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

The measurement that refers to the height at which the optical center of the lens sits is known as seg height. This measurement is crucial in ensuring that the optical center of a multifocal or progressive lens aligns properly with the wearer's pupil, providing optimal vision and comfort. When fitting eyewear, determining the seg height allows the optician to place the lens's optical center at the correct point relative to the wearer's line of sight, which is particularly important in bifocal and progressive lenses where different lens powers are utilized.

Frame width relates to the overall size of the eyewear frame and does not specifically address the position of the optical center. Pupil distance, while also an important measurement, refers to the distance between the center of the pupils, which is relevant for aligning lenses with the eyes but does not indicate the height of the optical center. Vertex distance refers to the distance between the back surface of the lens and the front of the eye, which impacts the effective power of the lens but, again, does not define the height of the optical center. Understanding seg height is essential for ensuring that vision correction is effective and comfortable for the wearer.