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128 Ophthalmic Lenses

7. The position of the segment top is usually specified in millimeter from
the marked straight line to the lower eye wire of the frame where the
bevel of the lens will fit into the groove (Fig. 10.31).

Fig. 10.31

VERIFICATION OF SEGMENT HEIGHT
Subjective determination is the most accurate means of determining the
segment height and of assuring the wearer that the bifocal segment will be
positioned properly (Fig. 10.32).
1. Ask the subject to stand and look straight ahead.
2. A card is held over the lower half of the frame with its upper border
coinciding with the proposed segment position.
3. The subject should be able to see the floor over the top of the card at a
distance of 15 feet.
4. Now ask the subject to look down at the reading material. The card is
held over the upper portion of the frame with its lower border at the
level of the proposed bifocal segment.
5. The reading material should be visible without major readjustments of
the subject’s posture or reading material.

Fig. 10.32