Laser-Tissue Interaction: Why Lasers Work on Skin
What Is Laser-Tissue Interaction?
Laser stands for Light Amplification by Stimulated Emission of Radiation. When laser light enters the skin, photons are absorbed by target chromophores and converted into other forms of energy. This process, called laser-tissue interaction, is the foundation of every aesthetic laser treatment.
The Four Fundamental Interactions
- Photothermal: absorbed light converts to heat, raising target temperature to coagulate or vaporize tissue. This drives hair removal, vascular and skin-tightening treatments.
- Photomechanical: nanosecond or picosecond pulses create transient pressure waves that shatter melanin and tattoo particles — the basis of tattoo removal and picosecond devices.
- Photochemical: light triggers chemical reactions, such as singlet-oxygen production in photodynamic therapy.
- Photobiomodulation: low-level red and near-infrared light influences cell metabolism via pathways such as cytochrome c oxidase, used in LED therapy.
Selective Photothermolysis
The theory proposed by Anderson and Parrish in 1983 underpins modern aesthetic lasers. Three conditions must be met: the wavelength must be preferentially absorbed by the target chromophore, the fluence must be sufficient to damage the target, and the pulse duration must be shorter than or equal to the target’s thermal relaxation time. Melanin in hair follicles, hemoglobin absorption peaks near 532/577/1064 nm, and water’s strong absorption at 2940 nm and 10600 nm are the classic examples.
Three Key Parameters
- Wavelength determines chromophore selectivity and penetration depth; longer wavelengths penetrate deeper.
- Pulse duration determines whether the target is heated (long pulse) or fractured (short pulse).
- Fluence (J/cm²) determines both efficacy and side-effect risk.
FAQ
Why do laser treatments require multiple sessions?
Hair follicles, pigment particles and vessels are only partially destroyed per pass, and skin needs recovery time between sessions. Most indications therefore require a course of 3-6 treatments.
How does skin tone affect parameter selection?
Darker skin contains more epidermal melanin, which competes with the target for absorption and raises the risk of burns and post-inflammatory pigmentation. Longer wavelengths, longer pulse durations, lower fluence and epidermal cooling are typically preferred.