Lesson 14 - Prisms and Decentration

Question 1

Apex

Answer 1

The tip of a triangle when drawing the prisms of plus and minus lenses

Question 2

In what directions does an image shift when light moves through a prism?

Answer 2

Towards the apex

Question 3

Prism diopter

Answer 3

A prism diopter is a unit that describes the power of a prism to produce a one-centimeter deviation in a ray of light at a distance of one meter from the lens.

Question 4

Slab off

Answer 4

To correct the client from seeing double from a vertical imbalance, you’ll use slab off prism. Slab off is always done on the most minus, or least plus lens, in the vertical meridian.

A slab off prism is something that you have to specify to the lab that you want and order it from the lab. It is ground using a bi-centric grinding process and added onto the lens surface by computer to create the effect of a larger base-up prism to match that of the right lens.

Question 5

Reverse slab off

Answer 5

Reverse slab off is a pre-manufactured, semi-finished molded lens made with base down prism in the bifocal segment rather than the regular base up slab off. In other words, it is applied to the most plus, or least minus lens, in the vertical meridian. The advantage of these is faster turnaround time from the lab.

Question 6

To avoid an induced prism, which is an especially serious problem when your client has high-powered lenses, you’ll want to what?

Answer 6

To avoid an induced prism, which is an especially serious problem when your client has high-powered lenses, you’ll want to make sure your client’s pupils are at or close to the optical center of the lenses. That’s the point in each lens where the light will go straight through the lens and not bend.

Question 7

Prentice’s Rule

Answer 7

Prism diopter (Δ) = Power of the lens (F) x the decentration distance in centimeters (dcm) between the optical center and the center of the pupil.

shortened formula: Prism = F x dcm

F is the power of the lens.
d is the decentration distance in centimeters.

Question 8

One use of Prentice’s formula is to calculate induced prism. Induce prism is what? How do you solve the following example?

Example:
Brad just got his new glasses, and the optical center is misaligned 2 millimeters nasally (or .2 centimeters) in each eye. How much induced prism does he have in each eye?

Here is Brad’s prescription:
O.D. +5.00 sph
O.S. -2.50 sph

Answer 8

Induced prism is the amount of prismatic effect a patient will experience if the lenses are not cut with the optical center at the patient’s PD.

Example:
Brad just got his new glasses, and the optical center is misaligned 2 millimeters nasally (or .2 centimeters) in each eye. How much induced prism does he have in each eye?

Here is Brad’s prescription:
O.D. +5.00 sph
O.S. -2.50 sph

For right eye
Prism = F x dcm
Prism = 5.00 x .2 cm
Prism = 5.00 x .2 = 1Δ

For left eye
Prism = F x dcm
Prism = 2.5 x .2 cm
Prism = 2.5 x .2 = .5Δ

You’ll indicate the direction of this effect according to the direction of the base. So it will be base out, base in, base up, or base down.

The right eye (OD) is looking through the part of the minus lens in which the base of the prism points toward his nose. This means that he’s looking through a base-in prism

The left eye (OS) is looking through the part of his plus lens in which the prism base points toward his temple. So on the left side, he’s looking through a base-out prism.

Brad’s right eye is getting a 1 prism diopter base-in effect because his right pupil is now looking through the part of the prism where the base is facing his nose, and his left eye is getting a .5 prism diopter base-out effect since his pupil is now looking through the part of the prism where the base is toward his ear.

Question 9

Alex just got a new pair of glasses. Their PD is 60 mm. The distance between the lens centers of the glasses is 64 mm. OD: +3.00 OS: -2.50. What is the induced prism in their right eye?

Answer 9

Alex just got a new pair of glasses. Their PD is 60 mm. The distance between the lens centers of the glasses is 64 mm. OD: +3.00 OS: -2.50. What is the induced prism in their right eye?

Remember:
Prism= (F x dmm)/10, where F = power of the lens and d= decentration distance of the lens in millimeters

OD is the right eye
OS is the left eye

Answer:
Prism= (3 x 2mm)/10 = .6Δ

Question 10

Let’s say you’re cutting down lenses for a client, and there’s a big difference between the client’s PD and the geometric centers of the frame the person chose. (This occurs if the frame they want is really too large, but they insist.) In this case, you may have to figure out how much to decenter the lenses so you won’t induce prism in the new glasses.

How do you figure out how much to decenter the lenses so you won’t induce prism in the new glasses?

Answer 10

First, you’ll need to calculate the difference between your client’s PD and the frame’s geometric center. Assume that the person’s pupils are exactly the same distance apart. When the eyes are symmetrical, you know that the distance from the pupil of one eye to the geometric center of the lens for that eye is the same as for the other eye.

As an example, let’s consider a client named Janet. Let’s say that Janet’s PD is 60, and the distance between the geometric centers of her frame is 70 mm. The total difference between them is 10 mm. But that’s for both eyes. So the amount of decentration for each eye to the geometric center of that lens should be half this number, which is 5 mm.

Question 11

If the doctor may want your client’s lenses to have a prism, and to achieve the correct prism, you’re going to need to decenter the lenses. How do you decenter a lens?

Answer 11

To decenter a lens, you’re going to move the optical centers of the lens away from the line of sight in order to create the prescribed prismatic effect.

Question 12

How to find the amount of decentration?

Find decentration for the following prescription:
O.S. -1.50 sph .5Δ BO (BO= base-out)

To achieve a base-out prism, you’re going to want to move the lens toward the client’s nose so they’re looking through the base-out part of the lens.

