fix and fit
Lens Part Two: Lens Measurements
By Alex Yoho, ABOM

In the previous Fix and Fit column, we covered the basics of measuring using a pupilometer and a PD ruler. This month, we'll cover the finer points of decentration, circumference, multifocal heights, and more.

Following are measuring guidelines for eyecare practitioners.

Decentration measurement. Selecting a frame demands that you know the patient's PD and the frame PD, since it impacts lens thickness. Calculating decentration requires the frame PD (A + DBL = Frame PD). Then we use the formula: Frame PD - Patient's PD = Decentration.

The decentration measurement is used by the lab to position the lens' optical center correctly before the eye. It should also be noted that lenses thicken with decentration. A good rule of thumb is to select a frame whose PD is within 6mm of the patient's PD.

Measuring circumference. Circumference, the distance around the lens, impacts the sizing of the lens for a good fit. If the circumference is 1mm too large, there will be a gap at the eyewire closure and you risk a chipped or cracked lens if you edge it down or try to force it. An accurate c-size is not an absolute guarantee that the lens will fit, however. If the angle of the lens bevel is not the same as the frame's groove, the lens could be too loose or too tight, depending upon the bevel angle.

Reading the c-sizer. This is fairly simple: Place the lens in the c-sizer box and pull the tape just firmly enough to surround the lens and remove all gaps between the tape and lens. Tugging on the tape with constant pressure, note the direct millimeter reading at the long reference mark. Have your calibration device checked by the lab doing the edging to ensure the same readings.

Multifocal heights. The MRP or major reference point is at the center of the pupil as the patient looks in a straight ahead gaze. MRP is useful for single vision or a progressive addition height. For bifocal heights, the lowest edge of the iris, or limbus, is the best place for the flat top, or round bifocal, to be measured. For trifocals, place the line at the lower edge of the pupil. Though this puts the bifocal part a little lower than the limbus, it covers all three focal distances.

Changing eye sizes. What if you measured the patient's bifocal with a different eyesize than they will wear? If changing the eyesize, add or subtract the difference in eyesize to the seg height originally ordered.

You may contact the author at ayoho@cox.net.