A progressive lens is a sophisticated multifocal design that provides a seamless transition between viewing distances, unlike traditional bifocals that have a visible line. This design eliminates the image jump, allowing for clear vision at far, intermediate, and near ranges within a single lens. Because progressive lenses correct vision for multiple distances, their prescriptions contain more details and measurements than standard single-vision prescriptions. Understanding the document requires deciphering the abbreviations and numbers that define the corrective powers and physical fitting requirements.
Decoding the Standard Vision Measurements
The first part of any progressive lens prescription addresses the patient’s distance vision needs using standard abbreviations. OD (Oculus Dexter) specifies the right eye, and OS (Oculus Sinister) specifies the left eye. Prescription data is often presented in a grid format, with the right eye information listed above the left eye.
The SPH (Sphere) column indicates the main lens power required to correct nearsightedness or farsightedness, measured in diopters (D). A minus sign signifies nearsightedness, while a plus sign denotes farsightedness. This SPH value serves as the base power for the distance viewing zone in the progressive lens.
The CYL (Cylinder) and AXIS columns are only filled if the patient has astigmatism, a condition where the cornea or lens is shaped irregularly. The CYL value specifies the additional lens power needed to correct the astigmatism, measured in diopters. The AXIS is a number between 1 and 180 degrees that indicates the precise orientation where the cylinder power must be applied. If these columns are blank or contain “PL” (Plano), no astigmatism correction is required.
Understanding the Addition Power
The defining feature of a progressive lens prescription is the Addition Power, abbreviated as ADD. This value represents the extra magnifying power needed for comfortable near and intermediate vision, correcting presbyopia. Presbyopia is the age-related loss of the eye’s ability to change focus up close due to the natural lens losing flexibility.
The ADD power is always a positive number, typically ranging from +0.75 D to +3.00 D, and is usually the same for both eyes. This power is physically integrated into the lens, starting near the center and gradually increasing as the eye moves down. This smooth increase creates the progressive corridor, which provides the intermediate and near viewing zones.
The level of ADD power influences the lens design and the size of the clear viewing areas. Higher ADD power, often needed as presbyopia progresses, requires a quicker change in lens curvature. This accelerated change can narrow the clear vision corridor and increase peripheral distortion. The specific ADD value is determined by measuring the smallest extra lens strength that allows the patient to read comfortably, typically at 40 centimeters.
Essential Measurements for Lens Fitting
Beyond the optical powers, a progressive lens prescription must include precise physical measurements to ensure the lens is fabricated and seated correctly in the frame. These measurements are crucial because slight misalignment can cause distortion or make the near-vision zone difficult to access.
The Pupillary Distance (PD) is the measurement, in millimeters, of the horizontal distance between the centers of the two pupils. For progressive lenses, PD is often measured for each eye separately (monocular PD) to ensure the optical center aligns perfectly with the pupil center for both eyes. This measurement is essential because the distance prescription zone is centered on this point.
The second physical measurement is the Fitting Height, sometimes called the segment height or fitting cross. This is the vertical distance, measured in millimeters, from the center of the pupil down to the deepest part of the lens within the frame. The Fitting Height determines where the progressive power corridor begins, aligning the distance, intermediate, and near zones with the patient’s natural line of sight. If the fitting height is too low, the wearer may have to tilt their head back to use the reading area. If it is too high, the reading area will be obscured.