Accuracy is the name of the game in lensometry and there are several stumbling blocks that lead to inaccurate results. Common problems include a smudged or dirty ocular lens, failure to use the lens table and lens clamp to assure the light rays are normal (i.e. at a right angle to the back surface of the lens being tested), failure to control accommodation, focusing on the cylinder mire lines first and analyzing the lenses in the incorrect order.
Keep Your Ocular Clean
Checking the ocular lens for smudges is best done with reflected light. Use a penlight to bounce light at a flat angle (like skipping a stone on water) across the ocular lens surface. Position your eye across from the penlight to easily see anything on the lens surface. Dust should be removed with a camel’s hair brush or a puff of clean compressed air (both available from a camera shop).For most instruments anti-reflective lens cleaner works well to clean oils from the ocular lens. A cotton swab moistened with the lens cleaner can be rolled against the direction of your stroke across the lens so that a clean portion of the swab is continuously in contact with the lens surface.
If the lensometer has a Risley prism it can be cleaned in the same way. Neither the ocular nor the Risley prism should be flooded with cleaner because its detergent action may gradually penetrate and interfere with the instrument’s lubricant. The ocular can be a source of eye infections, so clean it frequently.
Adjusting The Ocular
Calibrate the lensometer for your eye by adjusting the ocular for accurate readings. Here is an easy step-by-step method:
1. Hold a piece of white paper over the light aperture and look into the ocular. The paper gives good contrast for focusing the reticule marks.
2. Rotate the the ocular as far as possible into the plus. If your eyepiece does not have a plus/minus scale, the way to tell if you are coming out of the plus is to watch the reticule markings. If they come into focus and then get a little thinner, darker and brighter as you over-focus, you are coming out of the plus as you should be.
3. Remove the white paper, set the power wheel on plano and turn the ocular well into the plus.
4. Slowly turn the ocular focus out of the plus until the reticule just seems to be in focus, being careful not to go too far. If you go too far (i.e. over focus) the mire markings will get a little thinner, darker and brighter, and you will need to turn the ocular back into the plus and try again.
5. After focusing the ocular the mires should be in sharp focus. Move the power wheel into the low plus power (about +0.50) and then turn the wheel back toward the minus power until the mires are just in sharp focus.
6. Look at the power scale. It should read exactly plano. If it does not, repeat the procedure (starting with step 3).
7. If you get a second reading that does not end up at plano, then slightly adjust the ocular toward the minus and refocus the mires. If this doesn’t bring you to exactly plano, try slightly adjusting the ocular toward the plus and refocus the mires again. Non-presbyopic lensometer operators, especially if they are a little nervous, may find the ocular adjustment to be difficult. Non-presbyopes need to frequently check this adjustment because fatigue can affect their accommodative state. Trainees should be aware that experienced staff will (quite rightly) change the ocular setting for their own eyes, so this adjustment must become part of their own lensometer routine. Warn experienced staff a new user is working with the instrument so an ocular setting inappropriate for their use isn’t tripping them up. New users tend to over minus themselves causing them to tire quickly. New users often find closing one eye is helpful and less distracting, but experienced operators find this tiring. If you do lens analysis with one eye closed, do your ocular adjustment with one eye closed. If you keep both eyes open for lens analysis, keep them both open for the ocular adjustment.
Getting The Best View
The very best view when using your lensometer occurs when the entrance pupil of your eye is in the same position as the exit pupil of the lensometer. Turn the instrument on and hold a flat piece of waxed paper across the ocular aperture and then move it away from the instrument. Fifteen millimeters or so from the instrument a circle of light will come into focus. That circle is the exit pupil of the instrument, and the distance from the ocular to the paper is the eye relief designed into the instrument. Some instruments have such a short eye relief that users with long eyelashes are constantly smearing the ocular lens.
A short eye relief also interferes with wearing glasses when using the instrument because parallel rays of light issue from the ocular, the instrument’s user should wear their normal distance Rx when analyzing lenses. When purchasing a lensometer it is a good idea to measure the eye relief before you buy. It should be adequate for spectacle wearers and techs with long lashes.
Keeping The Light Path Normal
Errors in reading cylinder power, sphere power and axis occur when the light path is not normal to the lens surfaces, when the 180 meridian of the lens is not horizontal or when the lens is not against the light aperture of the instrument. To avoid these errors, use the frame table and lens clamp when taking your reading. Be sure both right and left eyewires touch the table evenly. “Right lens first” is how most of us learned to use the lensometer. A better rule is to analyze the strongest lens first. If you don’t, you may incorrectly reject jobs for unwanted prism even if they are within the ANSI standards. This is because centration of a lens is less exact with lower powers. If the right lens is of lower power, you may believe you have that lens centered and when you slide over to the stronger left lens you may find unwanted vertical prism. If unwanted vertical prism seems to be present, adjust the frame table to center the left lens and then recheck the weaker right lens.
