Segment height is a precise measurement used in prescription eyewear manufacturing. This vertical distance determines the exact placement of the reading or intermediate vision correction within a lens. Accuracy is foundational for achieving clear vision and visual comfort. If the segment height is incorrect, the specialized zones will not align with the wearer’s natural line of sight, leading to visual strain and discomfort.
What Segment Height Is
Segment height (Seg Ht) is a measurement taken in millimeters that specifies where the multifocal portion of a lens begins. This measurement is required for lenses that correct vision at multiple distances, such as bifocals, trifocals, and progressive addition lenses (PALs). Multifocal lenses divide the lens into different power zones, unlike single-vision lenses which use one prescription across the entire surface.
For progressive lenses, the segment height marks the vertical distance from the bottom of the lens to the fitting cross. This cross is the point directly in front of the center of the pupil in the wearer’s primary gaze. It establishes the distance prescription location and initiates the gradual corridor of increasing power for intermediate and near vision. For traditional bifocal lenses, the measurement is taken to the top line of the visible segment that holds the near-vision prescription. The segment height is unique to each individual’s eye-to-frame fit and cannot be universally applied.
The Process of Measuring Segment Height
Measuring the segment height must be performed while the patient is wearing the specific frame, ensuring it is adjusted to its final, comfortable position. The optician instructs the patient to stand or sit with a relaxed, natural posture, looking straight ahead at a fixed point at eye level. Maintaining this normal head and body position is important, as changes in posture can alter the eye’s vertical position relative to the frame.
The optician uses a specialized ruler or a digital measuring device to determine the distance. For progressive lenses, a dot is marked precisely on the lens directly over the center of the pupil while the patient is in the natural, primary gaze position. The vertical distance, measured in millimeters, is taken from this pupil center mark down to the bottom inner edge of the lens. Advanced digital systems, which often involve capturing an image or video, allow for highly accurate, sub-millimeter measurements, accounting for slight frame wrap or tilt.
Consequences of Inaccurate Segment Height
An error in segment height fundamentally misplaces the power zones, causing immediate visual problems and physical discomfort. If the measurement is too high, the intermediate and near-vision zones encroach upon the top portion meant for distance viewing. This forces the wearer to constantly tilt their chin down to look through the clear distance zone, leading to blurriness and potential neck strain.
Conversely, a segment height measured too low causes the wearer to struggle to access the reading or near-vision zone. The person must continuously tilt their head back, raising their chin high to bring the near segment into their field of vision. This unnatural head posture can result in chronic headaches, neck tension, and visual fatigue. Furthermore, an incorrect segment height can contribute to the “swimming” or wobbling sensation common with progressive lenses, often requiring the lenses to be replaced entirely.