INTRODUCTION — Laser refractive surgery is becoming increasingly popular as people are turning to alternatives to traditional spectacles and contact lenses for vision correction. 120 000 surgeries were performed in France in 2006

ANATOMY OF THE CORNEA — The transparent cornea is about one-half millimeter thick and has five distinct layers. The epithelium is the most exterior layer providing the smooth refractive surface and barrier against infection. The function of Bowman's membrane, which lies beneath the epithelium and its basement membrane, is unclear. The stroma, made up of intertwining lamellae of collagen fibrils, provides structure and accounts for 90 percent of the corneal thickness. The endothelium and its basement membrane (Descemet's membrane) form the innermost layers. Endothelial cells,  are responsible for the natural corneal dehydration necessary for corneal clarity.

OPTICS, REFRACTION, AND REFRACTIVE ERROR — Refraction is the bending of light rays as they pass from one transparent medium to another medium of a different density. Refraction is measured in diopters (D). The refractive power of a lens is the reciprocal of its focal length in meters. As an example, a 1D lens has a focal point of one meter; a 2D lens has a focal length of one-half meter.

The cornea and crystalline lens refract light entering the eye. The cornea is responsible for two-thirds of the eye's total focusing power while the crystalline lens accounts for one-third. Corneal power is fixed, but the lens can change its shape to become more powerful for viewing objects up close in a process called accommodation.

In emmetropia (an eye with normal vision), the focusing power of the cornea and lens is perfectly matched to the length of the eye. Parallel light rays from a distant object are brought into focus precisely on the retina and a clear image is perceived. The eye's focal point is at infinity.

Refractive errors occur when light rays entering the eye do not focus properly on the retina:

• In myopia (nearsightedness), the most common type of refractive error, the cornea is too curved or the lens is too powerful for the length of the globe. Distant objects cannot be seen clearly because light rays are focused in front of the retina (show figure 1). The myopic eye has a focal point close to the eye. As a result, near objects can be seen clearly.
• In hyperopia (farsightedness), the cornea and lens are too weak for the length of the globe. As a result, light rays reach the retina before they are focused to a single point . A distant object may be brought into focus by using accommodation, but clear near vision is difficult.
• With an astigmatism, the refractive power of the eye is not the same in different meridians. Light rays can never be brought to a single point and objects will appear blurry at any distance. Astigmatism may occur with myopia or hyperopia.

Presbyopia is a different type of refractive error in which loss of accommodation occurs as the lens hardens with age. This results in an inability to bring a near object into focus on the retina and requires the use of reading glasses, typically in patients in their 40s. Presbyopia is not directly corrected by laser refractive surgery.