Abstract
In natural science, the light which includes ultraviolet rays, visible rays, and infrared rays, is considered as one of the electromagnetic waves. The reaction of the light is differentiated photochemical reaction and photothermal reaction. In general, photochemical reaction is induced by ultraviolet rays, and photothermal reaction is induced by visible rays or infrared rays. As for the influence to the skin, ultraviolet rays are considered as cytotoxic, which may cause skin cancer and photoaging, so that immoderate exposure to ultraviolet rays should be avoided.On the other hand, the light is applied as the treatment of the skin diseases. The treatments using photochemical reaction such as ultraviolet light therapy and photodynamic therapy depend on the DNA destruction of abnormal cells and induction of apoptosis. As for the treatments using photothermal reaction, various lasers and light sources of visible and infrared wavelength are commonly used. They are utilized for destructing the target tissue, activating the cell by thermal effect and so on. Depending on the each individual wavelength, light absurption depth and the target tissues are different. In the treatment, we should select the most suitable lasers or light sources, therefore understanding of the skin suructure and making the accurate diagnosis are required.
