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8/18/2019 Contour and symmetry
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Contour plot is most common method used to represent PALs
It describes surface power of the front surfae of the lens
Identifes zones of constant surface power indicated by contour lines
It also show size of the distance, intermediate and near zone as well as the
extent and gradient of peripheral aberrations
2 types
1 Isosphere contour plots2 Isocylinder contour plots
Isosphere
! "epresents distribution of spherical power across the lens! It di#ides the lens along lines of e$ual dioptric #alues! %ach lines&shade represents increasing le#el of power at gi#en inter#al! Can easily describe location and size of near #ision zone
Isocylinder
8/18/2019 Contour and symmetry
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! 2 dimensional map that di#ides the lens into ranges of cylinder le#els
throughout the lens! 'oft design ha#e isocylinder plots located further apart! (ard design ha#e greater number of lines closed together
'ymmetrical and Asymmetrical design
'ymmetric design
In early of its generation, it employ single design that can be used as )left* or+right* lenses, with -, IP and .P are in #ertical order/ 0his causes problem in
accommodation as both eyes con#erged/
Its later generation was then rotated 1 degrees in 1 direction to create a +right*
lens and about 1 degrees in the opposite direction to create +left* lens/
isad#antages causes disruption of binocular #ision as the wearer gazed
laterally across the lens, since the astigmatism and power di3ered between the
nasal and temporal sides of the distance zone/ 0he binocular 4eld of #iew will be
se#erely restricted
8/18/2019 Contour and symmetry
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Assymmetric design
Amount of astigmatism can be ad5usted independently/
It reduces disruption in binocular #ision and pro#ides better o#erlap between the
distance, intermediate and near #iewing zones of the right and left lense
resulting in larger binocular 4eld of #iew
isad#antages 6nwanted astigmatism may be higher at the nasal side/ 0hus,
di3erences in prism, magni4cation, and power may still exit between
corresponding points on the two lenses as eyes mo#e across them