Laser hair removal terms and concepts
This is an extremely abbreviated description of the terms and scientific concepts needed for consumers to understand how laser hair removal works. The endnotes contain an overview of the basic mechanics behind lasers.  For a detailed technical discussion of terms and concepts in laser hair removal, please read the excellent article by Dr. E. Victor Ross and colleagues. 
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 U.S. 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 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. 
- Melanin, which 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).
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 the 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. [6, 7] 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. That's because the target is superficial and often even in depth and color compared to hairs. Hairs in any given treatment area can be widely variable in diameter, color, and depth. This poorly delineated target makes laser effectiveness hard to predict. The same amount of laser energy will have different effects on hairs with different widths. [6, 7] Some hairs are as deep as 7 millimeters . It's hard 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.
- Lawrence Livermore National Laboratory has a good definition which I've used here with slight modifications: The word "laser" stands for "light amplification by stimulated emission of radiation." Lasers are possible because of the way light interacts with electrons. Electrons exist at specific energy levels or states characteristic of that particular atom or molecule. The energy levels can be imagined as rings or orbits around a nucleus. Electrons in outer rings are at higher energy levels than those in inner rings. Electrons can be bumped up to higher energy levels by the injection of energy-for example, by a flash of light. When an electron drops from an outer to an inner level, "excess" energy is given off as light. The wavelength or color of the emitted light is precisely related to the amount of energy released. Depending on the particular lasing material being used, specific wavelengths of light are absorbed (to energize or excite the electrons) and specific wavelengths are emitted (when the electrons fall back to their initial level). If this happens in a mirrored chamber, it reflected light causes the same reaction in other atoms. The light increases in intensity as it oscillates between the mirrors. If the chamber has an opening, the resulting beam is highly monochromatic (nearly entirely one wavelength) and coherent (all the waves are in-step). It is also either well collimated (nearly parallel rays for most lasers including gas and solid state types) or appears to originate from a point source (diode lasers). In either case, the beam can easily be manipulated. Solid state lasers usually use optical pumping from high energy xenon flash lamps. Semiconductor lasers are most often pumped by DC current. For more information:, see also The Photonics Dictionary and Sam's Laser FAQ. See also an interesting discussion of lasers in tattoo removal.
- Ross EV, Ladin Z, Kreindel M, Dierickx C. Theoretical considerations in laser hair removal. Dermatologic Clinics.1999 Apr;17(2):333-55, viii.
- From three combined Greek words "destruction from heat caused by light" (Photo: light, Thermo: heat, Lysis: destruction). Originally proposed in: Anderson RR, Parrish JA. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science 1983;220:524-7.
- Littler CM. Hair removal using the Nd:YAG laser system. Dermatologic Clinics 1999 Apr;17(2):401-30, x.
- The Oregon Medical Laser Center has a good discussion of laser-hemoglobin interaction.
- van Gemert MJ, Welch AJ. Time constants in thermal laser medicine. Lasers in Surgery and Medicine. 1989;9(4):405-21.
- van Gemert MJ, Lucassen GW, Welch AJ. Time constants in thermal laser medicine: II. Distributions of time constants and thermal relaxation of tissue. Physics in Medicine and Biology. 1996 Aug;41(8):1381-99.