Answer 12

Prism = F x d
.5Δ = 1.50 x d (remember that d is the amount of decentration in centimeters)

(.5Δ/1.50) = d
.33cm = d

convert cm to mm
.33cm = 3.3mm

So you’ll want to decenter the lens 3.3 mm inward from the optical center. Of course, prisms can be vertical or horizontal. Just remember that the light bends toward the base, and the image will move in the opposite direction toward the apex.

Question 13

Imagine that you’re looking 5 mm (or .5 cm) above the optical center of a lens with this prescription:

+2.00 -0.50 x 180

How much verticle prism will you experience?

Answer 13

Imagine that you’re looking 5 mm (or .5 cm) above the optical center of a lens with this prescription:

+2.00 -0.50 x 180

How much verticle prism will you experience?

We’ll start with our optical cross. It has +1.50 on the vertical meridian and +2.00 on the horizontal meridian.

Prism = Power x decentration in centimeters
Prism = +1.50 x .5 cm
Prism = +0.75Δ BD

Question 14

Karen’s right lens is +2.00. How much will you need to move the optical center to create 1 prism diopter of base-out prism?

Answer 14

Karen’s right lens is +2.00. How much will you need to move the optical center to create 1 prism diopter of base-out prism?

Using Prentice’s Rule:
1 = 2 x dcm
.5 cm = dcm
5 mm = d

So, since it is the right lens, you will need to move the lens 5 mm to the right to create a base out (BO) prism.

Question 15

Let’s talk about Teddy, who has one amblyopic eye that’s always turned outward. He can’t use this eye, so the doctor will prescribe prism to make his eye look straight cosmetically. In this case, the doctor will prescribe what to produce a cosmetically corrected result?

Answer 15

In this case, the doctor will prescribe base-out prism for the eye to make the image move inward and produce a cosmetically corrected result.

Question 16

Let’s consider Betty. Both of her eyes turn too far inward, but only when she does certain tasks. In this case, the doctor wants to help her see more comfortably, so the doctor prescribes what?

Answer 16

In this case, the doctor prescribes a little base-out prism along with some eye exercises.

Question 17

What about Mary, whose eye muscles are weak? As a result, her eyes don’t converge well when she’s reading. In her case, the doctor may prescribe what?

Answer 17

In her case, the doctor may prescribe a base-in prism to relieve stress on her eyes and some eye exercises.

Question 18

What if a doctor wants to achieve a cosmetic improvement for Lucy, who has one eye muscle that’s too short, causing that eye to permanently turn inward?

Answer 18

In this case, the doctor can prescribe a base-in prism so it will appear to someone looking at Lucy that her eye is straighter.

Question 19

For a right hypertropia—that is, a right eye that turns upward— what is prescribed for that eye and what is prescribed for the other eye? If an eye is turned downward with two prism diopters of hypotropia, what is prescribed for that eye and what is prescribed for the other eye?

Answer 19

For a right hypertropia—that is, a right eye that turns upward—a base-down prism is prescribed for that eye, and an equal amount of base-up is prescribed for the other eye. The eye muscles are yoked, meaning that they usually move in the same direction by the same amount, so the amount of vertical prism is equally split between the two eyes. If an eye is turned downward with two prism diopters of hypotropia, one prism diopter of base-up prism is prescribed for that eye, and an equal amount of base-down prism is prescribed for the other eye. This is called splitting prism.

Question 20

Steve’s PD is 64, and the distance between the geometric centers of his frame is 70 mm. The total difference between them is 6 mm. But that’s for both eyes. What is the amount of decentration for each eye to the geometric center of that lens?

Answer 20

3mm

Question 21

How to calculate and correct vertical imbalance?

Example:
O.D. +6.00
O.S. +1.00

For the right eye, calculate how much vertical imbalance the patient would have when reading if the pupil moves 5 mm downward below the optical center.

For the left eye, calculate how much vertical imbalance the patient would have when reading if the pupil moves 5 mm downward below the optical center.

Answer 21

When you calculate vertical imbalance, you should draw the lenses to help you figure out these problems. Each type of lens should be drawn vertically: triangles with bases touching for plus lenses and triangles with apexes touching for minus lenses.

Example:
O.D. +6.00
O.S. +1.00

This example has plus lenses for each eye, so draw both the right and left lenses like the + lens illustration with the bases touching.

If the patient looks downward to read, they would be looking through the base-up portion of the prism, and the image would move toward the apex.

For the right eye, calculate how much vertical imbalance the patient would have when reading if the pupil moves 5 mm downward below the optical center.

Answer:
Prism = Power x Distance(in cm)
6 x .5 = 3 prism diopters

For the left eye, calculate how much vertical imbalance the patient would have when reading if the pupil moves 5 mm downward below the optical center.

Answer:
1 x .5 = .5 prism diopters

So the right eye would see the image 3 prism diopters down, and the left eye would see the image .5 prism diopters down. There is a vertical imbalance of 2.5 prism diopters (the difference between the two eyes). The result would be that the images for each eye would not be in the same position and the patient now sees double!

Think about how you would solve this issue.

Question 22

Mary has an amblyopic right eye that turns inward. Dr. Smith will prescribe a prism to make her right eye appear cosmetically straight. What type of prism will she prescribe for Mary’s right eye?

Answer 22

Base-in

Question 23

John’s right lens is -2.00. How much will you need to move the optical center to create 1 prism diopter of base-out prism?

Answer 23

John’s right lens is -2.00. How much will you need to move the optical center to create 1 prism diopter of base-out prism?

Using Prentice’s Rule:
1 = 2 x dcm
.5 cm = dcm
5 mm = d

So, since it is the right lens, you will need to move the lens 5 mm inward towards the nose to create a base-out (BO) prism.