Lesson 8 - The Spectacle or Glasses Prescription

Question 1

O.D.

Answer 1

Oculus dextrus, meaning right eye in Latin

Question 2

O.S.

Answer 2

Oculus sinister, meaning left eye in Latin

Question 3

Meridians

Answer 3

Imaginary lines that help us describe the location of the axis powers of a lens with a correction for astigmatism

Question 4

Spherical lens

Answer 4

In this type of lens, the power is the same in all directions.

Question 5

Spherocylinder lens (also known as a toric lens)

Answer 5

A lens that has an astigmatism correction and a spherical correction for either myopia or hyperopia, with the plane cut across the longest part of the sphere

Question 6

Pupillometer

Answer 6

A device for measuring the pupillary distance

Question 7

Pupillary distance (also called PD measurement)

Answer 7

The distance from the center of one pupil to the center of the other pupil (in millimeters). You’ll use this measurement when you order prescription lenses so you can ensure that the optical center of the lens is in front of the pupil when the person looks straight ahead while wearing the glasses.

Question 8

If a prescription only corrects for astigmatism, and a client isn’t nearsighted or farsighted, the prescription is written with the spherical number as ___ instead of writing 0.

Answer 8

If a prescription only corrects for astigmatism, and a client isn’t nearsighted or farsighted, the prescription is written with the spherical number as plano instead of writing 0. The surface curvature of the lens is either plano-convex or plano-concave

Question 9

The first number in a prescription is called what? What does it specify?

Answer 9

The sphere number and it specifies a single plus or minus power in all directions.

Question 10

What does it mean for the sphere power in a prescription to be either +/-?

Answer 10

A plus lens means that the eye is hyperopic or farsighted. A minus lens means that the left eye is nearsighted or myopic.

Question 11

The second number in a prescription is called what? What is it for? What does it specify?

Answer 11

The cylinder number and we use them if an eye has astigmatism. The cylinder power tells us the difference between the steepest axis of the eye and the flattest axis of the eye, which are generally separated by 90 degrees.

Question 12

What is written in a prescription if a patient does not have some form of astigmatism?

Answer 12

If there’s no astigmatism, you’ll see the abbreviation sph for sphere or D.S. for diopters of sphere after the number, as in these examples:

O.D. -3.00 sph
O.S. -4.00 sph

O.D. -3.00 D.S.
O.S. -4.00 D.S.

Question 13

By convention how is the cylinder component written?

Answer 13

By convention, the cylinder component is written with three digits. Example: .75 is written as 0.75. In the optical field, powers are measured in 0.25 diopter steps—for instance, 0.50, 5.25, and 4.75. You won’t see numbers like 7.31 or 9.45.

Question 14

The final number in a prescription is called what? What does it specify?

Answer 14

The third number is a prescriptions is called the axis or the axis of the astigmatism. This set of numbers describes the direction in which the person’s cornea is the flattest, causing the astigmatism.

Question 15

What does the x symbol before the axis of astigmatism value mean?

Answer 15

The x in our prescription denotes the word at. Example: O.D: +3.00 -2.00 x 090
O.S: -4.00 -0.75 x 110

Question 16

When a prescription has an add, it indicates that…

Answer 16

When a prescription has an add, it indicates that the prescription is for a bifocal, trifocal, or multifocal lens

Question 17

The total power of the lens through an add (also called the near portion of the lens, because it’s used for close vision) is equal to the…

Answer 17

The total power of the lens through an add (also called the near portion of the lens, because it’s used for close vision) is equal to the spherical power of the prescription in the distance portion of the lens plus the add power. (The astigmatism correction won’t change.)

Example: Let’s take a look at the numbers for the right eye (O.D., as you’ll recall) in this prescription.

O.D. -2.00 -1.00 x 134
O.S. +1.35 -0.75 x 035
Add +1.25

In this example, the total power of the lens the person will look through in the add portion of the right lens is:

-0.75

Question 18

In a trifocal lens, the midrange portion of the entire lens or the top portion of the segment is always…

Answer 18

In a trifocal lens, the midrange portion of the entire lens or the top portion of the segment is always half the power of the add power.
Example: trifocal prescription

-2.00 -1.00 x 090
Add +2.00
FT 7x028

The power of the midrange portion of the lens is equal to:

-1.00 -1.00 x 090

That’s because you’ll take half the add power (half of +2.00) and add it to the sphere number (which is -2.00). Again, you’ll keep the same cylinder and axis numbers.

Question 19

How to calculate the power in the near portion (lower portion of the segment) of a trifocal lens?

Answer 19

Add the full add power to the sphere power.

Question 20

Lensometer

Answer 20

A manual or automatic instrument used to find the prescription power in a lens when you don’t have a written prescription.

Question 21

Sometimes the PD measurement will be written on the prescription so the pupils will not match the optical centers of the lenses. Why?

Answer 21

Because the doctor wants to prescribe a prismatic effect for the patient. So if the PD written on the prescription does not match what you are measuring, it is best to contact the doctor to inquire if a prismatic effect was intended.

Question 22

There are two pupil distances you’ll need to measure when you’re fitting a person for glasses. What are they?

Answer 22

There are two pupil distances you’ll need to measure when you’re fitting a person for glasses. One is the distance PD—the distance between the pupil centers when the person is looking in the distance. The other is the near PD, which is the pupil distance when the person is looking close up (while reading, for example).

Question 23

What is a PD ruler (also called a PD stick) used for?

