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The possibility of using the energy of “light” to cause heat has been a fascination of physicists for almost a century, since Einstein conceived the concept of laser radiation in 1917. But the Laser Age was not launched until 1960, when a scientist used ruby crystals to observe what he termed Light Amplification by Stimulated Emission of Radiation.

Lasers may have entered the popular psyche through James Bond movies and military “Star Wars” scenarios, but its practical application is experienced primarily in industry (microchips may be etched by lasers, for example) and in medicine.

To many people, lasers seem like magic, something that’s smoke and mirrors. You may have seen the demonstration in which a red balloon is inflated inside a clear balloon and a laser pops the red one without damaging the clear one. In fact, mirrors do play a role in producing this very powerful “light” beam, but the concept follows some strict laws of the natural world.

You start by placing a material, called a medium, in an enclosed cavity. The medium could be a gas like argon or carbon dioxide, a liquid like a dye dissolved in an organic solvent, or a solid like a ruby crystal. If you introduce an electrical charge into the cavity, its atoms are “pumped up” to an excited state, and the orbiting particles called electrons achieve a higher energy level. The electrons quickly release that energy in the form of light particles, called photons. When a majority of the atoms are in the excited state, it is much more likely that an emitted photon of light energy will collide with other highly energized atoms, thus stimulating the emission of even more photons.

Here’s where the mirrors come in: Placed at each end of the laser cavity, they reflect the light back and forth, tremendously amplifying the process. In itself, this would not help doctors remove a tattoo or resurface the skin. But the energy emitted, coupled with the energy that stimulated the electron, yields two waves of light energy of the same frequency and wavelength traveling in the same direction.
All these processes cause a tremendous amount of light energy (or radiation) to be developed in that space. Then, a small portion (usually 5-10%) of the light is allowed to travel out of the cavity and be directed externally as a beam of laser light.

Unique qualities allow a laser to be medically important. Lasers are:

• Monochromatic. One “color” or wavelength. Medical lasers emit light in very narrow wavebands along the electromagnetic spectrum, from ultraviolet to infrared. But many lasers with medical uses are not visible. A laser using invisible light energy may also employ a lower power visible laser light to provide direction for the physician.
• Coherent. Laser light is orderly and highly directional with the light waves synchronous. It’s like orderly rows of a good marching band with its members marching in stride.
• Bright. Lasers have extremely high power of light, achieved by the amplification process, low level of beam “scatter”, and tight focusing qualities.

The beauty of lasers in surgical therapy is that they can be controlled so they damage biologic tissue in an extremely selective fashion. The power density and how long the tissue is exposed are two factors. The depth of penetration is generally proportional to the wavelength. The crystal, liquid or gas that is the laser medium can be chosen for their specific characteristics, including how it is absorbed by a particular pigment or tissue.

Most of a laser’s damage is caused by the heat it produces. If the laser is delivered in a pulsing mode, the damage may be partly mechanical as well, with the target essentially shattered to pieces. A third way lasers affect tissue is chemical, whereby a light-sensitive medicine reacts with the laser in a process called “photodynamic therapy”.

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