It is important to understand the principle first in order to choose rationally what laser you should do, instead of unknowingly going for a laser that may not be suitable for you and hurting your skin. The instrument itself consists of an energy source, a laser medium and an optical resonance cavity. The medium can be gas, liquid or solid, and the laser will be named by the category of the medium (e.g. ruby laser and CO2 laser). Lasers have three properties: monochromaticity, coherence, and parallelism. Monochromaticity: The characteristic wavelength of a laser is surrounded by a narrow band of normal distribution, which is determined by the atoms or molecules being excited. The wavelength of the laser determines the interaction between the laser beam and the skin or other tissues. Coherence: Laser light can be considered as a sinusoidal wave with temporal coherence and spatial coherence. It is in-phase, simultaneous and in-space. Parallelism: The laser is transmitted over long distances with very little dispersion, which facilitates focusing. The laser can be reflected, scattered or transmitted by tissues for the purpose of laser absorption in specific areas (target chromophores). The mechanism by which the laser interacts with a specific region is known as the principle of selective photothermal action. Selective photothermal actionPhotothermal action: This refers to the fact that the temperature (heat) (generated by the laser) has a series of effects on the tissues, above a certain temperature the skin will be damaged causing cell death. Simply put, the absorption of laser light in the target area is greater than the surrounding tissue, the greater the contrast, in dermatology treatment, the target chromophores in the skin are mainly melanin, hemoglobin and water (why is there transparent water, because above a certain wavelength water will vaporize), they have different absorption capacity of laser light at different wavelengths, the higher the absorption peak, because of these different absorption peaks, to facilitate doctors to have The laser is a good choice for the doctor to use different wavelengths, not only to fight the enemy, but also not to hurt the masses. Laser acne treatment: The broad spectrum of intense pulsed light has a very significant effect on eliminating inflammation, so some inflammatory acne that is progressing can consider trying this. Those acne marks left behind after the acne subsides, or what we call pigmentation, can also be solved with intense pulsed light. 4-5 times a session with an interval of 3 weeks – 1 month. As for acne pits, acne scars and other depressed or hyperplastic scars, fractional laser can be considered. Laser hair removal: the current more mainstream laser hair removal, this technology is more mature, as long as there is no intolerance to late temporary redness, rest assured to do on the line, generally also 4-5 times, can reach the extinction of hair (treatment endpoint: Cui hair). After that, there will be occasional stubborn individual hairs that won’t be reappearing, and then 1-2 times in the hair growth position will be almost enough. Laser mole removal: This may not have much to do with selective photothermal action, but mainly uses the collimation ability and instantaneous energy generated by the laser to peel and vaporize the pigmented mole on the skin surface, after which it will slowly heal. In addition to these more everyday applications, there is also the 308nm excimer laser that can treat vitiligo, the He-Ne laser that is anti-inflammatory and promotes healing, and the UVA, UVB, PUVA and other lasers that use ultraviolet light to treat psoriasis and so on. In fact, laser has been widely used in clinical treatment, in addition to skin and ophthalmology, there are also very mature applications in gynecology, urology, oral, swelling outside.