Page 8 - Practical-Refraction-English

Emmetropia, Ametropia - if emmetropic in one principal meridian and 1 3) Astigmatism: myopic in the other, the astigmatism is termed sim- An eye is said to be astigmatic when its optical power ple astigmatism and therefore its focus differs according to its different meridians. The eye thus presents an asymmetric ame- - if emmetropic and hypermetropic, it is termed tropia with different focal positions in different planes. simple astigmatism For example, a person with astigmatism who is looking - if myopic in all meridians, it is termed com- at a capital letter E may see the vertical line clearly but pound myopic astigmatism Practical Refraction the horizontal lines as blurred. - if hypermetropic in all meridians, it is termed compound hypermetropic astigmatism In an eye with astigmatism, there is always a meridian of - if hypermetropic in one principal meridian and maximum refractive power and another meridian of myopic in the other, it is termed mixed astigmatism minimum refractive power; these are termed the principal meridians. Between these, the refractive power varies The astigmatism is said to be with the rule if the most between the maximum and minimum limits. powerful meridian of the eye is close to the vertical (that is, located between 70° and 110°). It is said to When the astigmatism is regular, the principal meridians be against the rule if the most powerful meridian is are perpendicular to each other (that is, 90º apart) and close to the horizontal (located between 160° and the power varies in a regular fashion between these two 200°, or +20° and -20°). When the astigmatism is limits. Astigmatism may also be irregular, where the prin- neither with the rule nor against the rule, it is said to cipal meridians are not perpendicular to each other; be oblique. this may result from an injury, for example, and is not able to be corrected by spectacle lenses alone. The optical system of the astigmatic eye forms a com- plex light beam image of an object point. This beam is characterised by two small linear foci, each at one ext- reme of the interval, which are perpendicular to each other. These two foci correspond to the images formed by the principal meridians of the eye. Inside this Interval of Sturm, lies a particular location called the Disc of Least Confusion. At this location the section of the astig- matic beam is a minimum and at its smallest size. The © Essilor International disc of least confusion is dioptrically equidistant from the two foci, that is, near the midpoint of the interval; it is this location which is positioned on the retina when the best vision sphere is in place (see later). The principle of correction of the astigmatic eye is to introduce a lens of variable power so as to reposition the image onto the retina. The power of this lens varies according to its meridians, inversely to the astigmatism of the eye. The lens is called sphero-cylindrical, cylindri- cal, or toric. The difference between the refractive power of its maximum and minimum meridians (the cylinder) compensates for the astigmatism of the eye, thereby © Essilor International merging the two linear foci into a single point focus, while its spherical component places this image point on the retina. With the rule astigmatism is corrected by a minus cylindrical lens with an axis close to 180° and against the rule astigmatism by a minus cylindrical lens with an axis close to 90°. The axis of astigmatism varies Figure 5: The Astigmatic Eye and The Principle of its throughout life, generally from with the rule as a child to Correction against the rule as an older adult. Also, the axis of astig- Different types of astigmatism are possible, depen- matism in the two eyes is such that it is generally sym- ding on the location of the two principal foci (that is, metrical around the vertical meridian (the nose). the refractive power of the maximum and minimum meridia): 8 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.