Lesson 13 - Lensometry, Measurement Devices, and Multifocal Lens Considerations

Question 1

Lensometer (also called focimeter or vertometer; however, these are outdated terms)

Answer 1

A device used to measure the prescription of a lens—sphere, cylinder, axis, add, and prism

Question 2

Eyepiece

Answer 2

Used to focus the lensometer

Question 3

Lens holder lever

Answer 3

This holds the lenses in place on the instrument.

Question 4

Spectacle table lever

Answer 4

Used to raise or lower the glasses so the optical center is in the right position

Question 5

Power drum (or power wheel)

Answer 5

Dial used to read the lens power

Question 6

Lens stop

Answer 6

A lens is placed against the lens stop when placed in the lensometer.

Question 7

Cylinder axis wheel

Answer 7

Used to find the axis of cylinder power

Question 8

Reticule

Answer 8

The series of concentric rings you’ll see when you look through the eyepiece of the lensometer

Question 9

Mire lines

Answer 9

Perpendicular lines of three, used to calculate sphere and cylinder powers when provided a lens. Closely placed lines are the sphere, and wide lines represent the cylinder.

Question 10

Circumference gauge

Answer 10

A measuring tape that fits snugly around the lens perimeter and is measured in millimeters. It is often used when a replacement lens needs to be ordered and you don’t have the lens pattern. You would also need to measure the lens circumference if the new lens is falling out of the frame or the new lens is too big and you need to measure the circumference of the original lens.

Question 11

Start with the strongest lens first—and if you don’t know which one is strongest, start with the ____.

Answer 11

Start with the strongest lens first—and if you don’t know which one is strongest, start with the right lens.

Question 12

List the steps to find the lens power with a lensometer

Answer 12

1. Start by focusing the eyepiece. Do this before you place the glasses on the spectacle table. When you look into the eyepiece, you’ll see the reticle target. You may want to put a plain white piece of paper on the lens table and turn the eyepiece until the reticle lines are clearest for you. Remember that you can have a prescription in minus cylinder form or plus cylinder form. Most opticians use a lensometer to find the power of a lens in minus cylinder form, so let’s focus on that method in the remaining steps.
2. Pull the lens holder lever toward you until it locks in place away from the lens stop. Place the glasses into the lensometer, with the back surface of the lens against the lens stop and the temples pointing away from you.
3. Turn the power drum toward you in the high plus power direction (about +10.00) before you begin. This ensures that your initial point is well outside the actual power of the glasses.
4. Slowly turn the wheel in the opposite direction until the sphere lines are in focus. You may need to turn the axis wheel as well to get the best focus, especially if there’s a cylinder in the lens. If the sphere lines get more blurred instead of sharper, go back to step 1 and start at a higher plus power before moving the wheel back. If both the sphere and cylinder lines focus at the same time, you have a lens that is a sphere.

You’ll want to calculate your sphere power before you calculate your cylinder power. If the cylinder lines focus first, turn the axis wheel so the sphere lines focus first. Once the sphere lines focus first, note the power of the sphere. If both the sphere and cylinder lines focus at the same time, the lens has no cylinder, and you’re done!

If that’s not the case, continue to turn the power wheel in the direction of less plus until the cylinder lines focus, and note the power again. The difference between the power you see on the wheel when the sphere lines focus and the power you see when the cylinder lines focus is the cylinder power.

Question 13

If your client wears bifocals or trifocals, you’ll need to find out how strong the add is. This number will always be more ____ than the distance power.

Answer 13

If your client wears bifocals or trifocals, you’ll need to find out how strong the add is. This number will always be more plus than the distance power.

Question 14

When the add power in a pair of glasses is high, you’ll get a more accurate reading when you measure it from where?

Answer 14

When the add power in a pair of glasses is high, you’ll get a more accurate reading when you measure it from the front vertex of a lens. So you may want to turn the glasses around, placing the front vertex (or front of the lens) on the lensometer lens holder before you begin. If so, be sure to turn the glasses the same way each time.

Question 15

List the steps for measuring the add with a lensometer

Answer 15

First, read the distance power of the lens.

