Prior to the introduction of the lensmeter, the determination of lens power was accomplished by using lenses of known power (or focal length) to “neutralize” the power of the unknown lens. Neutralization was accomplished by determining the “motion” seen when moving the lens before the eye and finding a lens with opposite motion to neutralize it. Convex (plus) lenses exhibit “against motion,” meaning that an object viewed through them will appear to move to the right when the lens is moved to the left. Concave (minus) lenses exhibit “with motion,” meaning that an object viewed through them will appear to move in the same direction as the movement of the lens. Lenses that have astigmatism correction (sphero-cylinder lenses) may show either against, or with motion, or both depending on the total powers found in each of the two principle meridians of power. Some people describe the motion exhibited by the sphero-cylinder lens as “scissors motion.” This is due to the fact that the image of perpendicular lines viewed through the lens will be elongated or shortened in appearance as the lens is rotated in a clockwise manner before the eye, causing the lines to appear to move like the jaws of a scissors.
In the late 1800s the introduction of the lensmeter, also called a lensometer, focimeter, or vertometer changed the way the power(s) of lenses were neutralized. (For simplification, the term lensmeter will be used for the remainder of this article.) The lensmeter is essentially a centered telescopic optical system aligned with a standard optical lens and a rotatable target illuminated by a light source. It measures the focal length of a lens and converts it into diopters on a circular number line called a Power Drum. This is not however, the only function of the instrument. It is also used for: a) locating and marking the lens Optical Center, and orienting the axis of a sphero-cylinder lens; b) locating and “spotting” or “marking” the Prism Reference Point, or PRP (formerly called the Major Reference Point, or MRP), which is the point where the amount of prism prescribed is equal to the amount of prism found; c) verifying lens “add” power in bifocals, trifocals and progressives; d) locating areas of lens aberrations.
A General Description of the Lensmeter (see figure above)
1. The eyepiece — mounted in a screw-type focusing mechanism, the eyepiece plays an important role in the accuracy of your readings. It is essential that the eyepiece be focused to the individual eye of each user. It may be fitted with a rubber guard to prevent scratching of the user’s own eyewear.
2. Chrome Knurled Sleeve — used to rotate the Reticle to orient prism base.
3. Prism Compensating Device Knob — Used to read prism amounts greater than five prism diopters.
4. Lens Holder Handle — Used to hold a lens in place against the aperture.
5. Marking Device Control — Used to spot the lens at either the Optical Center or Prism Reference Point (PRP).
6. Gimbal — A pivoting holder that holds the lens in place.
7. Ink Pad — Holds the spotting ink.
8. Spectacle Table Lever — Used to raise, or lower the level of the Spectacle Table.
9. Spectacle Table — The resting place for the frame when neutralizing finished eyewear.
10. Power Drum — Hand wheel with numbered scale readings between +20 and -20 Diopters.
11. Locking Lever — Used to elevate or depress the position of the instrument for individual’s height or posture.
12. Prism Axis Scale — Used for orientation of prism axis
13. Prism Compensating Device — Used to verify or layout large amounts of prism.
14. Prism Diopter Power Scale — Displays prism amount.
15. On-off Switch — Power switch.
16. Lens Stop — Aperture against which the lens rests.
17. Cylinder Axis Wheel — Used to orient or neutralize cylinder axis.
18. Filter Lever — Used to engage or remove green filter.
19. Lamp Access Cover — Provides access to change the lensmeter bulb.
||The Reticle (see figure, left)|
The Reticle is a permanently etched series of concentric rings used to measure and locate prism base direction, it also contains orientation lines for each lens meridian and a protractor scale. On the Marco lensmeter, the first ring from the Reticle center represents one-half prism diopter and each further ring denotes one additional prism diopter each. Instruments made by other manufacturers may have the first ring be one prism diopter.
||The Target (see figure, left)|
The target is a cross hair, with the thin, closely spaced lines representing the sphere component of lens power and the thicker, widely spaced lines representing the cylinder power. The lensmeter actually does not read the prescription of a lens, but rather the powers of the lens in each meridian. In the case of a spherical lens all of the lines of the target will focus at the same time, while in the case of a sphero-cylinder lens, the lines will focus separately at different Power Drum Readings.
Operating the Instrument
The first thing to consider is the focusing of the eyepiece, in order to compensate for small uncorrected visual errors. It is important to eliminate the normal accommodation (focusing ability) of the eye. To do this you should try to keep both eyes open when using the instrument. Turn on the instrument and place a plain piece of white paper between the Lens Stop and Prism Compensating Device (#13). Turn the eyepiece counter-clockwise (into the plus numbers) and look into the eyepiece. Slowly turn the eyepiece in a clockwise direction until the concentric circles of the Reticle come into focus. I generally look at the numbers on the Reticle that indicate the prism amount value of each ring. Try not to turn past the point of clearest focus, as this will cause accommodation to come into play.
Once you have focused the eyepiece, it is important to verify the accuracy of your instrument by turning the Power Drum to +2.00 and while looking into the eyepiece, turning the Power Drum top away from you until the cross hair-like picture of the Target comes into sharp, clear focus. At this point, the Power Drum Reading should be at 0.00 or Plano. If not, repeat the process of focusing the eyepiece and try again. A good tool for verifying the accuracy of the instrument is the Gaugemaster, a calibrating tool produced by Younger Optics that can also be used to check the accuracy of the axis wheel, marking pins, surfacing tools, lens clock or a thickness gauge.
||Neutralizing an Uncut Spherical Lens (see figures, left)|
1. Focus the eyepiece and check the calibration.
2. Retract the Lens Holder Handle (#4) and place the lens between it, and the Lens Stop (#16) with the convex (front ) surface facing you. This will allow you to read the lens for Back Vertex Power (BVP).
