Laser Technical Details

Excited Dimer = Excimer

The excimer laser is an ultraviolet Argon-Fluoride laser at 193 nano metres (nm) It is a gas laser and requires refilling on a regular basis. The argon is an inert gas whereas the fluorine is highly excitable.

Photo 1: The excimer laser has a photon energy of 6.4 electron volts. The carbon-carbon links of the corneal structures are 3.5 electron volts. Each laser pulse lasts 18 nano seconds and takes off about 0.25 microns of corneal tissue with each pulse ( 1000 microns in 1 mm).

Photo 2: Hence, when the laser hits the cornea, it "blows apart" the carbon-carbon links and the noise that one hears is the particles from the cornea breaking the sound barrier as they come off the cornea in a mushroom cloud shape similar to an atomic bomb.

Photo 3: For a -3 Dioptre laser correction, the laser will take off about 40 microns. (The average cornea is about 540mm thick) Most modern lasers these days use a so- called "flying spot" At present I use the WaveLight 400 Hz laser which fires at 400 times a second. This enables one to sculpt fine details for more customised treatments. The fact that all the energy of the ablation is taken off the cornea in this supersonic plume means that the temperature of the cornea hardly rises at all and this means that there is virtually no "collateral damage" due to thermal effects on adjacent tissue. Hence the excimer laser gives a very keen cut. Below is a lasered hair of Professor Marshall from St Thomas's Hospital in London, who did some of the pioneer work with Professor Steven Trokel, the inventor of PRK.

Photo 4: This is a graph of corneal temperature change during a 20 second ablation of a human cornea. Normal corneal temperature is about 30-33 C and it rises to about 37 C, which is blood heat.

Photo 5: Finally, a cartoon sent by an optometrist friend of mine!