Next, in order to find the add power, just gently move the lens and the spectacle table upward so you position the reading portion of the lens in front of the lens stop. Be careful not to scratch the lens.

Read the spherical power of the lens in this position. The difference between the spherical power of the lens in the distance portion of the lens and the spherical power of the lens in the add portion is the add power.

Question 16

List the steps to find the optical centers of a pair of glasses with a lensometer.

Answer 16

Note that this time around, you’ll use ink and a special marking device to identify the location of your measurements.

After you zero the power drum and place the glasses on the spectacle table, look through the eyepiece and move the power drum until the sphere mire lines are clear. The mires may intersect at the center of the reticle, or they may not.

Use your lens-spotting device to spot the lens. Do the same for the other lens.

Remove the glasses from the lensometer, and use a PD ruler to measure the distance between the dots. If the prescription has no prism in it, the distance between the dots should match the PD requested on the prescription. If the prescription has prism in it, the distance you measure will be different from the patient’s PD.

Once you have your measurements, you can use Prentice’s Rule to calculate your induced prism and verify that you have the correct amount in the prescription.

Question 17

What does it mean to verify a lens?

Answer 17

When you verify a lens, you’re checking to make sure the optical lab did just what you told them to—from getting the power right to using the correct material and coatings.

Question 18

List the steps to verify a lens

Answer 18

1. Verify the power of the glasses. This includes distance and near power, as well as prism if indicated. When you’re checking your lenses, remember that they don’t need to be absolutely perfect. The American National Standards Institute (ANSI) sets error tolerances for lens prescriptions. If the power of a lens falls within the range of tolerance, you can consider the lens prescription acceptable, even if it’s a tiny bit off. For cylinder axes, the greater the cylinder power, the smaller the range of tolerance for the axis.
2. Verify the prism. Once you’ve verified the sphere and cylinder powers, here’s how you’ll verify the prism. After dotting the center of one lens in the lensometer, you’ll move over to the other lens. If the illuminated lines cross at one of the reticle circles in the target instead of in the center of the reticle, you have prism.

Here’s how to tell how much prism you have and whether it’s base up, base down, base left, or base right. The first ring of the reticle is .5 prism diopters, and each outer ring is 1 prism diopter. If the illuminated lines cross above the center of the reticle, you have base up or down. If the illuminated lines cross to the left or right of the reticle, you have base in or out. Refer to page 111 in System for Ophthalmic Dispensing, for pictures illustrating this.

Now we will use the lensometer to mark the optical centers of each lens in order to verify whether or not the lab correctly ground the lenses to match the PD if no prism was prescribed.

3. Confirm the PD. Verify that the distance between the optical centers of the lenses matches the patient’s PD if there was no prism prescribed. Using the lensometer, dot the optical centers of the lenses. Use a millimeter ruler or a PD ruler to measure the distance between optical centers and confirm that this distance is the same as the PD specified on the prescription.
4. Check the add for multifocals. If the glasses have multifocal lenses, check the add style, size, and height. If the add style is a segment, simply look at the segment to see what kind it is: for instance, flattop, round, ultex, or trifocal. To check the segment size, use your PD ruler. For a flattop segment, measure the width of the top of the segment. Measure the widest portion of the segment if it is a curved type of segment. Remember that your measurements will be in millimeters! To verify the segment height, use the PD ruler to measure from the top of the segment to the bottom of the lens bevel, which is typically about 1 mm deep in the frame rim. If the glasses have progressive lenses, look at the manufacturer’s etched symbol identifying the type of progressive lens and the power. It is seen by holding the lens up to a ceiling light and looking at the temporal edge or nasal edge of the lens.
5. Verify the base curve. Verify the base curve, especially if it is specified on the prescription. Place the lens clock prongs perpendicularly to the front surface of the lens. Remember that the base curve of a lens is the curve of the front surface of the lens as measured with a lens clock. And here’s a tip: There’s still another method for finding the base curve of a multifocal lens. You can measure the backside aspheric curve of the lens and the lens thickness. If you know the index of refraction of the lens, you can calculate the base curve from this information.
6. Verify the lens material. Verify the lens material. For instance, if you ordered photochromic lenses, did you get them? You can use a UV light or take the glasses outside to test if the lenses darken when exposed to UV. Verifying the material used in a lens is sometimes difficult, especially if you’re trying to tell the difference between a high-index plastic lens and a Trivex or polycarbonate lens. (It’s easy, however, to tell glass from plastic.) Sometimes you can identify the lens material by measuring the lens thickness. You can use a lens thickness gauge or calipers to do this.
7. Verify the lens tint. Verify that the lens is photochromic, if that’s what you ordered. Also check to make you the tints or coatings you ordered are correct.

