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Handling Prescriptions that Contain Prism
Phernell Walker, II, BSB, NCLC, ABOM
National Speaker and Author
Copyright 2003-2016, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Brief Biography:
Phernell Walker, II, BSB, NCLC, ABOM
Master in Ophthalmic Optics
Bachelor of Science in Business
Associate of Science in Opticianry
ABO Certified
NCLE Certified
Author of text-book, Pure Optics
Beverly Meyers Achievement Award in Optics
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Reference:
Pure Optics
by
Phernell Walker, II, BSB, ABOM
www.amazon.com
3
Part I Topics:
Define Ophthalmic Prism
Prescribed vs. Accidental Prism
Prism Symptoms
Calculate Prism
Redistribute Prism
Prism Verification
Calculate Bi-centric Grinding
Verify Bi-centric Grinding
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Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
Phoria vs. Tropia
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
Adverse Prism
Adverse prism is
prescribed to
strengthen a weak
rectus muscle.
The apex is placed
over the weak rectus
muscle, causing the
eye to turn toward
the image.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Therapeutic Prism
Therapeutic prism is
prescribed to relieve
the visual disturbance.
The base of the prism is
prescribed over the
weak rectus muscle,
which displaces the
image in the same
direction of the eye.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Fresnel Press-On
Prism
Temporary Prism
Used to Determine
if Prism will be
effective
Thin Flexible
Membrane
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Prism Dioptric Power
A 1.00 diopter displaces an light 1 centimeter at distance of
one meter.
1 Prism Diopter = 1 (cm) / 1 (m)
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Prism Occurs
Patients will experience prism if:
the Major Reference Point (MRP) in the 180th meridian
does not coincide with the interpupillary distance (PD)
a differential in the MRP in the 090th meridian
their eye rotates excessively behind the lens
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Avoiding Prism
Decentration (move from geometric center) is used by
every optical lab to avoid prism.
Decentration is calculated just before ophthalmic lenses
are edged (cut to the desired frames shape).
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Decentration to Avoid Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Direction of Decentration (In or Out)
If the patient’s PD is less than the sum of the “A” and
the “DBL” box measurements, the lenses will be
decentrated in (nasally).
If the patient’s PD is greater than the sum of the “A” and
the “DBL” box measurements, the lenses will be
decentrated out (temporally).
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Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Decentration to Create Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Decentration to Create Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Prentice Rule
The amount of prism experienced by the patient
can be calculated using Prentice’s Rule:
P = (hcm) (De)
P = prism
hcm = distance expressed in centimeters
De = dioptric power in the specific meridian
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Meridian of Dioptric Power
Calculate the exact amount of prism in a
given meridian (i.e. axis 180 and axis 090)
requires us to calculate the amount of total
power in a given meridian.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Meridian of Dioptric Power
Spherocylindrical lenses have only a percentage
of the total dioptric power present in each
meridian.
Consequently, each meridian has a different focal
length.
Either a meridian of dioptric power (MDP) chart
can be used or it can be calculated using a
formula.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Meridian of Dioptric Power Chart
The MDP chart can be used to determine the total
dioptric power of a lens in a specific 5 degree meridian.
Step 1:
Determine the difference from the prescriptions axis and
a given meridian.
Step 2:
Locate the difference on the MDP chart and multiply the
prescriptions cylinder power by the percentage
Step 3:
Algebraically add the amount in Step 2 to the
prescriptions sphere power.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Meridian of Dioptric
Power Chart
Degrees from Axis
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
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Percent of CYL.
0%
1%
3%
7%
12%
18%
25%
33%
41%
50%
59%
67%
75%
82%
88%
93%
97%
99%
100%
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
Example I:
Determine the dioptric power in the 060th meridian using the following:
Rx: +3.00 – 1.00 X 090
Step 1:
Compute difference in axis
090 – 060 = 030
Step 2:
Multiply cylinder power by ratio from MDP chart
-1.00 x 25% = -0.25 D
Step 3:
Add result to sphere power
+3.00 + -0.25 = +2.75 D
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Compounding Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Neutralizing
When two
prisms cancel
each other, we
call this net
effect
"Neutralizing or
Canceling
Prism".
