Ophthalmic lens design requires careful adherence to geometric principles to ensure visual comfort and clarity. Precision in locating specific points on the lens surface is paramount, as minor deviations can compromise the wearer’s vision. Specialized lens designs, particularly those incorporating two distinct powers, rely on an exact understanding of their internal geometry. The specific placement of the focal point for the added power is a fundamental consideration for effective vision correction.
Understanding the Round 28 Segment
A Round 28 segment is a specific style of bifocal addition used in prescription eyewear to provide reading correction for individuals with presbyopia. The “Round” designation refers to the circular shape of the near-vision segment, which is fused into the lower portion of the main lens. The number “28” indicates the segment’s diameter in millimeters.
This round design, often called a Kryptok, is characterized by a visible curved line separating the distance and near portions of the lens. Unlike flat-top or D-segments, the curved top offers a slightly less noticeable dividing line. The primary purpose of this segment is to provide the prescribed “Add” power needed for close-up tasks like reading. The physical size of 28mm offers a relatively wide field of view for reading.
The Role of the Optical Center in Lens Design
The optical center (OC) is the single point on a lens through which light passes without being bent or deviated, introducing no prismatic effect. For a single-vision lens, the OC is ideally aligned directly with the center of the wearer’s pupil, ensuring the sharpest, most comfortable vision. This point is the reference for measuring the lens’s prescribed power.
Any ray of light that passes through the lens away from the optical center will encounter an induced prismatic effect. This phenomenon relates the amount of induced prism to the lens power and the distance from the optical center. If a patient’s line of sight is displaced from the OC, the lens acts like a prism, shifting the perceived image. In multifocal lenses, the near segment has its own optical center, separate from the main lens, creating a second point of zero prism for the reading addition.
Precise Location of the Optical Center on the Segment
The optical center of a standard Round 28 segment is located precisely at its geometric center. Because the segment is a perfect circle 28 millimeters in diameter, the optical center is found 14 millimeters from the circumference. This means the point of zero prism for the reading addition power is located exactly in the middle of the segment’s width and height.
For fitting purposes, the optical center of the segment lies 14 millimeters below the highest point of the segment’s curve, which is the line separating the distance and near powers. When an optician measures the segment height for the patient, they determine the vertical position of this top line relative to the pupil. The fixed location of the optical center 14 millimeters below that line influences where the patient naturally looks while reading. This fixed OC is inherent to the round segment’s design and delivers the add power without vertical prismatic deviation.
Why Misplacement Matters: Induced Prism and Visual Clarity
The precise positioning of the Round 28 segment is necessary because misaligning its optical center with the wearer’s reading gaze introduces unwanted prism, leading to visual discomfort. If the patient’s eye looks through a point significantly above or below the segment’s geometric center, a vertical prismatic effect is induced by the reading addition power. This unwanted effect forces the eye muscles to work harder to fuse the two images.
Common complaints from improper segment placement include eyestrain, headaches, and a disorienting “swimming” sensation. The effect is magnified in patients with high-powered prescriptions, where even a small displacement from the optical center creates a noticeable amount of prism. Proper fitting involves ensuring the segment’s optical center aligns with the patient’s typical reading visual axis, usually a few millimeters below the distance optical center. The fixed 14-millimeter depth of the segment’s OC makes accurate measurement of the patient’s near visual needs important for achieving comfortable vision.