Answer 23

PD ruler is used to measure your client’s pupillary distance, to verify measurements of a frame, to measure the seg height needed for bifocals, to check the vertex of a frame, to measure the pupil height for a progressive lens, and to perform many other tasks.

Question 24

When you’re using a PD ruler, always be sure to…

Answer 24

When you’re using a PD ruler, always be sure to place the 0 of the ruler at one end of whatever you’re measuring

Question 25

If you’re measuring the eyewire size of a zyl frame, you’ll need to measure from…

Answer 25

If you’re measuring the eyewire size of a zyl frame, you’ll need to measure from the middle of the frame thickness and not from the outer edge of the frame

Question 26

When you’re measuring the seg height for a pair of bifocals, always measure to…

Answer 26

When you’re measuring the seg height for a pair of bifocals, always measure to the middle of the frame thickness and not the outer edge of the frame

Question 27

When you’re ordering rigid contact lenses, you’ll need to measure…

Answer 27

When you’re ordering rigid contact lenses, you’ll need to measure the size of the pupil, the lid aperture, the distance from the bottom of the pupil to the bottom of the eyelid, and the size of the cornea. You can also use your PD ruler to measure the diameter of a contact lens.

Question 28

Describe how to take PD measurement using a PD ruler

Answer 28

With the person facing you, have the person look at your left eye, and place the zero of the PD ruler at the very outer edge of the person’s right iris. Why the iris and not the pupil? It’s pretty easy to see the edge of the iris, so using this landmark instead of the pupillary light reflex in the center of the pupil can help. When patients have dark eyes, it can be difficult to see the pupillary reflex.

Then, without moving your hand or the PD ruler, look at the inner iris edge of the person’s left iris, and measure the distance in millimeters. The number you come up with should be somewhere between 50 and 70 millimeters. It’s typically about 50 to 60 millimeters in children and about 60 to 70 millimeters in adults. This would be the person’s near PD measurement.

Now—without moving your hand or the PD ruler—ask the person to look at your right eye, and read the new measurement at the inner edge of the person’s left iris. This is the person’s distance PD measurement.

Question 29

Describe how to take PD measurement using a pupillometer

Answer 29

When you use this device, the only thing your patient will need to do is look into the handheld instrument as if looking through a pair of binoculars. And all you need to do is slide the markers over the centers of the person’s pupils and read the LED reading off the instrument.

Question 30

Binocular PD

Answer 30

The distance between the centers of the pupils

Question 31

What is Monocular PD? When is it useful? How do you measure it?

Answer 31

The distance from the center of one pupil to the center of the nose. It can be very important when you need precise measurements for strong prescriptions or progressive lenses. The only way to take an accurate monocular PD is with a pupillometer.

Question 32

When optometrists write prescriptions, they usually use what’s called… When ophthalmologists write prescriptions, they usually use what’s called…

Answer 32

When optometrists write prescriptions, they usually use what’s called minus (negative) cylinder form. When ophthalmologists write prescriptions, they usually use what’s called plus (positive) cylinder form.

Question 33

What is transposition? List the steps to do it.

Answer 33

Converting minus (negative) cylinder form to plus (positive) cylinder form or vice versa.

The steps involved are:

1. Take the sphere number (including the sign) and the cylinder number (including the sign), and add them together to get your new sphere.
2. Take the cylinder number, and change its sign to the opposite sign to come up with the new cylinder.
3. Take the axis, and change it 90 degrees by either adding 90 or subtracting 90 from it. Remember that the axis cannot be 0 or greater than 180. This will tell you whether to add or subtract.

Example:
Old prescription = OS: -1.50 +3.00 x 005
New transposed prescription = OS: +1.50 -3.00 x 095

Question 34

Optical cross (or a power cross)

Answer 34

A system used for determining the power of a lens. To come up with the right numbers, you’ll add the front and back numbers for each meridian.

Question 35

Any time the prescribed axis of a lens is at 045 or 135 (called oblique astigmatism), you’ll determine the power of the lens in the 90-degree axis by…

Answer 35

Any time the prescribed axis of a lens is at 045 or 135 (called oblique astigmatism), you’ll determine the power of the lens in the 90-degree axis by taking half the cylinder and adding it to the sphere.

Example: For the prescription -2.00 -1.00 x 045, find the power of the lens in the 90-degree axis.

Answer = -2.50 D.S.

Again, just take half the cylinder, and add it to the sphere. So, the power at the 90-degree meridian is -0.50.

Question 36

How to calculate the power of the lens cylinder in a meridian that is a certain distance in degrees away from either of the principal axis meridians. There are two ways of doing this: One is the percentage method (quick), and the other is using a formula. What is the percentage method?

Answer 36

The percentage method is as follows:

• Remember that at 30 degrees away from the axis, you will have 25 percent of the cylinder power.
• Remember that at 45 degrees away from the axis, you will have 50 percent of the cylinder power.
• Remember that at 60 degrees away from the axis, you will have 75 percent of the cylinder power.

We are not talking about the power of the lens at axis 30, 45, or 60, but the power of the cylinder 30, 45, or 60 degrees away from either axis. Your answer to this question will be a percentage of the cylinder value added to the total power.

Example: Given a lens -4.00 -1.00 x 150, how would you correct a lens that was not cut correctly, and you need to know the power of the lens at the axis of 180?

Answer:
The difference in the axis between 180 and 150 is 30 degrees. You need to know the power of the cylinder 30 degrees away, which is 25 percent of the cylinder power. So, 25 percent of -1.00 is -0.25. Now add this -0.25 to the spherical power of -4.00, and your answer is -4.25 @ axis 180.