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Neutralizing Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Compounding Vs. Neutralizing Prism
Example I:
Combine the following Prism:
OD: -1.00 -0.50 x 175, 3 Prism D. Base In
OS: -1.50 -0.25 x 005, 3 Prism D. Base In
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Answer: Compounding Prism
Example I:
Combine the following Prism:
OD: -1.00 -0.50 x 175, 3 D. Prism Base In
OS: -1.50 -0.25 x 005, 3 D. Prism Base In
Solution I:
3 D Prism B.I. + 3 D. Prism B.I. = 6 D. Prism B.I.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Excessive Base In and Out Prism
Excessive Base In or
Out prism will result
in the patient seeing
objects at a 180
degree slant.
Example:
A patient observes your
dispensing table at a
slant, the side that is
too high is the prism’s
base location.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Excessive
Base Up Prism
Excessive Base Up
prism will result in the
patient noticing that
vertical objects
appear shorter, the
floor slants downward
and that horizontal
objects appear
convex.
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Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
Excessive Base Down Prism
Excessive Base
Down Prism will
result in the patient
noticing that
vertical objects
appear taller, the
floor slants upward
and that horizontal
objects appear
concave.
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Redistributing Prism
Rx I:
OD: -5.00 DS, 3.50 BI
OS: -5.25 DS
OD: -5.00 DS, 1.75 BI
OS: -5.25 DS, 1.75 BI
Rx II:
OD: +2.50 DS, 4 BU
OS: +2.25 DS
OD: +2.50 DS, 2 BU
OS: +2.25 DS, 2 BD
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Redistributing Prism
Practice #1:
OD: -2.00 -0.75 x 085, 3 Prism B.U.
OS: -2.50 -0.50 x 094
Practice #2:
OD: +3.00 -1.25 x 010, 1.50 Prism B.O.
OS: +3.00 -1.00 x 165,
Practice #3:
OD: -5.25 -0.25 x 093, 2.00 Prism B.I., 0.50 Prism B.D
OS: -5.50 -0.50 x 082,
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Answers: Redistributing Prism
Practice #1:
OD: -2.00 -0.75 x 085, 1.50 Prism B.U.
OS: -2.50 -0.50 x 094, 1.50 Prism B.D.
Practice #2:
OD: +3.00 -1.25 x 010, 0.75 Prism B.O.
OS: +3.00 -1.00 x 165, 0.75 Prism B.O.
Practice #3:
OD: -5.25 -0.25 x 093, 1.00 Prism B.I, 0.25 Prism B.D
OS: -5.50 -0.50 x 082, 1.00 Prism B.I, 0.25 Prism B.U
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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O.S. Lens: Base In Prism
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Base Up Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Base Down Prism
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Prosthetic Eye
Ocular Sinister = Prosthetic Eye
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Calculate Amount of Prism Needed for a
Prosthetic Eye
Example:
OD: -4.00 DS
OC 27
OS: Balance (Prosthetic Eye) OC 21
P = (hcm) / (1 / De)
? = (.6cm) / (1 / 4.00)
? = (.6cm) / (0.25)
P = 2.40 D Base Down would be ordered in the OS lens
Why would we use BD prism in the OS lens?
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Bi-Centric Grinding (Slab-off)
Slab-off is the process of changing the amount of prism in
the reading area of a lens without affecting the prism in the
distance portion of a lens.
Methods of Slab-off:
Traditional Slab-off
Reverse Slab-off
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Traditional Slab-off
Rx Imbalance =
> 1.50 Diopters x 090th Meridian
Lens Selection =
Weakest Plus Power
Lens Selection =
Stronger Minus Power
Ground (Surfaced) = Base Up Prism x 090th Meridian
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Reverse Slab-off
Rx Imbalance =
> 1.50 Diopters x 090th Meridian
Lens Selection =
Strongest Plus Power
Lens Selection =
Weaker Minus Power
Ground (Surfaced) = Base Down Prism x 090th Meridian
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Conclusion
Prism is the DNA of every ophthalmic lens design.
All lenses are composed of a
series of interconnecting prisms.
This Lens Geometry allows us to
"MASTER LIGHT".
"These Lens Arrays can focus light, magnify images, demagnify images and even
capture light and suspend it within a given material provided that the laws of internal
reflection are met.
Phernell Walker, II, ABOM
Master Optician
Copyright 2006, excerpt from Pure Optics textbook by Phernell Walker, II, BSB, NCLC, ABOM
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Reference:
Pure Optics
by
Phernell Walker, II, BSB, ABOM
www.amazon.com
120