Laser eye surgery. Artwork showing three stages of LASIK laser eye surgery for astigmatism. The eye is seen from above. Astigmatism is caused by a cornea (blue, left) that is not perfectly spherical. The non-uniform curvature means that light rays do not all come to a single focus point on the retina (back of eye). This can be repaired by LASIK eye surgery. A flap is cut in the cornea (top) using a special knife called a microkeratome. An excimer laser (middle) is then used to remove some tissue from the corneal stroma (pink). The flap is replaced and heals without stitches in a couple of weeks. This procedure can also be used to treat nearsightedness (myopia) and farsightedness (hyperopia).