Light is absorbed by dark objects. If there’s enough light, something dark can get pretty hot (like the hood of a black car in the summer sun). In a similar way, 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.
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. Laser and light-based methods are sometimes called phototricholysis or photoepilation.
Melanin is considered the primary chromophore for most lasers currently on the market.
Hair removal lasers selectively target one of three chromophores:
- Carbon, which is introduced into the follicle by rubbing a carbon-based lotion into the skin following waxing (this lotion is an exogenous chromophore). When irradiated by an Nd:YAG laser, the carbon causes a shock wave capable of mechanically damaging nearby cells.
- Hemoglobin, which gives blood its red colour. It preferentially absorbs wavelengths from argons, and to a lesser extent from rubies, alexandrites, and diodes. It minimally absorbs the Nd:YAG laser wavelength.
- Melanin, which gives skin and hair its colour. There are two types of melanin in hair: eumelanin (brown or black colour) and pheomelanin (blonde or red colour).
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 may be more effective with less side effects. Recently, very long pulse or super long pulse lasers have been theorized to be safer for darker skin, but this has yet to be demonstrated in published data.
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. 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: a newer experimental method
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 4 to 6 weeks apart, but theoretically, there is a point of diminishing return where additional treatments will not cause additional loss.
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 reflected. The amount of reflected light is called scattering. When this happens to a laser beam, it’s 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.
Variables in consumers that affect results.
Lasers can be useful for surface dermatological procedures like removing some kinds of tattoos, or birthmarks like port wine stains. This is because the target is superficial and often even in depth and colour compared to hairs. Hair in any given treatment area can be widely variable in diameter, colour, and depth. This poorly delineated target makes laser effectiveness hard to predict. The same amount of laser energy will have different effects on hair with different widths. Some hairs are as deep as 7 millimeters. It is therefore difficult for a laser to be effective at those depths without overheating the upper skin.
Obviously, if a laser targets melanin, the less melanin you have in your hair means the less effective a laser will be. That’s why someone with gray, red, or blonde hair is not as good a candidate for laser hair removal.
In addition, the more melanin in your skin, the darker it looks. Caucasians don’t have much skin melanin, while Africans have a lot. The laser doesn’t distinguish between melanin in hair and melanin in skin. That means the more melanin in your skin, the more the laser is going to target your skin. That’s why someone with darker skin is not as good a candidate for laser hair removal. Light skin and dark hair are the best combination for laser hair removal. The more closely your skin tone matches your hair color, the less likely you are to benefit from laser hair removal.