OpenOptix ABO Study Guide - Laramy-K Optical

Prism Thinning
Because of their design, the powers of a progressive lens vary from the top of the lens to
the bottom of the lens, leading to differences in thickness. To reduce the difference in
thickness, when the lenses are ground to specification, the lab will often incorporate
prism. This process is referred to as equi-thinning, prism thinning, or yolk prism. The
rule of thumb is to use 2/3 the add power in base down prism, however many labs employ
more sophisticated software that can take into account the frame size, power,
astigmatism, and segment height when making a determination for the amount of prism.
It is a good idea to establish your preferences for equi-thinning with your lab up front
then make alternate specifications should circumstances dictate. When changing one lens
it is also important to inform the lab of the amount of prism present in the current lens so
no imbalance is produced.
Progressive Design Generations
The Varilux design is the basis of all modern designs and as such we use their generation
to describe all progressives.
1 st Generation Designs: Outlined above; spherical distance zone, using a progression of
conic sections to arrive at a spherical near. The lenses were rotated nasally to allow for
inset. Sometime between the first and second generations the ideas of splitting the
intermediate zones into nasal and temporal to create asymmetry was employed.
2 nd Generation Designs: This generation moved away from spherical distance and near
zones to aspheric zones which allowed the aberrations to be managed slightly better by
moving some of the intermediate zone abberations into the distant periphery and some of
the aberrations down into the near periphery. This created a wider intermediate zone that
was more useable. This generation also focused on a concept called orthoscopy.
Orthoscopy is minimizing distortion, mainly in this case along horizontal and vertical
lines. This was accomplished by focusing on keeping the horizontal prismatic effect
from varying along the periphery both nasally and temporally, as well as keeping the
vertical prismatic effect from varying along the distance and near zones.
3 rd Generation Designs: This generation of progressive lenses was the first to employ
multi design principles in a series of progressive lenses. In the past the design was fixed
for the entire progressive series. This meant that the design was chosen to be either soft,
aberrations distributed throughout the entire lens, or hard, aberrations kept in the
periphery of the progression zone. At this time there were different designs on the
market with each manufacturer focusing their attention towards eliminating or managing
certain aberrations better than others. These design philosophies can be thought of and
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