ACCURACY AND THE LENSMETER

I am sure every operator thinks that the readings they obtain are accurate. But I also think it is instructive to understand exactly what degree of precision and accuracy is possible with any type of lensmeter. Luckily, we have the work of a noted lens AO engineer, Don Whitney (father of Dick Whitney, the current curator of the Optical Heritage Museum in Southbridge, Mass., and present chair of the ANSI Z80.1 committee) to draw on to help answer these questions.

A trained operator using a well calibrated instrument is capable
of obtaining lens powers featuring an accuracy of 0.03 diopters

In the late 1970s, the optical industry was under assault. With the rise of eyeglasses as true fashion, along with a growing consumerism movement, there was a feeling in some government circles that the ECPs were unfairly restricting the choices of eyeglass buyers. This first manifested itself in the FTC’s Spectacle Release law, which tried to address the practice of offices holding the consumer’s eyeglass prescription hostage. At the same time this law was passed, there was also movement to allow the sale of over-the-counter (OTC) reading glasses without a doctor’s prescription. Concurrently, the FTC was exploring whether there were tangible risks to the public in allowing wearers to have their present glasses duplicated via neutralization, i.e., without a written prescription. Don Whitney exchanged correspondence with the FTC regarding his team’s work into the area of operator consistency, precision and accuracy using a manually-focused lensmeter to record lens readings. AO had recently presented the Model 12620 Lensometer—a modification of the just introduced Model 12603—but with an electronic readout of lens power that could be set to the unheard of precision of 0.01 diopters! The use of Model 12620 informed AO’s conclusions on operator accuracy and precision.

Don Whitney showed the FTC that two factors were at play in Lensometry readings: 1. The accuracy of the manufactured instrument, and 2. The operator’s ability to consistently obtain similar readings of an individual lens.

What Whitney found was:

• Manufacturer accuracy in AO Lensometers—These were calibrated with lenses traceable to the National Bureau of Standards—were traceable to 0.03D at 95 percent certainty.
• The repeatability of operator readings was found to lie between 0.05D and 0.03D, depending on whether the operator was of “average” skill or a “trained” operator, respectively.
• Whitney further noted a trained operator could refine their power reading accuracy to as low as 0.01D by taking 10 separate readings and averaging them for the result.

Of course, the effort required using 0.01D precision requires a dedicated operator with a passion for excellence. Having owned and used the Model 12620 Lensometer extensively, I can personally vouch for the positive influence on my development as a skilled optician.

–BS  

CELEBRATING THE ORIGINS OF LENSOMETRY

The introduction of the Lensometer effectively marked the beginning of a 50 year “golden era” in American optics, when AO and its chief rival, Bausch & Lomb, elevated the United States to the position of world leadership in ophthalmic optics, instruments and eyeglass-related technologies. There’s no better time than this centenary to celebrate the lensmeter’s creation, development and history, and to discover how it became so completely woven into the daily life of an optical office and lab that its users—to a person—agree the only way they’d give it up is for someone to “pry it from their cold dead hands.”

Just what kind of instrument could engender this type of loyalty? Certainly only one that could create a deep feeling of intimacy with its operator. After all—you are in the “kissing zone” when you’re up close with a lensmeter’s eyepiece. But today, despite having sophisticated alternates for measuring lens power—the auto lensmeter and the lens mapper, which promise either more precise readings or a portrait of the lens surface in question—the standard lensmeter reigns supreme because it delivers what an ECP truly values: A visual and qualitative picture of exactly what’s going on in the lens under review. Because of this, the manual lensmeter is the most important instrument used by optician, prescriber or lab—and all agree it deserves the highest levels of respect. I can think of no other single tool that can make—or through misuse—effectively break an eyecare professional’s reputation. For an instrument capable of such power, we should visit the optical era from which it was born.

Early lensometers carried the Wellsworth name, as this was an instrument-making
company previously purchased by American Optical

THE AGE OF THE LENS EXPLORER
The 19th Century should rightfully be thought of as the explorer’s age for human vision. From Wollaston’s exploration, starting in 1804, into making lenses with wide views of view, to Young and Airy’s pursuit to solve the riddle of astigmatism, much empirical experimentation and thought was being put into the birth of the science we know as optics. The 19th century was also home to most of the familiar names of optics: Gauss, von Helmholtz, Listing, Monoyer, Nagel, Gullstrand, Badal, Abbe, Javal, Snellen, Donders, Percival, Ostwalt, Tscherning and Prentice. All of these visionaries worked tirelessly to elevate the understanding of human vision from its dark ages and created most of the scientific and mathematical foundations that comprise the discipline of ophthalmic optics.

DEFINING LENS POWER
In the early 1800s, a world based on a universal measuring standard was still a dream. As the use of lenses to correct vision progressed, one of the challenges was a lack of uniformity in defining and labeling lens power. The reference then used for lens focal length was based on the inch of the old English system, but the inch was not yet precisely defined and varied in regions around the world. Fortunately, a more uniform system of measurement based on natural objects and decimals—the metric system—was being proposed in France. But it would take until 1866, when the ophthalmologist Nagel suggested using this new metric system for measuring lens focal length in a more consistent manner. Then in 1872, Monoyer of France coined the term dioptre to create a relationship between lens power and focal length based on the metre. Together, these two innovations—the use of the metre and the dioptre—were the first steps in separating the unique requirements of ophthalmic lenses from those used in general, technical and photographic optics. Still, determining the power of an eyeglass lens remained an imprecise process. The typical optician of the day would assess an unknown lens by moving it across a distant object. Viewing the distant object’s resultant motion, the optician would then try moving various lenses of “known” opposite power across the lens in question, until the target’s motion was canceled, or “neutralized.” This became the skill known as hand neutralization, and even today, we continue to refer to verifying power as neutralization.

