The theory behind lasers was developed in the 1920s by Maxwell Planck and Albert Einstein. However, the first laser wasn't actually built until the 1960s. Lasers were first used to remove hair during this period, although hair removal lasers weren't truly practical until the late 1990s.
Theory
Most light sources such as the sun or a light bulb emit light with a range of wavelengths. A laser is a device that only produces light with one wavelength and is known as coherent light. Lasers may use a crystal, gas, liquid or dye to produce coherent light.
Challenges
The primary difficulty in using lasers for hair removal is delivering enough heat to the hair to destroy the hair follicles without damaging the surrounding skin. The early lasers such as the argon and ruby lasers weren't effective in removing hair safely and are no longer used for this purpose.
Q Switch
The first lasers produced a continual beam of coherent light, which is damaging to the skin. Dr. Leon Goldman began experimenting with a way to deliver a precisely controlled amount of laser light in the 1960s. His solution was a device called the Q switch, which turned the laser on and off in controlled pulses.
FDA Approval
The Food and Drug Administration approved the first laser for the removal of hair in 1995. This was a neodymium, yttrium aluminum and garnet (Nd:YAG) laser marketed under the brand name Soft Light. It required a carbon-based lotion to be applied to the skin in order to darken the hairs enough to be effective.
Modern Lasers
The Nd:YAG laser produces light with a wavelength of 1,064 nanometers (nm) and is most suitable for dark skin. A pulsed diode array laser produces light with a wavelength of 810 nm and is best for light to medium skin. An alexandrite laser produces 755nm light and is most effective on light skin.
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