Topic 5: Bifocals Flashcards
name the 3 types of bifocals
e-line (executive/franklin), d-seg, round seg
describe e-line bifocal
consists two separate lens where bttom half bears the reading power
describe d-seg bifocal
aka flat top, has (1) main body containing DIST RX & (2) small plus lens containing near ADD. (1) and (2) add up when wearer looks through (2)
28mm most common diameter, only blended style in sg by shamir
describe round seg
(1) main body containing DIST RX & (2) small plus lens containing near ADD. (1) & (2) adds up
varying diameter of 25-28mm, older bifocal, blended styles have cospicuous seg border
explain oc positions for the 3 bifocals
e-line: segment oc same as DIST oc
d-seg: segment oc 5mm below segment top
round seg: segment oc in the middle of segment
define image jump
(what) the Sudden Upward displacement of an object, (when) as the wearer’s visual axis crosses the segment top entering into the segment area. (why) this is due to the abrupt change in prismatic effect @ segment top (where)
further explain image jump
independent of DIST RX
*always BD prismatic effect. Calculated with Prentice rule
most prominent with round seg. the bigger the segment, the further the seg oc from seg top hence bigger image jump (P=cF, where c is seg radius)
less image jump in d seg than round (P=0.5F)
none in e-line: as wearer glances down towards seg, lesser & lesser prism. at the seg top where dist OC is, PE is zero. glancing further down entering the seg area, PE starts again slowly. hence no SUDDEN change in PE
state default seg height
from lower limbus to lowest point of frame
describe factors when modifying seg height
(1) height: lower seg ht for taller (>1.8m) people - downward glance is of greater degree hence normal bifocal line will interfere
(2) posture: lower seg ht for chin up posturers - seg interferes with dist vision
(3) occupation: lower seg ht for those who work outdoors - seg not needed most of time
(4) child: seg ht shld be as high as lower PUPIL margin - may unconsciusly use dist portion for near work
(5) seg style: slightly higher for round (1mm higher seg) than in dseg or eline - so that useful bottom half portion can be reached faster
(6) existing seg ht: segment position with reference to lid margin to be repeated in new bifocal - eg. 2mm lowered from limus from old bifocal to be repeated
(7) VD & PT: excessive VD results in “high” default seg - if vd cannot be reduced lower seg ht, excessive PT results in “low” default seg - higher the seg ht
explain dis/advantages of eline
:) - wide field of view for near task, no image jump. good for architects/engineers who look at big drawings at near
:( - chips easily, heavy, line presence, accumulation of dirt at ledge
explain dis/advantages of round seg
:) - cosmetically nicest due to thin seg edges, lighter than eline, cheapest
:( - greater excursion of eyes, image jump increases with seg diameter, smallest FOV
explain dis/advantages of dseg
:) - cosmetically nicer than eline, lighter than eline, more advanced lens design where seg oc is closer to NVP
:( - line presence, accumulation of dirt at seg top, smaller reading area compared to eline
