When measuring bifocal seg heights, draw a horizontal line on the lens at which landmark?

• A. At the level of the patient's lower eyelid margin
• B. At the level of the patient's pupil center
• C. 2 mm above the lower eyelid margin
• D. 1 mm below the lower lid margin

Answer: (D)

In bifocal measurements, the top edge of the reading segment is positioned relative to a stable facial landmark, the lower eyelid margin. Placing the horizontal line about 1 mm below that margin keeps the segment just beneath the lid in primary gaze, so it isn’t obscured by blinking or lid movement, while still being easily seen when the wearer looks slightly downward to read. This distance provides a practical balance: the reading portion is in the wearer’s lower field of view without sticking up too high and interfering with distance vision or being blocked by the lid.

Using the pupil center as a reference isn’t ideal because the pupil moves with gaze and lighting, so it doesn’t provide a consistent landmark for the fixed position of the bifocal segment. Placing the line exactly at the lid margin risks being covered by the lid, and placing it higher (above the lid margin) would position the segment too high for comfortable reading without lids interfering. Therefore, 1 mm below the lower lid margin is the standard landmark.