eye disorders
The Eye Examination
Refraction
Refraction is the procedure examiners use to assess focusing errors. Problems with visual acuity (sharpness of vision) that result from refractive errors, such as Causes , One important visual acuity test uses theSnellen chart(eye chart), which is a large card or lighted box that displays rows of letters in smaller and smaller sizes. The chart is read from a standard distance. The degree of visual acuity is determined by the size of the row of letters that the person can read. For people who are unable to read, a modified chart can be used in which the letters are represented by an upper case "E," which is rotated randomly. People are asked to describe which way the "E" is facing. If a person cannot read anything from a chart, the examiner may see whether the person can count the examiner's fingers or see whether the examiner's hand moves. Examiners also test near vision by asking the person to read a standard near card or newsprint at a distance of about 14 inches (35 centimeters). is done with a device that determines the refractive error of the eye by measuring how light is changed when it enters the eye. The person sits in front of the autorefractor, a beam of light is emitted from the device, and the eye's response is measured. The machine uses this information to calculate the Corrective Lenses needed to correct the person's refractive error. This measurement takes only a few seconds. Aphoropteris the device commonly used, in conjunction with a Snellen chart, to determine the best corrective lenses for a person being assessed for eyeglasses or contact lenses. The phoropter contains a complete range of corrective lenses, allowing the person to compare different levels of correction while viewing the chart. Typically, the eye doctor uses the phoropter to refine the information obtained from the autorefractor before prescribing lenses. 