Optometric Management Tip # 218   -   Wednesday, March 22, 2006
A Guide to Lensometry for Techs

Last week’s tip offered ideas for increasing delegation of clinical procedures to staff. In my experience, one of the trickiest instruments for a technician in training to master is the lensmeter. Here, I will provide a simple, step-by-step guide that can be printed and kept near the lensmeter as an aid.

A lensmeter measurement of the habitual eyeglasses is a basic element of the eye exam that should always be delegated to staff. The procedure takes a bit of practice but any good employee can master it, saving a significant amount of doctor time.

Autolensmeters

Of course, the guide in this tip is for manual lensmeters, but I should acknowledge that automated lensmeters are wonderful instruments that take much of the skill out of the process. I think every practice should have an autolensmeter, but the reality is that it may be one of the instruments on that future purchase priority list. Autolensmeter technology is improving rapidly, but many models have their own nuances, so a manual lensmeter is still a very valuable tool.

I’m still amused by a hiring experience I had years ago, soon after opening my practice. During a job interview, I asked an applicant if she could perform lensometry. She answered yes, and assured me that she did it all the time in her previous job. I hired her and on the first day of work I showed her our manual lensmeter. After looking at it for a moment, she said “Oh – I only know how to use the kind where you press a button.”

A challenging technique

Lensometry is second nature to those of us who have mastered it, but when you think about the steps involved, one can see why it can be tough for a newbie. To find a cylinder axis requires both the power wheel and the axis wheel to be turned at the same time. The number line concept and some basic algebra can be a little beyond those who haven’t used it much. And if you skip a step or focus the wrong mire pattern first, the result will be wrong. These are some of the areas where I’ve found the guide useful.

Disclaimer

This guide is written as a very practical and quick guide that will actually be used on the job. It assumes that the technician has been given some training and practice on a lensmeter by an experienced user. The guide is merely a refresher – a cheat-sheet, if you will. It is not textbook complete and it does not delve into PD or prism. I think those aspects are best trained in person. A technician who is not skilled in those areas can be taught to ask for help when a mire pattern will not move to the center of the reticule.

Print the section below


Lensometry Guide

This guide is to assist you in accurately measuring the lens prescription in a pair of glasses. It assumes you will record the Rx in minus cylinder format using a manual lensmeter with an American style target. The target that you focus on is called a mire pattern and it looks like a cross pattern of illuminated bars. They may be green, white or some other color. If the Rx has cylinder, the bars running one direction will focus at a different power reading than those running in the opposite direction. Every step below is important and you must follow this guide exactly.
  1. Release the lens holding mechanism, place the eyeglasses on the stage platform so both lenses rest squarely, center the mire pattern for the right lens, and carefully secure the lens holder.

  2. Always start with the power wheel in very high plus.

  3. Turn the power wheel to reduce plus power, while turning the axis wheel, until a set of mire stripes come into focus and the bars appear straight and unbroken. Refine the axis and power wheels until the set of stripes is very sharply focused and unbroken.

  4. The single line mire (3 stripes close together) must come into focus first when coming down from high plus in the previous step. If the triple line mire (3 stripes far apart) came into focus first, turn the axis wheel 90 degrees and the single line mire will be in focus.

  5. Write down the sphere power reading and the axis wheel reading.

  6. Continue to turn the power wheel reducing plus (or adding minus, which is the same thing) until the triple line mire comes into focus. You should not have to turn the axis wheel during this step. The amount you turned the power wheel is the cylinder power. It is the difference between the reading at the single mire and the reading at the triple mire. You may wish to write down the reading of the triple line mire and subtract it from the single line mire reading.

  7. Bifocal add power is determined by turning the power wheel to focus the single line mire through the bifocal portion of the lens. Move the stage up so the lensmeter aperture is placed in the bifocal seg area. Do not move axis wheel from the location found previously. You will always need to add plus power. If the single line mire is not visible or will not focus, move the glasses until the problem is corrected. Progressive lenses may show distorted mires if the seg area is not positioned properly on the aperture. Subtract the distance sphere power from the near sphere power to get the add power.

  8. Repeat sequence for left lens.

Best wishes for continued success,

Neil B. Gailmard, OD, MBA, FAAO
Chief Optometric Editor, Optometric Management