How Lasers Work

Laser is not a word but an arcane acronym for "light amplification by stimulated emission of radiation." (Radiation refers simply to energy traveling in waves, and most forms of radiation are not harmful; see "Lasers 101," left.) Light, including laser light, is created when atoms release energy in the form of photons. They have different wavelengths, corresponding to the colors in the electromagnetic spectrum, and contain various amounts of energy. Most light sources release photons randomly. In a laser, by contrast, all the atoms are pumped up (the amplification) to an energy level higher than normal and when they release their photons, the light emerges as one specific wavelength and the photons move in unison and in one direction. This gives the laser its power. The fact that the beam can be varied in its intensity, speed, and duration has made lasers useful in processes as disparate as playing CDs and cutting the $20,000 timber floors for Sean "Diddy" Combs's retrofitted SUV (this according to the New York Post). The wavelength and pulse of a particular laser typically determine its medical application.

"Laser is just very high-energy light that can be focused to a specific point," explains Richard Fitzpatrick, M.D., the director of the dermatology division at the LaJolla Cosmetic Surgery Centre in California, who has trained thousands of physicians in laser techniques and is one of the preeminent researchers in the field. "A laser may not put out more energy than a lightbulb, but the lightbulb is lighting the whole room. A laser can heat a dot to the energy of the sun for a millionth of second, and it's only going to affect tissue that will absorb that energy." Light is avidly absorbed by dark objects, which is why lasers work so well on dark pigments (known as chromophores) such as hemoglobin (the protein that makes blood red) and melanin, which gives skin its beige or brown color.

If the development of cosmetic lasers were seen on a time line, there would be a big red star at 1983. That's when R. Rox Anderson, M.D., came up with the concept of "selective photothermolysis." In lay terms, that means getting the right amount of the right wavelength of laser energy to a specific tissue with minimal disturbance to surrounding tissue. The tissue that absorbs the laser's particular wavelength will be "energized" and heated to a greater degree than the surrounding tissues that tend to reflect, scatter, or bend the wavelength. The longer the laser's wavelength, the deeper the penetration: A larger capillary deep in the skin requires a different laser than a small, superficial one.