The shape of the front surface of the eye (the cornea) may be measured with a keratometer. The patient sits in front of the instrument with his or her head on a headrest. A light pattern is reflected off the cornea and by making the proper adjustments, the doctor can measure the curvature of the eye surface. This is useful in helping determine the amount of astigmatism, fitting contact lenses, and determining the cause of decreased visual acuity, even with the best spectacle prescription.
A more sophisticated device is a corneal topographer. This instrument captures a photograph of a light pattern reflected off the cornea. The image is then analyzed with a computer, which produces a detailed map of the shape of the eye. This device is often used to diagnose certain corneal diseases, aid in contact lens fitting, and in conjunction with refractive surgery techniques.