TECH TIPS – AUGUST 2012

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EACH ISSUE, TECH TIPS WILL EXPLORE SOME INTERESTING ASPECT OF OPTICAL TECHNOLOGY. THIS MONTH LOOKS AT PUPILLARY DISTANCE.
What are you measuring when you take a PD (pupillary distance)? Actually, it’s a bit more complicated than you might expect from the way most eyecare professionals (ECPs) measure it. The concept behind PD is to measure from the center of a patient’s right pupil to the center of the left pupil. This measurement was fine 200 years ago but if you’re looking for accuracy, this isn’t what you want to measure.

Of course, it’s the only thing you can conveniently estimate using a ruler (by measuring from the outer edge
of the right pupil to the inner edge of left pupil), which explains why ECPs are still measuring it.

VISUAL AXIS
Look at Fig. 1 and you’ll notice that vision doesn’t pass through the center of the eye; it passes at a slight angle. This is because the retina’s fovea (the most sensitive portion of the retina) is not directly in the back of the eye; it’s slightly off center (out and down). The line along which vision passes is called the Visual Axis, and as Fig. 1 illustrates, a person’s line of vision emerges out of the eye at an angle to the optic axis (which runs directly down the center of the eye).

Fig 1. Axis of the Eye

Fig 2. Visual Axis Crossing the Spectacle Plane

Fig 3. Visual Axis Location at the Cornea



CROSSING SPECTACLE PLANE

So what should you be measuring when you take a PD? What you want to measure is where the Visual Axis of the two eyes cross the spectacle plane for a specified fixation distance (Fig. 2). Translating that into simpler language, you want to measure where the lines of vision of each eye (the Visual Axis) cross the lenses when a person looks at some specified point in front of her like distance (infinity) or a 23-in. (about 60cm) intermediate distance.

Notice how the “lines of vision,” the Visual Axes, of these eyes cross the lenses for an object far away. The points on the lenses where these lines cross are where the lenses’ optical centers (or distance prism reference points) should be located. That’s what you should be measuring—this separation in millimeters.

LOCATION OF CORNEA
Fig. 3 illustrates the center of the pupil (cross mark) and the point where the Visual Axis crosses the cornea (spot). Notice that the Visual Axis is located in and up from the corneal center. This is because the fovea is off center as mentioned earlier. If you’ve ever used a corneal reflex pupilometer, you have seen this white spot of light.

You really don’t want to measure a patient’s PD–—the distance between their pupil centers. If you do, the wrong thing is being measured. What you want to measure is the separation between the Visual Axis as they cross the lenses.

HOW IMPORTANT IS THIS?
Many eyes have a minimal displacement from the center of the cornea of 0.6mm or so in each eye (1.2mm total). This amount can be much higher per eye, especially after refractive surgery or IOL implantation where it can be 3mm or more…per eye! In progressives where the eye needs to navigate down the center of the corridor and use the reading zone properly, not using a corneal reflex method to measure PD can lead to non-adaptation due to inaccurate measurement.

Remember, you really don’t want to measure the patient’s PD.



Ed De Gennaro is Director, Professional Content of First Vision Media Group.

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