For Asian skin color, the mainstream hair removal laser for hospitals and beauty salons is the Alexandrite: 755 nm and the semiconductor Pulsed diode array: 810 nm. In addition to the need to endure pain, the clinical application is quite satisfactory.
The light of IPL is a band, and the household is generally 475nm+ (meaning 475-1000nm), 530nm+, 550nm+.
The adverse reactions of optical depilation of dark skin are relatively large, and improper operation may cause blisters, pigmentation or scarring. The longer the wavelength, the better the safety, especially for dark skin. Most household color bands have insufficient penetrating power and cannot reach the hairy protrusion or hairball for treatment. The penetrating power of the laser larger than 600nm can reach the hair root, and the hair follicle selectively absorbs well, so that the energy is enough to achieve the hair removal effect.
Energy density or Fluence (J/cm2) is an important indicator of hair removal effectiveness. The higher the energy density, the more effective the hair removal and the longer the effect. However, high energy is more destructive to the hair follicle and will be more painful.
Studies have shown that 5J/cm2 is the minimum energy density required for hair follicles to begin to produce damage, and [the minimum damage to the entire hair follicle of the bulge is 10-15 J/cm2. The insufficient energy density only causes the hair follicle to be slightly damaged, and the hair enters the rest period, which is only a temporary growth delay, but not long-term hair removal. The color energy density of household color light is low, and most of them play a role in delaying hair growth.
Compared with professional hair removal equipment, it produces an energy density of 10-60 J/cm2, but it has a high pain and must be used by professionals. Otherwise, safety cannot be guaranteed. This is why hair removal equipment with high energy density can only be used in professional hospitals. Household instruments have lower energy densities, so keep an eye on whether their effects can permanently destroy hair follicles or just temporary growth delays. Many of the available clinical studies are only returned in a short period of time, such as 1 to 6 months.
There are hair removal instruments with more than 10J energy on the market. Currently, I have research on Smoothskin and Tria. (But the latter is very energetic, up to 22J, so the pain is slightly prominent, and the possible side effects are short-term erythema and edema.)