Thursday, August 4, 2011

How to use a manual Lensometer!

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. 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.
Manual Lensometer
Descriptions and examples of what you see in a lensometer:
Reticle: The reticle, the guide displaying numbered rings shown in the drawing below, is seen when looking through the eyepiece. It allows you to place the lens’s optical center in the correct position, allows the displacement of the optical center for filling prescriptions that require prism, and can be used to check axis positions in sphero-cylinder lenses for verification. The rotational position of the reticle can be moved by turning the chrome knurled sleeve.
The “Target”:  The center of the reticle is a small circle. The circle has lines that divide it in to perfect quarters. You use the lines and the center circle to assure that you have the lens’s optical center correctly positioned in the lensometer for accurate verification (or layout) results. When the point where the sphere lines and cylinder line cross each other is perfectly centered within the circle, the lens is said to be in target. The center of the target is where you want your sphere and cylinder lines to cross when Rx does not require prism.
Sample Display of lensometer Reticle

Operating Instructions
1.      Set the lensometer on a stable surface. Adjust the height of your chair and the angle of lensometer so that you can easily see through the eyepiece on the lensometer.
2.      Turn the eyepiece slowly counterclockwise until the black cross becomes completely clear to focus the eyepiece on the display area.
3.      Rotate the measuring wheel to set the scale at zero. The dividing line in the center of the display area should be at its clearest at this setting.
Display sample of measuring scale.

4.      Turn the power wheel to its highest power for best light infiltration and focus through the eyepiece.
5.      Set the right lens of the eyeglasses on the seat of the lensometer with the front facing up toward you. Gently release the pressing piece on the seat so that the feet of the piece hold the lens in place. This will ensure that the lens will not shift while you work, but will not apply so much pressure as to damage the lens.
6.      Rotate the measuring handwheel so that the dividing line is at its clearest. While you work on this focus, shift the lens side to side so that the dividing line and the center of the black cross line up. You must move these two pieces simultaneously to accurately bring the lens into focus for measurement.
7.      Read the scale at the point where the dividing line crosses the scale. This reading gives you the spherical diopter, or the outer power of the lens.
8.      Turn the power wheel to lower the power slowly. As you do this, the dividing line will go out of focus and two other similar lines will move in and out of focus. Turn until the third line is completely in focus and read the scale.
9.      Subtract the second reading from the first to determine the cylindrical power of the lens, which is the inner power.
10.  Remove the right lens and readjust your eyepiece and setting as you did in the beginning. Repeat all steps for the left lens.

Things to remember:

  • ·         Work right lens first
  • ·         Place glasses in correct position in lensometer, placing the bottoms of the eyewires evenly on any spectacle table.
  • ·         Center target in reticle or display target before reading power display.
  • ·         Be sure glasses remain level and in contact with spectacle table when you switch between right and left lenses.
  • ·         Read lens by correct type (single vision, lined multifocal, progressive).
  • ·         Read the prescription directly from the display.
  • ·         The colors of the lines and scales within the viewing area of the lensometer vary by manufacturer. The placement of the wheels may also vary by model. Consult your product's owner's manual for details regarding your specific lensometer.
  • ·         Many manufacturers produce automatic lensometers, which require nothing more than setting the eyeglasses in place and pushing the "start" button.
  • ·         Measuring power and angles for bifocals or astigmatic lenses involves more steps than measuring for simple eyeglasses. Follow your manufacturer's instructions for these details.

A General Description of the Lensmeter

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.
 Lamp Access Cover — Provides access to change the lensmeter bulb.
Note: All lensometer use (manual and auto) and verification skills will take time and practice to become a master. 

Ref: ehow, opticianworks, amconlabs, optometric, 2020mag

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