Lasers are optical devices which produce intense coherent, collimated and mono-chromatic beams of light. A laser consists of an active medium such as a crystal, gas or liquid that amplifies light when excited by an external energy source (a flash amp or electric discharge, for example). When the appropriate medium is employed, the laser can be fine-tuned to generate a very narrow band of light wavelengths (such as the individual colors of the visible spectrum).
Lasers designed for permanent hair reduction emit wavelengths of light designed to be absorbed by the pigment in the hair (melanin). If the surrounding skin is relatively light compared to the color of the hair, then the entire energy of the laser will be concentrated in the hair shaft, effectively destroying it without affecting the skin or follicle. Hair removal lasers target the dark pigment in the hair. That’s why laser hair removal works best on light skin (so laser passes right through) and dark coarse hair (has most pigment).
The ability of the laser to produce a very narrow bandwidth on a consistent basis is the key to a safe efficient treatment. The types of lasers used for permanent hair reduction include the ruby (old machines only safe for very pale skin types - not recommended), Nd:YAG, diode, and alexandrite.
While the laser emits a beam that only heats the hair shaft, heat is transmitted from the hair shaft to the surrounding tissue for several milliseconds after the laser pulse. Several lasers possess cooling attachments which cool the surrounding skin to fully absorb any heat transmitted from the destroyed hair shafts.
Be aware that hair removal systems that use traditional light for treatment (Intense Pulse Light machines or IPLs) are not true hair removal lasers. These devices use a highly concentrated beam of traditional incoherent light, often in conjunction with a cream or gel, to burn the hair shaft. A serious flaw with these systems is that they lack the laser's ability to produce a selective bandwidth of light that will only affect the hair shaft (selective photothermolysis). These devices produce a wide bandwidth of light that can heat up all of the surrounding tissue. IPLs are generally cheaper devices and are used for various skin procedures first and foremost, with hair removal as more of an afterthought. Judging from consumer feedback, true lasers usually achieve better and faster results for hair removal purposes.