DISPENSING TIPS FOR BIFOCAL FITTING

DISPENSING TIPS FOR 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.

1.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.

2. 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.

3. 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 thesegment top in case of first time wearers. However, if the bifocal prescribed is mainly for near vision, then the segment top might be fitted little higher, say, midway between the lower edge of the iris and the lower edge of the pupil (Fig. 10.24B). If the lenses have been prescribed for some vocational purpose and are to be designed for only occasional near vision use, then the segment top might be fitted 2-3 mm lower than the normal (Fig. 10.24C) These suggested positions of the segment top assume that the head is held in the subjects primary position with the eyes viewing a distant object. But if a person who habitually holds his head high will need a lower segment height than a person who slouches a bit and keeps his head downward.

    A tall person who habitually keeps his head down may need a little higher segment top whereas a short height person who has to keep his head up may prefer a little down segment top. Past wearing habit of the subject should also be given importance while positioning the segment top. If a wearer is used to wearing bifocal segment up, fitting a lower segment in the new spectacle will create trouble for him to adjust immediately.

    Various kinds of dispensing aids have been proposed from time to time to assist in the fittingof bifocal lenses, but most practitioners obtain excellent and consistent results by simplymeasuring the segment height with a millimeter ruler. Ideally the segment height should beconverted into a segment top position in relation to the horizontal centre line of the frame.

A typical routine procedure for taking the measurement is:

1. Choose the final frame and adjust it to fit the subject correctly.

2. If the frame is without demonstration lens, attach a vertical strip of transparent adhesive tape to each eye of the frame to enable reference points to be marked.

    

            

3. Replace the frame on the subjects face and direct the subject to look straight into your eyes. If necessary, adjust the height of your chair so that your eyes are on exactly the same level as those of the subject.

4. Direct the subject to look straight into your open left eyes and using a fine tip marking pen, place dots at the same height as the lower edge of the subject’s right iris. This point often coincides with the line of the lower lid. Similarly, without moving his head, place a second mark in front of the subjects left lower iris margin

                            

5. Remove the frame and put the frame face down with dots coinciding with a straight line. Draw a straight line from one dot to another.

                      

6. Put back the frame on the face and verify.

                

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.

                    

VERIFICATION OF SEGMENT HEIGHT

    Subjective determination is the most accurate means of determining the segment height and ofassuring the wearer that the bifocal segment will be positioned properly.

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 withthe proposed segment position.

3. The subject should be able to see the floor over the top of the card at a distance of15 feet.

4. Now ask the subject to look down at the reading material. The card is held over theupper portion of the frame with its lower border at the level of the proposed bifocalsegment.

          5. The reading material should be visible without major readjustments of the subject’s posture or reading material.