What measurement refers to the height at which the optical center of the lens sits?

The height at which the optical center of the lens sits is referred to as the seg height. This measurement is essential in ensuring that the optical center aligns correctly with the wearer's line of sight, particularly in multifocal or progressive lenses. Proper seg height placement is crucial for optimal vision and comfort, as it allows for clear sight through the intended area of the lens when the wearer is looking straight ahead.

Frame width refers to the overall width of the eyeglass frame and does not specifically address the positioning of the lenses in relation to the eyes. Pupil distance measures the distance between the centers of the pupils and is important for aligning lenses correctly but does not indicate the height of the optical center. Vertex distance is the space between the back surface of the lens and the front of the eye, which can affect magnification and other visual factors but does not pertain to the height of the optical center itself.