Checking the weakest lens first can also lead to errors in determining whether the PD is within the ANSI tolerance. If the PD appears to be incorrect, locate and dot the MRP of the stronger lens, then use a marker and PD rule to dot the weaker lens at the correct PD. Replace the weaker lens in the lensometer so the instrument’s center pin falls on your mark. If the amount of lateral prism is then within the ANSI standard, the PD is correct. When considering possible vertical prism errors, compare the powers in the vertical meridians of the lenses. For lateral errors you must compare the powers in the horizontal meridians to determine which is the “stronger lens.”
Know Your Sphere Mire
The sphere mire is the mire that comes into focus first if your axis wheel is set to the axis of the test lens and if you are turning the power wheel from high plus (if you want a minus cylinder reading) or from high minus (if you want a plus cylinder reading). If the cylinder mire comes into focus first, your reading will be in error by 90 degrees. (See Figure A for visually locating the plus and minus axes of lenses.)
Front vs. Back Vertex Powers
Spectacle lenses have a front vertex power and a back vertex power. To know the effective power for distance vision, measure the back vertex power. For near vision, measure the front vertex power. Back vertex power is measured with the back of the lens touching the lensometer aperture. Front vertex power is measured with the front of the lens touching the lensometer aperture. To check the add, place the front surface of the lens against the lens aperture. Position it so the light is passing through the distance portion of the lens. Rotate the mires so they are on axis and can be brought into sharp, clear focus. Note the power when the mire nearest the vertical is in focus. Then raise the lens until the light passes through the near MRP and refocus on the mire nearest the vertical. The difference between these two readings is the power of the add. Mo Jalie, in his book, “Ophthalmic Lenses & Dispensing,” suggests another method of measuring the effective add power. In this method the minus side of the lens is against the lensometer aperture. The mire nearest vertical is brought into focus. The lens is then raised until light passes through the near MRP. A thin, flat trial lens is placed against the front of the lens being tested. The power of this lens should be the same amount of minus power as the amount of add desired (e.g. if a +2.00 add is wanted, use a -2.00 trial lens). If the mire remains in focus, the add is correct. In this technique the eyewear does not have to be flipped over, mire positions don’t need changing and no calculation is required. This should be easier and faster for technicians who do a lot of eyewear verification.
Quality of Optics
Lenses sometimes have waves or distortions that are difficult to locate. The lensometer can locate these distortions by bringing the mires into focus and then carefully (to avoid scratches) moving the lens while watching the mires. When the light path passes through these optical distortions the mires fall out of focus or may be otherwise distorted.
You can dot the 180 meridian in your lensometer before removing the old lens or a demo lens and sending it to your lab. This permits a more accurate trace than if the lab relied on axis, seg line or laser marks for orientation. When replacing one lens in a rimless mounting, the eyewear can be put on the lens table upside down and then the remaining lens can be dotted to establish the 180 line.
Clues For Instructors
When teaching lensometry, start with naming the parts of the instrument and explain the terms that are used when analyzing lenses. Show your student how to carry the instrument, change a bulb and how to keep the ocular clean. These simple basics are second nature to someone with experience, but knowing them will help orient a beginner. Lensometry is easy once you learn it, but the stress and confusion early on can be quite frustrating. The lens in Figure A is being held at about 16 inches from the tech’s eye. She is looking at a line that extends above and below the lens and she has moved the lens so the line appears to pass unbroken as it passes through the lens. To achieve this the line must pass through the optical center of the lens. Her lensometer reading for this lens was -1.00 for the sphere, -1.25 for the cylinder power and 090 for the axis. To be sure the axis was correct, she watches the portion of the line that falls within the lens and rotates it a few degrees clockwise (against the direction of the lens rotation).The part of the line seen in the lens rotates counterclockwise as in Figure B, indicating the plus axis is at 90. Since she has read the lens as a -1.25 axis 090, the lensometer reading is incorrect. If the lens had a minus cylinder axis 090 she would have seen the line rotate clockwise as in Figure C. If the lens were a sphere, the image of the line would not rotate.
Lensometry as a Practice Builder
Once staff members have lensometry skills, they can routinely open a record of the Rx’s of drop-ins who need eyewear realignment or minor repairs. Many of these drop-ins are really scouting your practice. They often want a feeling of caring and competency before they commit to an appointment or eyewear purchase. Opening a record with the eyewear information (including frames, tints, etc.) lets you be of help if they lose or destroy their eyewear, but you must let them know you have done this. It’s a nice service. It opens the door to offer to call or send a reminder when care is needed. It’s a great way to start a practice/ patient relationship. LT
Palmer R. Cook, OD is director of education for Diversified Ophthalmics in Cincinnati.