For bifocal lenses, which reference point is often used for measurement?

The lower limbus is often used as the reference point for measuring bifocal lenses because it is a consistent anatomical landmark that assists in ensuring proper placement of the reading segment of the lens. When fitting bifocal lenses, it is vital that the segment aligns correctly for optimal visual performance at near vision tasks.

Using the lower limbus as a measurement point takes into account the natural position of the eye and how it relates to reading distance. Given that the reading portion of a bifocal lens is positioned to provide maximum comfort and efficiency for near vision, aligning it with the lower limbus helps ensure that the segment is appropriately placed relative to where the wearer typically reads.

Other reference points, while considered in optical measurements, may not provide as reliable or standardized a positioning for bifocals compared to the lower limbus, which maintains a uniform baseline across different individuals.