Question 19

Always record the ____ of your client’s glasses on the patient chart or in your store records, since the person may not remember. This is especially critical if the person should be wearing what?

Answer 19

Always record the lens material of your client’s glasses on the patient chart or in your store records, since the person may not remember. This is especially critical if the person should be wearing an impact-resistant lens.

Question 20

List the steps for obtaining accurate measurements for progressive lenses.

Answer 20

Take measurements for each eye individually. You’ll need to specify the exact horizontal distance each eye travels when going from distance viewing to near viewing (monocular pupillary distance, or monocular PD) and the seg height for each eye.

1. Measure your client’s monocular pupillary distances with a pupillometer. Each eye has to travel within the progressive corridor, and you need to tell the lab exactly where the progressive corridor should be. If the monocular PDs aren’t measured correctly, the person’s eyes may move too quickly outside of the progressive corridor, making side vision blurry.
2. Fit and completely adjust the frame. If the frame height or pantoscopic tilt has to be significantly adjusted after the lab makes the lenses based on the initial measurements, the prescription and adaptation to the prescription may be affected.
3. Accurately measure the fitting seg height (also known as the fitting cross) using a permanent marker and a penlight. With your client sitting directly across from you at arm’s length and at the same head height as you, shine the penlight from the height of your temple by your eye toward your client’s eye so you can see the penlight’s reflection in the other person’s pupil. Make sure your client’s head is not tilted up or down. Using the pen with your other hand, draw a cross on the reflection of the penlight that appears on the lens in front of your client’s eye. Do the same with the other eye. When you take this measurement, be as accurate as possible. If you dot the fitting cross too high, your client will have a difficult time seeing clearly in the distance because he or she will be looking through the intermediate portion of the lens!

Question 21

Each progressive lens manufacturer has a centration card that is used for what?

Answer 21

Each progressive lens manufacturer has a centration card that will help you know exactly where the reading portion will fall on the lens if you line up the chart with the optical cross of the lens you drew your dots on. You’ll see an example of a centration chart on page 456 of, System for Ophthalmic Dispensing. You can also use this card to see if the lens can be cut to fit the frame your client wants. If you see that any part of the lens won’t be enclosed by the frame after the lens is cut, you’ll know you should select a different frame.

Question 22

List the steps to verify a progressive lens

Answer 22

Remember the lensometer? You’ll use this piece of equipment to make sure the lens you get back from the lab has the correct distance power, add, seg height, and pupillary distance.

The job will come back from the lab with removable ink lens markings on the lens or a sticker placed on the lens to indicate the fitting point, the prism reference point, the distance reference point, the near reference point, and the alignment reference point.

The lens will also have an etched logo, and the near add power will be etched onto the lens.

To verify the add power of the progressive lens, you’ll simply need to look at the number etched on it. Remember that only the first two numbers of the add are etched on the lens. So a +2.00 add will appear as 20, a +1.75 add will appear as 17, and so on.

When you verify your prescription, remember to place the lens against the lensometer correctly so you’re reading the powers using the front vertex powers of the lens. You’ll want to do it this way, because the add is usually placed on the front surface.

Question 23

Distance reference point

Answer 23

Where you’ll verify the distance power of the progressive lens on the lensometer.

Question 24

Fitting point

Answer 24

Where you’ll want the patient’s pupils to be when dispensing the lens.

Question 25

Prism reference point

Answer 25

Where you’ll look through the lens on the lensometer to verify prism.

Question 26

Near reference point

Answer 26

Where you’ll verify the near power of a progressive lens on the lensometer.

