Lesson 13 - Lensometry, Measurement Devices, and Multifocal Lens Considerations Flashcards
Lensometer (also called focimeter or vertometer; however, these are outdated terms)
A device used to measure the prescription of a lens—sphere, cylinder, axis, add, and prism
Eyepiece
Used to focus the lensometer
Lens holder lever
This holds the lenses in place on the instrument.
Spectacle table lever
Used to raise or lower the glasses so the optical center is in the right position
Power drum (or power wheel)
Dial used to read the lens power
Lens stop
A lens is placed against the lens stop when placed in the lensometer.
Cylinder axis wheel
Used to find the axis of cylinder power
Reticule
The series of concentric rings you’ll see when you look through the eyepiece of the lensometer
Mire lines
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.
Circumference gauge
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.
Start with the strongest lens first—and if you don’t know which one is strongest, start with the ____.
Start with the strongest lens first—and if you don’t know which one is strongest, start with the right lens.
List the steps to find the lens power with a lensometer
- 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.
- 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.
- 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.
- 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.
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.
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.
When the add power in a pair of glasses is high, you’ll get a more accurate reading when you measure it from where?
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.
List the steps for measuring the add with a lensometer
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.
List the steps to find the optical centers of a pair of glasses with a lensometer.
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.
What does it mean to verify a lens?
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.
List the steps to verify a lens
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
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?
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.
List the steps for obtaining accurate measurements for progressive lenses.
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.
- 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.
- 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.
- 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!
Each progressive lens manufacturer has a centration card that is used for what?
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.
List the steps to verify a progressive lens
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.
Distance reference point
Where you’ll verify the distance power of the progressive lens on the lensometer.
Fitting point
Where you’ll want the patient’s pupils to be when dispensing the lens.
