Bifocal lenses provide a solution for presbyopia, the age-related condition where the eye’s lens loses its ability to focus on near objects. These lenses contain two distinct zones: the upper portion corrects for distance vision, and the lower segment offers magnifying power for reading and close-up tasks. Accurate measurement of the segment height ensures the visual transition between distance and near is smooth and effortless for the user. If the near-vision portion is incorrectly positioned, the wearer may experience constant eye strain or be forced to adopt unnatural head postures.
Defining the Bifocal Segment Location
The bifocal segment height is the vertical distance measured from the lowest internal edge of the frame’s bezel to the top line of the near-vision segment. This measurement determines precisely where the reading power begins within the lens area. This vertical measurement is unique to the individual wearer and the specific frame chosen, as frame shape and how the frame sits on the face can significantly alter the required height.
The segment height should not be confused with the optical center height, which is a measurement related to the distance prescription’s optical center. For traditional lined bifocals, the goal is to position the dividing line so that the wearer looks over it for distance viewing and drops their gaze below it for reading. The standard fitting height for a lined bifocal is typically placed at the level of the wearer’s lower eyelid margin.
Essential Equipment and Setup
Accurately measuring segment height requires a fine-tipped, removable marker and a millimeter-graduated ruler or a specialized segment measuring device. A measuring gauge is often preferred over a standard ruler, as it is designed to align precisely with the frame’s lower edge to prevent parallax error.
Before any measurement is taken, the chosen eyeglass frame must be properly adjusted to the wearer’s face. This involves ensuring the frame sits comfortably and level, with appropriate pantoscopic tilt, which is the angle of the lens plane relative to the face. The wearer must then be positioned in a natural, relaxed posture with their head completely level, looking straight ahead at a distant target. The person taking the measurement must also be positioned at the same eye level as the wearer to eliminate vertical measurement distortions.
The Segment Height Measurement Process
The measurement process begins with the wearer looking straight ahead at a distant object at eye level, ensuring their eyes are in a primary gaze position. The measurer, positioned at the same height, then uses the fine-tipped marker to place a small, horizontal line directly onto the demo lens of the frame. For bifocals, this mark should align precisely with the upper edge of the lower eyelid, which serves as the target fitting height for the segment line.
Once the initial mark is placed, the glasses are carefully removed from the wearer’s face without disturbing the frame’s position. The next step is to measure the vertical distance from this temporary mark down to the deepest internal curve of the frame’s lower bezel. A millimeter ruler or segment measuring gauge is held vertically against the lens, with the zero mark aligned exactly at the bottom edge of the frame. The measurement, recorded in millimeters, is taken at the point where the marked line intersects the measuring tool.
It is important to understand the typical industry adjustments applied to this measurement. While the lower eyelid is the fitting reference, the final bifocal line is often set to be at or slightly below this point. For instance, some practices may use the center of the pupil as a starting point and then subtract a standardized amount, often 1 to 2 millimeters, to ensure the distance portion is not obstructed during normal viewing. The recorded measurement represents the segment height for that specific eye in that particular frame, and the process must be repeated for the second eye. Any discrepancy must be noted and used in the final lens fabrication to guarantee that the reading segment is placed at the correct height for each eye.