The primary principle behind laser hair removal is selective photothermolysis. Lasers can cause localized damage by selectively heating dark target matter in the area that causes hair growth while not heating the rest of the skin. Light is absorbed by dark objects, so laser energy can be absorbed by dark material in the skin (but with much more speed and intensity). This dark target matter, or chromophore, can be naturally-occurring or artificially introduced.

Hair removal lasers selectively target one of three chromophores:

• Carbon, which is introduced into the hair follicle by rubbing a carbon-based lotion into the skin following waxing (this lotion is an "exogenous chromophore"). When irradiated by a Q-switched Nd:YAG laser, the carbon causes a shock wave capable of mechanically damaging nearby cells. This method has been abandoned since it does not cause permanent loss. (It has been replaced with the long-pulsed Nd:YAG laser which targets endogenous melanin in the follicle.)
• Hemoglobin, which occurs naturally in blood (it gives blood its red color). It preferentially absorbs wavelengths from argons, and to a lesser extent from rubies, alexandrites, and diodes. It minimally absorbs the Nd:YAG laser wavelength. Hemoglobin is not a primary target for laser hair removal.
• Melanin is considered the primary chromophore for all hair removal lasers currently on the market. Melanin occurs naturally in the skin (it gives skin and hair its color). There are two types of melanin in hair: eumelanin (which gives hair brown or black color) and pheomelanin (which gives hair blonde or red color). Because of the selective absorption of photons of laser light, only black or brown hair can be removed.

Any laser light beam intended for topical use can only penetrate skin tissue two millimeters deep. ^{[citation needed]} As such, there has been great controversy surrounding the laser industry claims to what most people think of when they hear the word "permanent".

The dermal papilla is the only appropriate target of destruction, as it is the only substructure of the hair shaft which can reproduce a new root system, and therefore a new hair shaft. ^{[citation needed]} The dermal papilla is located at a depth of 7-8 millimeters for coarse hair, and so is beyond the reach of laser.

For a short list of approved lasers, the Food and Drug Administration permits the use of the phrase "permanent reduction." The use of the phrase "permanent removal" is not permitted for any laser whatsoever. The intended meaning of "permanent" may also be confusing to consumers. Most consumers understand "permanent" to mean that the hair will stop growing forever. However the FDA permits the use of "permanent" as long as the laser reduces some visibily growing hairs for as short a time as one growth cycle (a matter of a few months for most body parts). In fact, many clinical studies have shown the rate of hair regrowth from laser epilation to be only slightly slower than from traditional methods. No long term clinical studies (i.e., follow-up greater than 12 months) have been done to test the true permanency of laser hair removal.

Clinics performing laser hair removal usually recommend at least six treatments. For this reason treatments are often sold in packages.

Laser parameters that affect results

Several wavelengths of laser energy have been used for hair removal, from visible light to near-infrared radiation. These lasers are usually defined by the lasing medium used to create the wavelength (measured in nanometers (nm)):

• Argon: 488 or 514.5 nm
• Ruby: 694 nm
• Alexandrite: 755 nm
• Pulsed diode array: 810 nm
• Nd:YAG: 1064 nm

Pulsewidth is an important consideration. It has been observed in some published studies that longer pulsewidths are safer in darker skin.

Wavelength is a critical factor. Longer wavelengths are safer in darker skin.

Spot size, or the width of the laser beam, affects treatment. Theoretically, the width of the ideal beam is about four times as wide as the target is deep. Most lasers have a round spot about the size of your little finger (8-10 mm).

Fluence or energy level is another important consideration. Fluence is measured in joules per square centimeter (J/cm²).

Repetition rate is believed to have a cumulative effect, based on the concept of thermal relaxation time. Shooting two or three pulses at the same target with a specific delay between pulses can cause a slight improvement in the heating of an area.

Epidermal cooling has been determined to allow higher fluences and reduce pain and side effects, especially in darker skin. Four types of cooling have been developed:

• Clear gel: usually chilled
• Contact cooling: through a window cooled by circulating water
• Cryogen spray: immediately before/after the laser pulse
• Air cooling: forced cold air at -34 degrees C (Zimmer Cryo 5 unit)

Multiple treatments have been shown in numerous studies to be more effective for long-term reduction of hair. Current parameters suggest a series of treatments spaced at 1 month for the face, 2 months for the chest and 3 months for the legs.

Laser energy also gets less effective the deeper into the skin it must travel. Think of it like putting your hand over a flashlight. A little light penetrates the thinner skin (the reddish glow), but can't penetrate the thicker areas. Light that enters the skin is either absorbed or scattered and reflected back out of your hand. When this happens to a laser beam, this scattering is called attenuation. The more tissue light has to travel through, the more attenuation will occur. That means at deeper levels, less energy reaches the target.

Number of sessions

The number of sessions necessarily depends on various parameters, the most important of which being the localization of the area to be treated and gender. In addition, resting follicles are not damaged by the laser. In general, it is necessary to foresee between three to six sessions, but it also depends on initial pilosity and the degree of hair removal required. Certain areas (back and face in men, for example) may require a prolonged treatment whose exact duration cannot be predicted in advance.

Intervals in between sessions

The intervals between sessions depend on the area of the body to be treated and gender and are spaced progressively. Concerning the moustache and chin, an interval of 6 to 8 weeks is usual.

Other uses

Hair removal lasers are effective treatment for pseudofolliculitis barbae (commonly called ingrown hairs or "shaving bumps"). For darker skin patients with black hair, the long-pulsed Nd:YAG laser with a cooling tip can be safe and effective when used by an experienced practitioner.

They have recently been reported as helpful treatment for pilonidal cysts, since they eliminate the ingrown hairs that produce the troublesome foreign body reactions in this congenital malady.