The optician is diligently positioning this clip so it will provide highly accurate results.
This patient is being told to look at a chart on the wall across the room in order to create a natural head posture.

Each issue, Tech Tips will explore some interesting aspect of optical technology. This month we look at taking accurate digital eyewear fitting measurements.

Here are some tips to follow when using measuring devices.

You’ll find units for digital eyewear, information, measuring, and selection systems (DEIMSS) in freestanding versions, desktop models, and many have emerged as apps for iPads. While these devices are designed to provide many features, their measuring aspect requires the user to understand what the device does and how it takes its measurements in order to get accuracy intended by the designer. Here are some guidelines to follow if you have one of these units or you’re planning to buy one.

One of the biggest errors in using one of these devices is that the optician does not pre-adjust the frame or does so haphazardly. In years past, pre-adjustment meant doing a little adjusting to get the frame close to where it would sit when delivered. Some opticians have been known to hold the frame front in place when taking fitting measurements. That might have worked years ago for bifocals, but if you’re recommending premium free-form progressives, that’s not going to work. DEIMSS can provide digital measuring accuracy, but only if the frame is sitting in the location where it will be when the eyewear is dispensed.

This means that you have to adjust the frame enough so that it sits in the correct position in front of the eyes. It doesn’t have to be perfectly adjusted behind the ears, and the nosepads don’t have to be perfectly fitted either, but the frame’s horizontal alignment to the face, the vertex distance to the eyes, and the pantoscopic and face-form (wrap) adjustments must be as they will be in the delivery of the eyewear. If you don’t pre-adjust the frame as suggested, you’ll obtain highly precise inaccurate measurements.

Nearly all DEIMSS use a clip that fits onto the frame. This device provides markers for the DEIMSS that are utilized in creating measurements. The unit assumes the clip is perfectly fitted vertically and horizontally. If it is not, errors can occur. For example, let’s assume the right side is fitted higher than the left. This will give a fitting height error. Clips are similar but different, so be sure to know the one you’re using well. Follow the manufacturer’s instructions diligently when installing the clip. Some are more user-friendly than others so make sure you understand how to install it precisely and what a properly installed clip looks like.

Digital eyewear units expect the person to hold their head in a natural position when taking measurements. Some people naturally hold their head high while others hold it low, and still others have a natural head tilt. Whatever their natural head posture is, that’s what you want when taking measurements. The mistake some opticians make is in telling the patient to hold their head in a natural position. As soon as they say that, of course, the person becomes self-conscious and begins hunting for their natural head position having no idea what that is. Instead, I suggest you simply ask them to look across the room at some distant object. As they do, they’ll automatically assume a natural head position. Do the same thing for near if your unit asks for a reading position measurement. Handing the patient something to read is the easiest way to do this.

Another head position error to avoid is a head turn. If the patient turns their head unnaturally during the measuring process, even a slight amount, this will induce a monocular PD error. Most clips have a way to determine if the patient’s head is turned so you can rectify the situation.

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


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