Meridians of the Eye

Eye care providers, whether optometrists or ophthalmologists, require standardized methods of recording the visual acuity of their patients. Their findings are detailed on a paper known as the eyeglass prescription which is used by opticians to make lenses. A meridian, or imaginary arc of 180 degrees, is one of the mechanisms utilized. Understanding the eyeglass prescription requires knowledge of the meridian of the eyes.

Anatomy

• The perfectly functioning eye receives light through the cornea and sends that light to the lens. The light then transforms into a clear image that is received by the retina in the back of the eye. As the Mayo Clinic explains, a problem with eye anatomy generates blurred images. If you are nearsighted, or cannot see clearly at distances, your cornea is curved too much or your eye is too long. The image appears in front of the retina. If you are farsighted, or have trouble with close-up vision, the opposite occurs: your cornea is curved too little or your eye is too short, producing the image behind the eye. Astigmatism, or blurry vision, occurs when the cornea is curved more in one direction than the other.

Protractor

• A protractor is an instrument used in mathematics and geometry to measure and draw angles. The protractor represents the top half of a circle. It measures angles in degrees, with 180 degrees being the total number from one end to another. A perpendicular line drawn from the middle of the protractor to the top of the circle measures 90 degrees. An eye care provider measures degrees based on the protractor being placed on the front surface of the eye, with the center point lined up with the middle of the eye's pupil.

Readings

• Meridian of the eye readings are only used in cases of astigmatism. When the eye is spherical, that is, shaped evenly as in a sphere, no need exists for measuring degrees. All meridians of the eye are equal. Eyes can be spherical and have perfect vision, be nearsighted or demonstrate farsightedness. It is only the irregularly shaped eye that causes the blurry vision of astigmatism that requires use of the protractor lines.

Cylinder Power

• Cylinder power refers to the term used to measure the lens power necessary to correct astigmatism, which may exist in one or both eyes. The lens power is noted and then the degrees are used to explain how the lens will correct the irregularly shaped eye. Basically, the meridian locates the most and least curved portion of the cornea so the optician can shape the lens. For example, a report at 180 degrees meridian means the horizontal meridian receives no extra corrective power while the vertical 90 degrees receives the necessary power.