What does the term 'segment depth' refer to in lens design?

Segment depth is a critical measurement in the design of multifocal lenses, particularly in segment or bifocal designs. It refers specifically to the longest vertical dimension of the segment used for near vision, which represents how deep the segment extends from the top edge of the segment to the bottom of the lens. This dimension is crucial because it influences how effectively the wearer can see close up through the lens segment without having to tilt their head excessively or strain their neck.

When determining the appropriate segment depth, opticians must consider the visual needs of the wearer, including their working distance for tasks such as reading or crafting. A segment depth that is well-suited for the individual’s activities ensures optimal vision and comfort. Proper segment depth is essential for balancing aesthetics and functionality, contributing to a seamless visual transition between distance and near vision areas in the lens.

While other aspects of lens design, such as the radius of curvature, vertical height of the lens, and overall thickness, are important in creating a fitting and effective lens, they do not specifically define the segment depth.