Home » LASIK » Glossary



When you are considering the option of laser refractive surgery – LASIK or PRK – it’s important to understand the terminology and the process.

Cornea: The transparent tissue – the clear window – on the front part of the eye that also overlies the iris (the colored part). The cornea provides about 2/3 of the eye’s focusing power. Both LASIK and PRK refractive surgery utilize a laser to reshape the cornea to change its refracting power.

Epithelium, stroma, and endothelium: The cornea has layers to it. The outermost surface layer is the epithelium. The stroma is the middle layer, and provides the strength and shape of the cornea. The endothelium is a layer of cells that are the inner lining of the cornea. With both LASIK and PRK surgery, the corneal stroma layer is re-shaped by the laser to provide focused vision.

Nearsightedness or Myopia: The cornea and the lens of the eye bring light into focus in front of the retina. When light doesn’t come into focus on the retina, this is called “refractive error.”

Farsightedness or Hyperopia: This is the opposite refractive error to near-sightedness, and means that the cornea and the lens of the eye bring light into focus “behind” or “far” to the retina.

Astigmatism: The word “astigmatism” literally means “out of focus”, and in eye care generally means that the cornea has different focusing powers in different meridians. The classic example is that if you looked at the side of a basketball, the curvature would be the same in all directions (no astigmatism). If you looked at the side of a football, one direction would have a steeper curvature, and the other direction would have a flatter curvature. Because curvature is part of what determines the focusing power, having different curvatures means that instead of light coming to one sharp focus, there are two focus points. Most people simply see this as blur, but people with higher levels of astigmatism may actually see “ghosting”, a faint second image.

What is a Diopter? A diopter is a unit of measurement of the strength of a lens. Glasses and contact lens power are measured in diopters.