In measuring FT trifocals, where is the top of the near segment placed?

For FT (Flat Top) trifocals, the placement of the near segment is a critical factor in ensuring that the wearer has comfortable and effective vision for close-up tasks. The standard practice is to position the top of the near segment at the superior limbus, which is the border of the cornea and the sclera of the eye. This placement aligns the optical zones of the lens with the anatomical landmarks of the eye, providing the best ergonomic access to the near vision segment when a wearer looks down.

The superior limbus position ensures that when the wearer transitions from distance to near viewing, the near segment is easily accessible and positioned at a comfortable height for reading or other nearby tasks. This anatomical reference helps in facilitating eye movement and maintaining head posture during these activities.

Other placements, such as being at the lower pupil margin, one millimeter below the pupil margin, or at the lower lid margin, may not provide optimal positioning for visual performance and comfort. Each of those positions could result in inadequate positioning of the near vision segment, potentially leading to discomfort or strain during near vision tasks. Therefore, positioning the near segment at the superior limbus is considered the standard practice for maximizing the efficacy of FT trifocal lenses.