Question 27

Alignment reference point

Answer 27

Used by the lab to align the lens during finishing, as well as to verify the prescription axis and placing the finished lens in the frame.

Question 28

If you don’t educate clients upfront about their progressive lenses, “they will be back with complaints.” Remember this advice: “The better you educate, the better the patient adapts to the lenses.” What are some important tips to remember when dealing with clients who have new progressive lenses?

Answer 28

Explain to clients that they’ll need to turn their heads to get a clear view of something to the side. Let them know that their vision will blur or “swim” a little when they look through the sides of the lenses.

Check-in with clients a few days after they get their glasses. If they’re having problems with them, you may be able to offer helpful tips.

Question 29

How to dispense glasses with progressive lens

Answer 29

You’ll want to make sure there’s a short vertex distance between the client’s eyes and the lenses and check to see that their eyelashes don’t touch the lenses. There should be a small amount of face form if the frame isn’t a sport face-wrap style. There should be an adequate amount of pantoscopic tilt, and his frame should sit level on his face.

Once you’re confident your client’s frame fits properly, make sure they can read comfortably with the glasses. Can they move their eyes from a straight-ahead gaze down to the reading level without tipping their head back? They should be able to.

When you’re happy with the fit of their glasses, you’ll want to talk with them about what to expect while wearing them. It takes a little while for people to get used to progressive lenses, and they’ll make this adjustment faster if they’re well prepared. That’s especially true when it comes to short-corridor progressives.

Question 30

If your client has a prescription for progressive or bifocal lenses and decides to purchase a single-vision pair for reading only, how will you arrive at the new prescription?

Answer 30

It’s easy: Simply add the value of the add to the sphere for each eye.

For example, let’s say your client has this original prescription:
O.D. +3.00 -0.50 x 080
O.S. +2.00 -1.00 x 010
Add: +2.00

The single-vision prescription for reading only is:
O.D. +5.00 -0.50 x 080
O.S. +4.00 -1.00 x 010

Question 31

If your client has a prescription for progressive or trifocal lenses and decides to purchase a single vision pair to use while working on the computer, how will you arrive at the new prescription?

Answer 31

Typically, you’ll want to change the prescription to allow the person to see things at the intermediate or midrange distance, since computer monitors are usually about 22 to 24 inches from the eyes instead of the 16 inches used for reading books.

So the add for computer distance is one-half the add for the reading prescription.

what would the new prescription for a single-vision computer or intermediate-distance pair of lenses be?

For example, let’s say your client has this original prescription:
O.D. +3.00 -0.50 x 080
O.S. +2.00 -1.00 x 010
Add: +2.00

What would the new prescription for a single-vision computer or intermediate-distance pair of lenses be?
O.D. +4.00 -0.50 x 080
O.S. +3.00 -1.00 x 010

Question 32

If you see that your client has to shift their head slightly to one side to see straight ahead with their progressive lenses, what may be the issue?

Answer 32

If you see that your client has to shift their head slightly to one side to see straight ahead, the monocular PDs may be off. Recheck this with the pupillometer.

Question 33

If you see that your client has to shift their head up or down to see in the distance with their progressive lenses, what may be the issue?

Answer 33

If you see that your client has to shift their head up or down to see in the distance, the seg height may be based on an inaccurate measurement. (Also, some people have eyes that are slightly different heights.) Remeasure the seg heights.

Question 34

When your patient has several complaints, you might need to what?

Answer 34

When your patient has several complaints, you might need to write them down so you are sure to address each one. Repeat or paraphrase what your patient said so they can confirm that you heard them correctly.

Question 35

When dispensing a new pair of lenses what should you do?

Answer 35

Always ask the client about their vision with the new lenses first. Give them a reading card and ask them if their vision is comfortable and clear. If not, assess the fit of the frame and adjust accordingly. If the frame is adjusted correctly and the patient still has vision issues, you will need to refer them back to their doctor for a check.

The frame should be adjusted each time the patient is dispensed their new glasses. The lab may have tweaked the frame or nose pads out of alignment while inserting the lenses. Ask your patient if their frame is too tight, too loose, if it hurts behind their ears, or is too close or far from their eyes.