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Module 31 |
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Module 31: |
Measuring Prism in Glasses |
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Section 3: |
Mixed Prism | ||
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Module 31 |
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Module 31: |
Measuring Prism in Glasses |
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Section 3: |
Mixed Prism | ||
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Measuring
a Mixed Prism Correction A
mixed prism correction is simply a combination of vertical and
horizontal prism correction in the same lens. Recognizing
the presence of mixed prism correction in a pair of glasses As
discussed in the previous Sections, the presence of a prism correction can
be easily recognized when performing manual lensometry.
If the lensometer port is lined up with the point in the lens
through which the patient views, then the intersection of the mires must
center with (or be close to) the target of the lensometer.
This condition would
indicate that there is no
prism correction.
The presence of a horizontal prism correction would be indicated by a horizontal shift of the mires away from the lensometer target.
The
presence of a vertical prism correction would be indicated by a vertical
shift of the mires away from the center of the lensometer target.
As you might guess, the presence of a mixed prism correction would be indicated by both a vertical and a horizontal shift of the mires away from the lensometer target.
Measuring
the mixed prism correction In
order to accurately measure the prism correction, you will need to mark
each lens at the point through which the patient views. Be sure that the
glasses are fit properly to the patient’s face before you make your
marks. As discussed in
previous Sections, if the lens has a flat-top bifocal, you can usually
assume that this point is near the center of the seg line and just The
figure below illustrates an example of a mixed prism correction.
Let
us assume that this is a right lens that we are measuring.
From just a glance at the image we know that we are dealing with
a combination of base-up and base-in prism.
The intersection of the mires is above the center target of the
lensometer, indicating base-up prism.
The intersection of the mires is also to the right of the center target,
indicating base-in prism.
If this was a left lens, then the horizontal prism direction
would be base-out. The
amount of vertical prism can be measured by determining how far the
intersection of the mires is above the lensometer target, by means of
the diopter scale.
In
the above illustration, the intersection is 2 scale units above the
target, indicating
2 diopters of base-up prism correction. We
now rotate the scale, using the knob of the eyepiece, so that the
horizontal distance can more easily be measured.
As
you can see in the figure below, the intersection lies 2 units to the
right of the target,
indicating that there are 2 diopters of base-in (remember that this is a
right lens) prism correction.
Depending
upon the make of the lensometer, you will not always have to rotate the
scale in order the accurately visualize the scale distance from the
target to the intersection. Keep
in mind that the lens stage must not be moved up or down when measuring
the other lens. If
both lenses show base-up prism, or if both lenses show base-down prism,
then adjust the lens stage up or down so that one of the lenses shows no
vertical prism. If
there is astigmatism present, you may have to focus “in-between” the
single line mire and the triple line mire in order to locate the
intersection of the mires. If the axis of the astigmatism is oblique, the mires will be at an angle, making it more difficult to estimate the distance of the intersection from the center target along the vertical axis and the horizontal axis, as illustrated below.
Mixed
prism correction that is off the scale You may encounter a mixed prism correction that is off the scale of your lensometer, illustrated below.
If
the intersection of the mires is off the scale vertically, you will need
to use a loose prism in front of the glasses lens with the base oriented
in the opposite direction of the vertical prism orientation
in the glasses to bring
the intersection back to a position where it can be read from the scale.
Be
sure to add the power of the loose prism to the scale reading power to
arrive at the correct prism power.
If
necessary, a loose prism can be used in a horizontal base orientation to
bring the intersection into the field of the horizontal scale, as
illustrated below.
Test Yourself
What is are the prism corrections in the right and left lenses illustrated below?
Right lens
Left lens
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