photothermolysis led to the concept of a laser-induced injury confined to microscopic sites of selective light absorption in the skin, such as blood vessels, pigmented cells, and unwanted hair with minimal damage to the adjacent tissues. To achieve this selective effect, lasers would need to fulfill three requirements:
1. They should emit a wavelength that is highly absorbed by the targeted structure.
2. They should produce sufficiently high energies to inflict thermal damage to the target.
3. The time of tissue exposure to the laser should be short enough to limit the damage to the target without heat diffusion to the surrounding tissues. This is known as the thermal relaxation time (TRT).
These concepts revolutionized cutaneous laser treatment and led to the development of successful laser and light-based hair removal devices.
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