Corrective lenses require precise measurements to ensure light focuses correctly on the retina. While simple nearsightedness or farsightedness only needs uniform spherical correction, astigmatism introduces complexity. For people with astigmatism, a specific measurement known as the axis is required for their glasses prescription. This number instructs the lab exactly how to orient the corrective power within the lens material.
Understanding Astigmatism and the Axis
Astigmatism is a common condition where the eye’s cornea or lens has an irregular curvature, causing light to focus at multiple points instead of one clean point on the retina. Instead of a perfectly spherical shape, the astigmatic eye is shaped more like a football, possessing two different curves or meridians. One of these meridians is steeper than the other, and the difference in curvature between them is what causes blurred or distorted vision.
To correct this, a special lens power called cylinder power neutralizes the irregular curve. This cylinder power must be applied along a specific orientation, or meridian, known as the axis. The axis measurement indicates the exact angle at which the cylinder power must be placed within the lens, not the strength of the correction. The greater the cylinder power, the more precisely the axis must be aligned for clear vision.
This orientation is standardized using a protractor-like scale ranging from 1 to 180 degrees. The horizontal meridian of the eye is represented by 180 degrees, while the vertical meridian is 90 degrees. An axis of 180, for example, means the cylinder power is placed horizontally in the lens, correcting a vertical curvature error in the eye.
Clinical Determination of the Axis
The eye care professional determines the axis through objective and subjective testing methods. An initial, automated measurement is taken using an autorefractor, which shines light into the eye and measures the reflection to estimate the degree and orientation of astigmatism. This provides an objective starting point for the final prescription.
The subjective refinement of the axis occurs while the patient looks through a testing device called a phoropter, which contains a series of lenses. The professional uses a specialized lens called the Jackson Cross Cylinder (JCC) to fine-tune the precise axis. The JCC is a combination of two equal but opposite cylindrical powers, typically 0.25 or 0.50 diopters, with their axes separated by 90 degrees.
The professional places the JCC in front of the patient’s eye and rotates it, asking the patient to compare the clarity of an image in two different positions. The patient’s response guides the eye care professional in rotating the axis of the corrective cylinder until the patient reports that the view is equally blurred in both JCC positions. This point of equal blur indicates that the cylinder axis in the phoropter is precisely aligned with the patient’s astigmatic meridian, resulting in the final axis measurement.
Verifying Axis Alignment in Finished Glasses
Once the prescription is finalized, the lens manufacturing laboratory uses the axis number to grind the corrective cylinder into the lens. The optician then performs a quality control check on the finished glasses using an instrument called a lensometer, also known as a focimeter, to ensure the lens was correctly made and mounted.
The lensometer measures the optical properties of the physical lens, including the power and the orientation of the cylinder axis. The optician places the lens against the lensometer’s stage and rotates the axis wheel until the internal measurement lines appear straight and sharp. This reading confirms the exact angle at which the corrective cylinder power has been placed in the finished lens.
For verification, the measured axis must fall within accepted tolerance standards established by regulatory bodies, which allow for a deviation of only a few degrees, depending on the cylinder power strength. The lensometer is also used to mark the optical center and the cylinder axis on the lens surface with tiny dots. These markings ensure the lens is centered correctly in the frame before the final cutting process, maintaining the precise axis alignment determined during the eye exam.