By Jeff Hopkins

It was Lord Kelvin, the great British physicist and thermometer enthusiast who said, “To measure is to know. If you cannot measure it, you cannot improve it.” Perhaps this is obvious, but it sounds more impressive when it’s said by a great scientist. (In the interest of full disclosure, he also said “Heavier than air flying machines are impossible,” which is obviously wrong. But having been forced to fly coast-to-coast in an economy-class middle seat, I don’t think he was that far off.)

By now you’ve most likely recognized the application of Kelvin’s statement to eyeglasses. From the prescription itself to fitting the eyewear, it’s all about measurement. Precise PD and fitting height values are crucial to progressive lens performance, and additional measurements, done with high accuracy, can make the visual experience even better.

Which brings us to devices for measuring patients. Since the virtues of the ruler are already well-established, we will move directly to the high-tech variety, which are sometimes called “digital centration systems.” They come with varying capabilities and range from freestanding units to tablet-based. But they all fill one important role: taking the measurements your lab needs to create the best possible eyewear for your patient.

Given that there have long been ways of taking patient measurements that didn’t require batteries or a wall socket, why would you want a digital device? Let’s explore the question.


Using a ruler to take PD and fit height measurements is effective but not ideally precise. The more sophisticated lenses become, the greater the precision needed to get the best visual performance. And no matter how good you are at taking the measurements, there are limitations to how accurately a ruler can measure, and how consistent the results can be when different opticians use them.

An internal study by Carl Zeiss Vision showed that the average variance among measurements taken by different opticians using a ruler was almost 3 mm. Pupilometers are much more accurate, but different models showed measurement variance ranging from 1.2 to almost 3 mm. By contrast, a recent European study of four different digital measurement systems showed an average variance of 0.09 to 0.24 mm, depending on the device used. (“Comparison of PD Measuring Devices,”, Feb. 12, 2010.)

Are a couple of millimeters here and there really going to cause problems? Yes. A 2-mm centration error can reduce the size of the binocular field of view by 25 percent (“i.Terminal by ZEISS Frequently Asked Questions,” Carl Zeiss Vision, 2007). Progressive performance is highly dependent on the pupil being positioned the right distance above the corridor (good distance vision and comfortable access to the near) and aligned with the corridor (comfortable movement through the channel as the eyes converge for near vision.) These factors, in turn, depend on precise monocular PD and fit-height measurements. Errors in these measurements are among the leading causes of redos.

Find out more about types of digital measurement devices and their capabilities with the CE Taking the Measure of Digital Centration Systems, Part 1: What Does "Better" Mean? at