The term "axis" in eye care refers to the orientation of astigmatism in the eye. Astigmatism occurs when the cornea or lens has an irregular shape, causing light to focus unevenly on the retina. The axis is expressed in degrees, ranging from 0 to 180, and indicates the direction of the astigmatism, where the curvature is greatest. In other words, the axis defines the angle at which the cornea or lens is steepest.
In a prescription for eyeglasses or contact lenses, the axis is used alongside the cylinder (CYL) value to correct astigmatism. The cylinder value indicates the degree of astigmatism, while the axis specifies its orientation. For example, an axis of 90 degrees means that the steepest curvature of the cornea or lens is vertically aligned, while an axis of 180 degrees indicates that the curvature is aligned horizontally.
Correcting astigmatism requires lenses that have varying refractive power in different meridians. These lenses, known as toric lenses, compensate for the irregular curvature of the eye, allowing light to focus properly on the retina and providing clear vision. Accurate measurement of the axis is crucial for effective correction, as even small errors in the axis value can result in blurred or distorted vision.
