Page 134 - PowerPoint 演示文稿

124 Ophthalmic Lenses

ordinary round bifocal. Since the blending is wider than the actual
demarcation line, the bifocal may require either slightly higher placement
of the top of the segment or greater depression of the gaze for reading. The
manufacturer recommends fitting this bifocal 1 mm higher than other types
of bifocals. The invisible bifocals only cater to the wearer’s vanity. Some
bifocal wearers are ready to accept the blurred area and the comparatively
small reading field in order to avoid the stigma of wearing bifocals.
BIFOCAL FITTING

Bifocal segments must be positioned so that the distance and near positions
of the lens provide adequate fields of view for distance and near vision
respectively. The fitting position of the bifocal segments should be decided
according to the purpose for which they are to be worn. However, it is
necessary to consider the positioning of segments in vertical and horizontal
meridians. So while fitting the bifocal into a spectacle frame three factors
must be considered:
1. Segment Inset
2. Segment Width, and
3. Segment Height.

Segment Inset
Segment inset is specified as the difference between the subjects distance
PD and near PD. In the normal range of PDs, the near PD for a reading
distance of 40cms is 4mm less than the distance PD. Segment inset, therefore,
is usually specified as 2mm for each lens. There are two reasons for insetting
bifocal segment – to ensure that the subject’s line of sight will go through
the segment at its optical centre and to ensure that the reading fields for
the two segments will coincide with one another.

Segment Width
Different segment widths are usually available for straight top shaped
segments in 22, 25, 28 and 35 mm widths. Some manufacturers also provide
this option in round shaped segments. E-style bifocal encompasses the full
width of the lens, so it is obvious not to specify segment width when this
lens is prescribed. Given a choice wider segment widths are preferred.
Segment Height
In the vertical meridian, bifocal lenses prescribed for general purpose use
are usually mounted before the eyes so that the segment top is tangential
to the lower edge of the iris (Fig. 10.24A) .In most cases the position of the
lower edge of the iris also corresponds with the line of the lower eyelid
when the head is held in the primary position. This is usually normal for
great majority of bifocal wearers and is certainly the safest position for the