CLE 170 – Physiological Optics
Notetaker: Nilou Masheli
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Date: 04/ 25/2013
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We have been talking about prisms and how they work and how they can affect eye
positions, both monocularly and binocularly.
A bi-convex lens can be thought of as two prisms stacked together base to base (this is
due to the nature of plus lenses that have their thickness in the middle)
A bi-concave lens can be thought of as two prisms stacked together apex to apex (minus
lenses have their thickness on the edges)
All ophthalmic lenses have some amount of prism inherited in them by the way they are
manufactured
 If patient looks through the top part of a plus lens, it is as if they are looking through
some amount of base down prism
 If patient looks through the bottom part of a plus lens, it is as if they are looking
through some amount of base up prism
 If patient looks through the top part of a minus lens, it is as if they are looking
through some base up prism
 If patient looks through the bottom part of a minus lens, it is as if they are looking
through some base down prism
 Patient does NOT experience any prismatic effect if they look at the center of the
lens, specifically the optical center of the lens (this is one of the properties of the
optical center)
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Prentice’s rule: must memorize the equation
 Used to determine how much prism a patient experiences when they are not looking
through the optical center of a lens
 P is the amount of prism in prism diopters
 F is the power of the lens (power in diopters)  we do NOT plug in a +ve or –ve sign
 d has its units in centimetres (measured from the optical center to the point the patient
is looking at)  d= zero when patient looks through the optical center of the lens.
This explains why prismatic effect is not induced when one looks through the optical
center of the lens).
 The formula does not tell us any information about the direction the image is
shifting, but purely calculates the magnitude of the shift
 If P is really big, it takes a little movement from the optical center (d) to induce a
large amount of prism (they are directly proportional to each other)
 F refers to the power in the optical cross of a lens  for example, if patient is looking
though a pair of glasses that have a different power in the 90th and 180th meridian,
then when pt looks horizontally, they will ‘use’ the power in the horizontal meridian
(180th). If patient looks vertically, they will ‘use’ the power in the vertical meridian
(90th).
 In-class Demo:
Suppose patient is looking through this plus lens  all the
power is vertical
CLE 170 – Physiological Optics
Notetaker: Nilou Masheli
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The arrow is pointing to the optical center of this lens
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Case 1: if the patient looks through the optical center of
the lens, zero prism is experienced
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Case 2: if the patient is looking above the optical center
of the lens, they’d be looking through the top part of
the lens. From a side profile of the lens, one can
conclude the patient is looking through a base down
prism (the arrow is pointing to the base of the prism)
and the amount of prism induced depends on how far
they are looking through from the optical center.
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Case 3: if the patient is looking below the optical
center, then they’d be looking through a base up prism
(squiggly arrow is pointing to the base of the prism).
Minus Lenses Demo:
- Case 1: If patient looks above the optical center of a
minus lens, they’d be experiencing base up prism.
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Case 2: if patient looks below the optical center of a
minus, they’d be experiencing base down prism.
Date: 04/ 25/2013
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CLE 170 – Physiological Optics
Notetaker: Nilou Masheli
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The large circle represents patient’s face
The triangle represents the patient’s nose
 Because the patient is looking horizontally, it does
not matter which eye we work with
After choosing an optical center, use triangles to draw
the plus lens
The patient is looking through the ‘left’ triangle, which
has its base away from the nose (base shown with a
couple of squiggly lines). Thus, Base out.
Date: 04/ 25/2013
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Nose
Pt’s eyes looking
nasally
Arbitrary chosen
optical center
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We measure the PD so when fitting glasses, we can put the optical center of the lens right
in front of the pupil. This ensures that patient is not looking through any prism.
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Example:
 Since the patient is looking vertically, then my
choices are base up or base down
 Since they are looking below optical center, then
the pt is experiencing base down prism.
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Clicker Question:
 Answer: 1

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Prism Position:
Can be orientated 360 degrees around pt’s eyes
Many times, we don’t specify the degrees. But, it is understood that a base up prism is at
base positioned at 90 degrees
CLE 170 – Physiological Optics
Notetaker: Nilou Masheli
Date: 04/ 25/2013
Page4
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For example, prism applied at 210 degrees in OD translates into:
 Patient experiencing a little base out prism effect
 Patient experiencing a little base down prism effect
 This is an example of an obliquely positioned prism
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Part A of this diagram shows a high hyperope, who is
looking through the optical center of the lens when
viewing at distance. They are converging their eye and
are experiencing prismatic effect, specifically base out
OS and OD  Patient experiences a total base out
(additive). This causes the eye to turn in even more
(i.e. more convergence required)
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Part B shows a myopic patient, who while converging will experience base in prismatic
effect OS and OD. This gives some convergence bonus to the myope
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CL do not induce prismatic effect because they move with the eye
NOTE: Dr. Goodfellow did not go over the solutions to the following 4 clicker questions.
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Clicker Question:
 Answer: 1
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Clicker Question:
 Answer: 3
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Clicker Question:
 Answer: 1
CLE 170 – Physiological Optics
Notetaker: Nilou Masheli
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Clicker Question:
 Answer: 2
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An eye is 4 mm nasal to the optical center = optical
center is 4 mm temporal to the eye
Date: 04/ 25/2013
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Fresnel Prism:

The size of the prism does not really matter. The apical angle and the index of
refraction of the prism are the two important factors

Fresnel Prism allows one to get a large amount of deviation without introducing
thickness to patient’s glasses.

It is applied to the ocular surface of glasses

Bigger stripes on these prisms hint to the fact that more prism power is present.