Ophthalmic Prism 5.2014

Handling Prescriptions that Contain Prism

Phernell Walker, II, BSB, NCLC, ABOM National Speaker and Author

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

Reference:

Pure Optics

Phernell Walker, II, BSB, ABOM

www.amazon.com

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

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.

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.

Fresnel Press-On Prism

Temporary Prism Used to Determine if Prism will be effective Thin Flexible Membrane

28 Prism Dioptric Power

A 1.00 diopter displaces an light 1 centimeter at distance of one meter. 1 Prism Diopter = 1 (cm) / 1 (m)

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

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).

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 36 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).

Decentration to Create Prism

40 Decentration to Create Prism

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

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.

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.

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.

Meridian of Dioptric Power Chart

Degrees from Axis Percent of CYL. 0 0% 5 1% 10 3% 15 7% 20 12% 25 18% 30 25% 35 33% 40 41% 45 50% 50 59% 55 67% 60 75% 65 82% 70 88% 75 93% 80 97% 85 99% 90 100%

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

Neutralizing

When two prisms cancel each other, we call this net effect "Neutralizing or Canceling Prism".

Neutralizing Prism

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

60 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.

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.

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.

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.

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

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,

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

Base Up Prism

Base Down Prism

Prosthetic Eye

Ocular Sinister = 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?

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

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

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

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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

Reference:

Pure Optics

Phernell Walker, II, BSB, ABOM

www.amazon.com

120