|Modern optical principles for an objective determination of eye refraction errors
Most of us will have some vision problems. These difficulties in seeing may be minor inconveniences or major short comings in our ability to see clearly. They may appear in or late in our lives. Fortunately, many methods from eye glasses and contact lenses to surgery are available to make corrections to our natural optical (ocular) system. However, before eye specialists can make corrections they must know very precisely what the problem is. In this unit we will investigate a variety of modern technique for examining the optical properties of the human eye.
Figure 1: The Snellen Eye Chart is commonly used for eye examinations requiring patient feedback. (http://www.i-see.org/eyecharts.html)
ye specialists use many methods to measure the performance of the ocular system and to determine the combination of lenses which optimizes visual acuity for distant objects. The most widely used and well established method relies on subjective patient feedback. A patient looks at a target eye chart; while an optometrist moves test corrective lenses into the patient's field of view. The patient is then asked to verbally compare the quality of the perceived image as afforded by different corrective lenses. Some patients have difficulty articulating the comparison of the quality of vision with the various lenses. Thus, eye specialists need ways to estimate the optical properties of the eyes without collaboration of the patient.
Methods which do not require a subjective response from the patient assume that visual acuity is maximized when the quality of the retinal image is maximized, i.e. when the retinal image is optimally focused. Using this principle elaborate approaches have been developed to measure refractive error2. Two distinctive procedures are available:
An optical system projects a picture on the retina and the light coming from this picture is examined with a second optical system. The second system looks for changes in the image, such as distortions.
A ray of light is directed into the eye. Its path and the path of the light scattered by the retina is investigated.