How the Laser Works

The laser, which is controlled by computer, emits gentle and invisible beams of light that actually vaporize the tissue from the surface of the eye. The laser is extremely accurate and precise, and the process removes only a fraction of the thickness of the cornea—about the thickness of a human hair. Excimer Laser - Background The excimer laser is one of the most exciting new developments to occur in all medicine and surgery. The excimer laser reshapes the cornea to change the refractive focal length of the eye. The excimer laser may be the most important advance in eye care since the advent of the contact lens. The excimer laser was developed at IBM in 1976 for producing sophisticated sub-micron computer microchips. The concept of applying it to the cornea was developed in 1983 by Stephen L. Trokel in cooperation with R. Srinivasan, a physicist from IBM. Trokel received a patent in 1983. The first sighted eye was treated with the excimer laser by Theo Siler on January 14, 1987. Between 1987 and 1989 several U.S. companies worked toward pre-market approval to use the argon fluoride excimer laser. Trokel received the first investigational device exemption from the FDA. The excimer laser generates ultraviolet light of a very specific wavelength (193 nanometers) when high-voltage electrical energy is discharged into a cavity containing argon fluoride gas. The term excimer is derived from two words 'excited' and 'dimer', which are used in describing the reaction in which the laser transfers energy through the ultraviolet beam of light. The pulses of light, with controlled and specific wavelength, are the basis for the excimer laser's use in sculpting the surface of the eye. Ultraviolet light with wavelengths less than 300 nanometers will not penetrate through the surface of the eye. Because of this particular phenomena, the excimer laser with a wavelength of 193 nanometers does not transmit energy through the cornea to the internal tissues of the eye. Instead, the ultraviolet light is absorbed in the surface cells of the eye giving the excimer its unique ability to reshape these surface tissues. As the ultraviolet light is absorbed by the surface of the eye, tissue is vaporized. No heat is generated in the process, therefore, no scarring of the corneal surface occurs. The ultraviolet light emitted by the excimer laser will ablate only about 0.25 microns of corneal tissue per pulse. About 200 pulses would be required to etch through a typical human hair which is about 50 microns in diameter. Such precise control is the basis of laser vision correction. To understand the excimer laser's ultra-high accuracy and precision, some explanation of basic terminology provides greater clarity. One micron equals .001 mm One thousand microns equal 1 mm. Central corneal thickness equals 520 microns. Fifty microns equal the thickness of the corneal epithelium. Ten microns equal the height of 1 human epithelial cell. Fifty microns equal the diameter of the human hair. Tissue ablated equals 0.25 microns per pulse. Forty pulses are required to ablate 1 cell thickness. When the laser beam touches the cornea, the intermolecular bonds are broken and the molecules are dispersed into the air. This leaves a clear and smooth underlying corneal surface. In order to induce a refractive change to correct myopia, more tissue must be removed from the center of the cornea than the periphery.