3. Turn the Power Drum top toward you until you reach a number +10.00 or greater.
4. While looking into the eyepiece, turn the Power Drum top away from you until the image of the target begins to become sharply imaged.
5. Once you have brought the both the spherical lines (three thin, closely spaced) and the cylinder lines (three thick, widely spaced) of the target into clear focus, lift the Lens Holder Handle, and reposition the lens until the cross hairs of the target are centered on the Reticle rings. It is important to lift the Lens Holder to avoid scratching the lens surface.
6. You have now located the lens Optical Center, and should note the Power Drum Reading.
7. If you want to spot or mark the lens Optical Center, depress the Marking Device Control (#5) and press it forward against the lens surface. This will leave three marks, two that are guides to axis orientation on sphero-cylinder lenses and the center dot that represents the Optical Center or Prism Reference Point (PRP).
Neutralizing an Uncut Sphero-Cylinder Lens - In Minus Cylinder Form
(see figures, left)
1. Focus and calibrate.
2. Place lens in position and set the Power Drum at about +10.00 D.
3. Set the Cylinder Axis Wheel (#17) to the desired cylinder axis.
4. Turn the Power Drum top away from you and look for the sphere lines (the thin, closely spaced ones) first. As you are moving the Power Drum in the minus direction, you want the sphere lines to come into focus first. If perhaps you find the thicker cylinder lines coming in first, rotate the lens 90 degrees and repeat the process. Neutralizing the spherical power first insures that your prescription will be read in minus cylinder form. It is possible that you will find that the central area of the sphere lines are not straight and continuous (indicating that you have not properly aligned the axis). If this is the case, adjust the lens until the lines are clear, straight and continuous. (See illustration #8, showing a lens off axis).
5. Once you have located the sphere lines, center them on the Reticle and note the power.
6. Turn the Power Drum top away from you until you now see the thicker cylinder lines clearly, note the position of the Power Drum and center the lines on the Reticle.
7. Recheck the centering of each set of lines alternately until both are perfectly centered. I tend to use the etched in lines of the Reticle to bisect the center line of both the sphere and cylinder targets.
8. Mark the Optical Center or Prism Reference Point.
If you are interested in marking the lens in plus cylinder form, start with the Power Drum in a high-minus position and turn the Power Drum top away from you and look for the sphere lines first. Inevitably, the cylinder lines will be more plus than the reading of the sphere lines.
Converting Power Drum Readings into an Rx.
Because the lensmeter is actually measuring the total powers of the principle meridians, rather than the actual prescription, it is necessary to do a conversion, using the following steps.
1. The sphere reading (PDR #1) will be your sphere power in the Rx.
2. The sign of the cylinder is determined by the direction that you have moved the Power Drum from the sphere reading (PDR #1) to the cylinder reading (PDR #2). In this case it will be minus due to the procedure that we used.
3. The amount of the cylinder is determined by the “distance” traveled from PDR #1 to PDR #2. This distance is in fact the total number of diopters (or fractions of diopters) that you have moved on the number line etched on the Power Drum.
4. The Axis is determined by simply noting the position of the Axis Wheel.
Examples of Converting Power Drum Readings to an Rx
Example #1 PDR #1 = -2.00 (sphere)
PDR #2 = -3.50 (cylinder)
Axis Wheel Reading = 180 degrees
The sphere power is -2.00, the direction you have moved from PDR#1 to PDR#2 is minus, making the sign of the cylinder minus and you have moved a “distance” of 1.50 diopters.
Example #1 = -2.00 -1.50 X 180
Example #2 PDR #1 = +1.00 (sphere)
PDR #2 = -1.25 (cylinder)
Axis Wheel Reading = 90 degrees
The sphere power is +1.00, the direction you moved from PDR #1 to PDR #2 is minus, making the sign of the cylinder minus and you have moved a “distance” of 2.25 diopters. In this case it is important to note that you have moved from a plus number (+1.00) to a minus number (-1.25). You must take care to count the total distance moved. From +1.00 to Plano is 1.00 and from Plano to -1.25 is an additional 1.25 diopters, making the total 2.25 diopters.
Example #2 = +1.00 -2.25 X 90
Neutralizing a Finished Pair of Single-Vision Eyeglasses
in Minus Cylinder Form
When neutralizing a pair of finished single-vision eyeglasses there are several small procedural differences that you must follow. It is important that you place the lower rims of both eyewires rest on the Spectacle Table (#9). This may require the elevation or depression of the table utilizing the Spectacle Table Lever (#8). Generally, it is best to center the lenses by eye and subsequently adjust the table to center the target of the first lens on the Reticle. The rest of the process is very similar to neutralizing an uncut lens.
1. Focus and calibrate.
2. Start at a high-plus power, and turn the Power Drum top away from you until you see the first set of target lines. While doing so, you may have to adjust the Axis Wheel to make the sphere lines straight and continuous. This requires you to work both the Power Drum and Axis Wheel simultaneously.
3. You want to bring the sphere lines into focus first, but if you find the cylinder lines first you must rotate the Axis Wheel by 90 degrees to establish the correct axis.
4. Note PDR #1.
5. Note PDR #2.
6. Note the final position of the Axis Wheel.
7. Construct the Rx as explained above.
8. Repeat with second lens.