A New Look at Visual Acuity

A critical tool in every optometrist’s toolbox is the phoropter or trial lens kit. Whether it is used for performing a subjective refraction or simply measuring a patient’s visual acuity (VA) with a given prescription, these tools are essential.

A manual phoropter.

The phoropter is the current standard of care, and has a dazzling array of features and capabilities, ranging from standard corrective lenses to special diagnostic lenses like Maddox rods and polarizing lenses. Phoropters cover a large range of refractive errors, and have precision machined metal and glass components for the highest standard in accuracy.

The downside of a phoropter is that it is large, heavy, and requires a special mount or stand to hold it in front of a patient’s face. For situations where more mobility is required from the tool (or for budget constrained practitioners), a trial lens set is often used.

A trial lens set.

The trial lens set consists of a range of basic spherical and cylindrical corrective lenses that can be placed in a trial frame. They are more cumbersome to use than a phoropter, lenses are easily misplaced or mixed up, and the set is still quite heavy, weighing up to 15 lbs. All it takes is one clumsy moment, and you’ll spend the next hour sorting lenses back into the right slots and making sure you haven’t scratched any.

We set out to fill the gap between these two products. What if we could take the ease of use and compact design of a phoropter and somehow make it more portable? Could we leverage our experience with plastics design and production to make a portable, lightweight, simplified phoropter that could be taken out in the field?

Enter the Netropter

Our answer is the Netropter. The Netropter operates for both distance and reading tests as well as monocular and binocular readings and fully replaces trial lens kits. It is designed to be used in the patient’s hand instead of on a supporting structure. It is light-weight and the lenses are molded and polished with high-precision tools to get them the closest to their labeled power.

When designing the Netropter, we started with a phoropter design and started trimming. We minimized the spherical range (to -10.75 -> +5.00) and removed all of the auxiliary lenses to reduce the size and weight of the device. While all of the auxiliary lenses are important, they are only used in a tiny fraction of cases, for the vast majority of patients, the sphere and cylinder lenses suffice. In cases where a Jackson Cross cylinder is required, an external JCC can be used just as when using a trial lens set.

We reduced the lens diameter a small amount in order to dramatically reduce the size of the device (even a 1mm change in lens diameter results in a 10mm increase in the total width of the device because of how the lenses have to be arranged). We settled on a 15mm lens diameter, compared to 17–20mm on a classical phoropter. Our aperture matches that of classical phoropters which have apertures ranging from 15–19mm. When you take into account how thin the Netropter is (30mm), the field of view remains similar to that of conventional phoropters.

A single molded part allows us to pack the lens surfaces directly next to each other, putting more lenses in a smaller form.

Another place we were able to reduce costs, weight, and size was in our material choices. Except for fasteners, everything in the Netropter is made of plastic. This allowed us to pack more lenses in a smaller space by molding the spherical lens carousel as a single piece rather than inserting separate lenses into a holder. Much has been said about the importance of Abbe value, but upon closer examination, this has been shown to be of little importance, especially in the context of a phoropter.

A patient holds the Netropter in one hand and a reading chart in the other while assessing near VA.

Instead of a stand, which would add to the bulk of the device and be difficult to implement in the widely varied environments encountered outside the office, we chose to design the Netropter to be held by the patient themselves. This allows the patient to automatically align the device to their face, easily positioning the apertures in front of each eye. With such a light weight, even elderly patients hold the device for the duration of a VA check with little difficulty. Patients are also able to hold the device in one hand with a reading chart in the other for checking near VA.

One of the 16 cylindrical lenses in each Netropter, molded with integrated gear teeth which allow for a smaller device.

Go Forth and Measure

Whether you’re going on a mission trip to a remote village or just going across town to a school or nursing home, the Netropter takes the usability of a phoropter and combines it with the portability of a trial lens set. The Netropter integrates a full-range of sphere, cylinder and axis values to validate and refine refractive measurements for distance and reading. It also includes a pupillary distance adjustment, lock and occluders for monocular testing. Running monocular and binocular visual acuity tests for distance and reading glasses has never been so simple. Interested in learning more? Find more information on EyeNetra’s online store here.