Generally speaking, there are four possible interactions between light and skin:

•Reflection
•Absorbtion
•Scattering
•Transmition

Usually around 4% to 7 % of light is reflected by the ski, and the penetration depth of the light is inversely related to the wavelength of light. Principal skin chromophores inlucde hemoglobin ,melanin ,intracellular water.

The tone of human skin can vary, Von Luschan's chromatic scale is often used to classify skin color for the purpose of assessing sun tanning risk.  6 skin types are distingguished as follows:

    Type I: von Luschan 1-5 (very light).
    Type II: von Luschan 6-10 (light).
    Type III: von Luschan 11-15 (intermediate).
    Type IV: von Luschan 16-20 ("Mediterranean").
    Type V: von Luschan 21-28 (dark or "brown").
    Type VI: von Luschan 29-36 (very dark or "black").

Laser stands for Light Amplification by Stimulated Emission of Radiation. Currently Laser has been widely used for different medical applicaitons including dental surgery, dermatology, oral surgery, ear surgery, nose surgery, head and neck surgery, ophthalmology, urology, neurosurgery, podiatry, and certain cancers treatment. It works by generating a highly focused and powerful thin beam that can selectively interact with tissue, and vaporize, coagulate, cut, or remove tissue. Different type of laser can generate different light with different wavelength, pulse duration and spot size, thus causing different skin interaction results for different purpose.