This is a reprint of a 2020 Continuing education divided into four parts.

Measuring PD

For eye care professionals, it wasn't until the arrival of progressive lenses in the 1960s that the importance of an accurate monocular PD for obtaining optimal intermediate and reading utility was fully appreciated. Before that time, measurements primarily centered on obtaining binocular PDs. Using a millimeter ruler, monocular values, if desired, were derived by halving this value.

Measuring Methods and Accuracy

Several studies in the last twenty years have attempted to scientifically analyze the accuracy and repeatability of obtaining PDs using three primary methods:

  • PD Ruler - Used to measure the distance between pupils, corneal reflections, iris margins and particularly in the case of small children, the inner and outer can-thus. A study completed in 2002 assessed that the ruler method can yield values accurate to within 1.54 mm of a gold standard (see below) with 68% confidence.
  • Pupillometer - Primarily measures the distance between corneal reflections. Overall values were found to be within 0.74 mm accurate with 95% confidence.
  • Digital Centration Devices - These may use either corneal reflections or iris borders for determining PD. The user should be aware if the device's software employs an internal calculation to extrapolate at the visual axis location from the pupil/iris center. DCDs were found to be very accurate instruments, with binocular PDs found to be within 0.40 mm with 95% confidence.

There's a pattern here: Pupillometers appear more accurate than millimeter rulers and digital centration devices appear more accurate than pupillometers. But before you ditch your PD ruler, remember that the ruler and manual measurements will always play an important role in an ECP’s toolkit. Both objective and subjective verification and troubleshooting of prescription eyewear will continue to require expertise in the use of ruler and pen. Because of this, some practitioners prefer to use an enhanced manual method, which employs a mirror to subjectively determine the visual axis at the spectacle plane. Each method has to consider variables that may impact accuracy and efficacy.

Below is a summary of the most important factors affecting pupillary measurement:

  1. Device Calibration - While obviously important when using instruments, parallaxic errors can arise when using the ruler method if there are differences between the PDs of tester and subject. These are avoided by using Jalie's Rule of Sixteenths, which states that every millimeter difference PD requires an adjustment of 1/16mm to the end result.
  2. Instrument Operator skill/experience -
    1. Repeatability - Can a single operator take several similar measurements from the same patient?
    2. Serial - Can multiple operators take similar measurements from the same patient?
  3. Near and Intermediate Considerations-
    1. Vertex distance - Generally based on an average value of 13 mm. However, if the fitted VD departs from this number by 4 mm or more, then values for near and intermediate use will not be optimal without recalculation.
    2. Pantoscopic Tilt-Improperly measured values can result in deviations that may require compensation.
  4. Frame Considerations - A person’s physiognomy or fitting preference may place the frame markedly off the facial median plane, a deviation that, can make the efficacy of the found values suspect.
  5. Head Cape - Defined as the habitual departure from the orthogonal facial plane, head cape compensation is important for both optimal binocularity and multifocal utility.
  6. Rounding Errors - present in all methods, rounding errors can affect accuracy.
  7. Patient Cooperation - This is one of the least controllable of all variables affecting PD measurements.

The Gold Standard in PD

In the 2009 study, 8 different devices, both pupillometers and DCDs, were used to help answer an important question:

Considering all the variables affecting a PD measurement, can we be confident that using any method or instrument will result in the complete truth of a precisely accurate PD? The answer is no. Faced with determining the truth of an objectively unverifiable parameter, statisticians often turn to averaging all the measurements to a mean value, and then observing the distribution of all the values found to this mean. This “best possible compromise" is then referred to as the gold standard. The 2009 study sought to establish both device repeatability (9 subjects tested in 5 rounds by 8 different instruments) and serial consistency (80 subjects measured once by 8 different instruments). When the data was adjusted for various corrective factors, the average for all devices was found to be within 0.6 mm of the gold standard with 95% confidence.

PD and the Consumer as King

In our political system, consumers are not always active participants. Nevertheless, government officials recognize that consumer influence can be great if an issue surfaces with enough sizzle to stir these voters out of bed on election day. It's clear today that lawmakers increasingly tend to consider regulatory issues before them in “consumer-friendly” terms. In Utah for example, legislation is pending that makes room for advancements in technology that facilitate “alternate channels of distribution” for medical services, some of which had previously been done only by licensed professionals. Therefore, as consumers increasingly feel comfortable taking their own blood pressure and heart rate in a kiosk at a local mall, lawmakers are also considering allowing contact lenses and even eye exams to be made available in a similar manner.

Only two states, Kentucky and Massachusetts, mandate that a PD be entered on every eyeglass prescription. But if eye care professionals continue to ignore the growing demand for freedom of choice in buying prescription eyewear, they might just see their state government become both a consumer advocate and a professional adversary. More than ever, eye care professionals must reformulate their business strategy to successfully compete in an economy where consumers are infatuated with the low prices, wide selection and convenience of online.

Barry Santini, ABOM, graduated from New York Technical College in 1975 with an AAS in Ophthalmic Dispensing. He is a New York State licensed optician with contact lens certification, is ABO certified and was awarded an ABO Master in 1994. As sales manager for Tele Vue Optics from 1987 to 2003, Santini developed his knowledge of precision optics and has been an owner of Long Island Opticians in Seaford NY, from 1996 to present. Recently The Society to Advance Opticianry named Barry Santini, OO, as the 2016 Ophthalmic Optician of the Year. In addition, Santini is an amatuer astronomer and lecturer and plays bass trombone in the Brooklyn Symphony.