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The effect of light on skin depends on the wavelength of the light. Light in the UV region (100-400 nm), which is invisible to the human eye, is known to cause deleterious effects such as erythema, hyperpigmentation, and cutaneous carcinoma. Light energy in the visible spectrum (380-700 nm) is mostly innocuous, but it can be absorbed and cause thermal damage when it is delivered to the skin at a high intensity. Light in the near IR region of the spectrum (780-3000 nm), which is also invisible to the human eye, causes skin and retinal defects. In general, the effects of light in the mid-to-far IR region of the spectrum (3-1000 µm) are limited to the superficial layers.

The degree of absorption and its thermal effect on skin vary with the amount and type of chromophores that are present in the recipient. As stated earlier, hemoglobin and melanin are natural endogenous chromophores. An example of an exogenous chromophore is tattoo ink. Different chromophores have different absorption coefficients. The absorption coefficient is a measure of the degree of absorption by the chromophores at a particular wavelength. Because the laser is monochromatic and because it has a very narrow bandwidth, it permits selective targeting of chromophores in the tissue for treatment. This property is one of the underlying principles of selective photothermolysis (SP).

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