Fix and Fit
part one: Optical
Measuring
By Alex Yoho, ABOM
This multi-part guide to basic measurements is designed to serve as a review for those who have been dispensing for years and as a training tool for those new to optical. This month, we'll look at taking PD and frame-related measurements.
So, where to start? At the beginning, and the fact that all optical measurements are based on the meter--there are millimeters and there are centimeters.
Even the diopter (a unit of measure of power or curvature) focuses a light ray at one meter. For example, if the doctor has prescribed a +2.50 add for near work at 40cm, and you have a patient who wants a single vision computer Rx at 60cm, you can calculate the add for the new working distance by using the standard formula: D=1/F, with D representing diopters and F, the focal length in meters. So 1/.60=1.66D.
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B&S's digital pupilometer |
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MEASURING THE PATIENT'S PD
That being said, let's move on to some of the most basic measurements. And none is more basic, or more important, than taking the patient's PD or interpupillary distance. Several methods are available to accomplish this.
Using pupilometers. Probably the most accurate method available is the pupilometer, sometimes more accurately called a corneal reflection pupilometer or CRP for short. Pupilometers measure the corneal reflex and hence the distances between the visual axes, not just the pupil centers. This is why the pupilometer appears to read narrow.
Pupilometers are more consistent and more accurate than the PD rule. As a result, they reduce the reject rate for progressive lenses. Their accuracy can be attributed to the elimination of parallax errors, an issue when your PD differs appreciably from the patient's.
With a pupilometer, one can easily take monocular PDs quickly, easily, and accurately, as well as near PD measurements for a range of working distances.
Another benefit is, that for someone with strabismus, you can also occlude one of the patient's eyes while taking measurements.
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Hilco's multi-use metal PD ruler |
Using a ruler. While holding the ruler in your right hand near the patient's eyes, have the patient look at your left index finger (which should be positioned just under your left eye). Then, align your ruler's zero mark with the nasal-most edge of the patient's right iris. Next, move your left index finger under your right eye (without moving the ruler), and instruct the patient to follow your finger with their eyes.
While the patient is following your movement, observe the measurement of the temporal-most edge of the left iris. This is the binocular distance PD. The theory is that if you get your eyes aligned with the patients' in a straight-ahead gaze and then measure the center of the patients' pupils, you will obtain a good distance PD.
The challenge is that when looking at someone who is a foot or two away, you tend to concentrate on a single point of reference. That's fine if you want a near PD. By having the patient look from side to side, however, you simulate a straight-ahead gaze and obtain an accurate PD.
To obtain a near PD, have the patient look at a near point such as your nose and, without moving anything, obtain a measurement at the same landmarks as described above.
What about taking an infant's PD? Measure from the inner canthus (corner of the eye) on one eye to the outer canthus on the other eye.
FRAME-RELATED MEASUREMENTS
Some offices edge their own lenses and for optimum thinness, they need to provide a few critical measurements known as box measurements. Many of the newer online ordering systems require this information. Here are tips for handling these measurements.
A measurement. This is the longest horizontal measurement of the lens shape.
B measurement. This is the longest vertical measurement of the lens shape.
ED measurement. The ED or effective diameter measurement is also critical in order for the computer to calculate lens blank size and thickness needed to produce the finished lens. Many people wrongly take the longest measurement of the shape and consider it the effective diameter. The true ED is twice the longest radian of the lens shape from the geometric center.
It, therefore, tells us what diameter lens blank is needed to cut the shape, but it does not take decentration into account. When you can't find the true ED, for a reasonable approximation, a guesstimate can be substituted by adding 3mm to the longest diagonal measurement.
DBL measurement. This stands for distance between lenses at their closest point. You can measure this with a millimeter ruler at the narrowest point between the lenses. Measure at the narrowest nasal area from the inner edge of one eyewire to the outer edge of the other eyewire. You may also choose to use a pair of lens calipers that are capable of measuring in the range of 3mm to 26mm or more. To measure, place the tips in each eyewire at the narrowest point.
Frame PD. The DBL added to the A measurement yields what is commonly called the frame PD. This measurement is useful in correctly sizing a frame to the patient.
Next time, we'll continue to measure all things optical with a focus on key lens measurements.
You may contact the author at ayoho@cox.net