KNOWLEDGE IS POWER
In the early years of the 20th century, ophthalmic lenses began transitioning from an empirical science to one more firmly grounded in technically sound mathematical and optical principles. To that end, it became clear that work needed to be done to ensure that both vision testing and prescription eyeglass fulfillment precisely met the needs of the wearer. In the world of technical optics—telescopes, binoculars, microscopes and the like—it was routine to measure focal lengths using the concepts of principle points and planes as the reference points. However, in the world of ophthalmic lenses, the location of a lens’ principle points varies with lens curve, form and thickness. Therefore, a more suitable way of defining ophthalmic lens power—one which encompassed the requirements of both trial and eyeglass frame—was created called vertex power. This defined lens focal length as measured from the peak curvature of the rear lens surface—the vertex—regardless of the lens’ form. In this way, the shape and thickness of a lens placed in front of the eye were no longer factors influencing its dioptric power. All the lens forms in use at the time—plano convex/concave, double convex/concave, and the manifold forms of meniscus lenses—periscopic, coquille, Wollaston, Ostwald, etc., could all enjoy a uniform way of defining their effective power in front of the eye. Vertex power is also referred to as back vertex power or effective power. Now all that was left was to find a convenient way for manufacturer, prescriber and lab to measure vertex power.

THE CHALLENGE OF MEASURING VERTEX POWER
The three main challenges facing the creation of a practical vertex power lensmeter were:

1. Translating the measurement of focal length from geometric to linear—Clearly, measuring the actual back focal length of a 0.25D lens, which is 4 meters, would dictate an instrument of impractical proportions. What was needed was a way to compress the size of the instrument and make the focal length readings readable using a linear rather than geometric scale.
2. Make the target mire comfortable to view, uniform in size and stable in luminance—determining the focal length, i.e., power of an unknown lens requires varying a target’s distance to discover where its focus lies. Along with the lens’ power, these two factors would clearly impact the observed size and brightness of the target image. Therefore, a way of ensuring the target image would remain a comfortable size and uniform in scale and brightness became important.
3. A way of inspecting the target image formed by an unknown spectacle lens that would not be influenced by either operator eye error or variations in operator accommodation.

How all three of these challenges were met was revealed in Edgar L. Tillyer’s 1920 paper for the first commercially successful AO Lensometer. By employing a standard/field lens of specific power—traceable to a known standard—positioned in compliance with a principle first outlined by the ophthalmologist Dr. Badal in 1877, and along with a telescope for enlargement and inspection of the target’s image, Tillyer easily met all three of these challenges. This new Lensometer was the culmination of a decade’s efforts. It featured a compact and linear dioptric power scale, uniform target size and luminance, and reduced the impact of operator eye error and accommodation on the accuracy of the ultimate power readings. The instrument presented a consistent 2.5 mm linear distance between full diopter steps and measured a lens using a 6 mm aperture—a value chosen because it approximates the eye’s fully expanded pupil. The instrument’s power range of plus/minus 20 diopters would encompass 99.99 percent of all prescriptions countered.

THE STAGE IS SET FOR SUCCESS
For a number of years, the new Lensometer gained wide acceptance for its ability to quickly determine lens power, axis and prism, and when equipped with a marker, to precisely mark lens centers. Competing manufacturers such as B&L developed their own model, which they trademarked Vertometer. In 1938, American Optical introduced the model 620 Lensometer, which included a frame table, or stage, upon which completed eyewear could be neutralized and verified. Analyzing eyewear for vertical prism imbalance and astigmatic axis became a breeze. And this wonderful instrument has remained more or less unchanged in form and function—right through until today.

ENTER THE AUTO
In 1956, American Optical became the first company to create an electronic automatic focusing Lensometer featuring a precision digital readout. The team that designed this instrument included Donald Whitney, who would later use this experience in the development of the Model 12620 digital reading manual Lensometer released in 1978. Over the years, automatic lensmeters have also become important tools to the optical office and lab. Here’s where auto lensmeters excel:
1. Power reading precision to 0.01 diopter.
2. Ability to enter the abbe value of the lens material in order to further refine the power reading.
3. Ability to toggle between the two competing world standards of 546 nm and 587 nm in selecting the preferred reference wavelength value for defining diopter power.

But electronic circuits, as audiophiles have long known, can influence the final result obtained because they have their own unique imprint that standard makers refer to as an instrument’s fingerprint. So despite promising an accuracy of 0.01D, most automatic models, because of their unique fingerprint, are actually no more accurate at 0.03D than a well calibrated manual instrument used by a skilled operator.

THE TRIAL OF TRIAL LENSES
All these efforts regarding effective lens power and accuracy would be for naught if there wasn’t a clear correspondence between the lens power used to prescribe and that used to fill an Rx. For successfully meeting this challenge, we have to thank Charles Prentice—the first to design a trial set for refraction whose lenses possessed a form and labeling that would permit them to be stacked together and their values summed for an accurate final prescription. In 1900, American Optical introduced the first trial set based on Prentice’s design, and these lenses represented an early attempt toward the fuller concept of vertex power, formally introduced later by Von Rohr of Zeiss.

ONE INDEX TO GUIDE THEM
It is interesting to note that lenses designated by vertex power are also free of influence from the lens material’s index of refraction. This is important because in the early years of the 20th century, crown glass was transitioning from “old” ophthalmic crown having an index of 1.53, which was actually 1.528 index rounded to 1.53, to modern ophthalmic crown having an index of 1.523.

–BS