A refraction eye exam determines the exact lens power required to correct a person’s vision, aiming for the clearest sight possible. This examination measures how light rays bend (refract) as they pass through the cornea and lens to focus on the retina. Since refractive errors like nearsightedness, farsightedness, and astigmatism cause light to focus incorrectly, the refraction test pinpoints the specific degree of correction needed. This test is necessary to accurately determine a prescription for corrective lenses, whether glasses or contact lenses.
Preliminary Measurements
The process begins with objective measurements that do not rely on the patient’s verbal feedback. The primary tool is the auto-refractor, a computer-controlled machine that provides a fast, estimated starting point for the refractive error. The auto-refractor projects infrared light into the eye, which reflects off the retina. Sensors measure how this reflected light is distorted, calculating the necessary lens power to bring the image into sharp focus.
This measurement is completed quickly, with the patient focusing on a distant image within the machine. The instrument may also incorporate keratometry, which measures the curvature of the cornea. While the auto-refractor provides an objective estimate, it serves only as a guide for the eye care professional and is not the final prescription.
Objective Refinement
Following automated measurement, the eye care professional refines the prescription using retinoscopy. This technique is particularly useful for patients who cannot communicate clearly, such as young children, but it is a standard step for most patients. The practitioner uses a retinoscope to shine light into the patient’s eye while observing the “red reflex” reflecting off the retina.
As the practitioner moves the light, the direction and speed of the reflex’s movement within the pupil reveal the refractive error. The doctor places trial lenses in front of the eye until a lens neutralizes the movement, making the pupil appear to fill with a steady glow. This neutralization point objectively determines the spherical and cylindrical correction needed, providing a highly accurate, foundational prescription.
Subjective Determination
The subjective determination phase involves the phoropter and the familiar question, “Which is better: one or two?”. This process starts with the estimated prescription loaded into the phoropter’s lens chambers. The initial goal is to relax the patient’s focusing muscle (accommodation) using a technique called “fogging,” which temporarily makes vision slightly blurry with plus power. This prevents the eye from over-accommodating, which could lead to an inaccurate correction.
The doctor begins monocular testing, covering one eye to refine the spherical power by presenting clearer lens options. Once the best spherical power is found, the cylinder power and axis for astigmatism are refined using the Jackson Cross Cylinder (JCC). The JCC presents two views, and the patient indicates the clearer image, allowing precise adjustment of the astigmatic correction’s angle and strength. Finally, binocular balancing tests both eyes together to ensure they work comfortably and equally.
Translating Results into a Prescription
The final outcome of the refraction exam is a written prescription that translates the refined lens powers into specific parameters for the lens fabricator. This prescription is organized with separate rows for the right eye (OD) and the left eye (OS). The prescription contains several key components:
- Sphere (SPH): Indicates the main lens power for nearsightedness or farsightedness, measured in diopters. A minus sign (-) means the patient is nearsighted, while a plus sign (+) indicates farsightedness.
- Cylinder (CYL): Provides the power needed to correct astigmatism. If this column is blank, no astigmatism correction is required.
- Axis: A number between 1 and 180 that specifies the precise orientation of the cylinder correction on the lens.
- Add: For patients with presbyopia, this power is included to be added to the distance prescription for reading.