June 18, 2018
The EyeQue Personal Vision Tracker™ (PVT) has been commercially available since Jan 2017. Since then over 11,000 individuals have used it to conduct over 40,000 refraction tests.
The question most people considering the EyeQue PVT have is just how accurate is it? Can a simple low-cost, DIY device give results comparable to those achieved by sophisticated auto refractors or experienced optometrists. The answer is yes!
Optically speaking the Personal Vision Tracker is an ophthalmic refractometer, a class 1 medical device as defined by the FDA. Based on an MIT patent, the EyeQue refractometer has proven to be highly accurate and objective over a wide range of spherical (+8 to -10 diopters) and cylindrical (0 to -5 diopters) refractive error values. With a high-resolution phone, for example a Samsung Galaxy phone with over 500 pixels per inch, the optical resolution using the PVT is 0.15 diopter. By comparison eye glasses are prescribed to the nearest 0.25D. So, what does this all this mean? What is a diopter?
To start, a diopter is a measure of an optical system’s focusing strength. The larger the diopter, the higher the focusing strength. The human eye is an optical system with a focusing strength of about 50 diopters. That is, the eye lens or cornea takes incoming light and focuses it on the retina which is about 20 mm behind, which corresponds to 1/20 mm, or 50 diopters. If someone has a refractive error, the eye lens does not focus light exactly on the retina, and this can be corrected, for example, with eye glasses. If the focus is before the retina this is called myopia and behind the retina, hyperopia. The other refractive error is called astigmatism, and represents a non-spherical aspect of the eye. Astigmatism is approximated to be a cylinder component oriented at a particular angle from the horizontal called the axis. EyeQue measures refractive error and expresses these values in the terms used for ordering prescription eye glasses. The amount of correction is typically small compared to the 50 diopter strength of the eye. If someone’s eye has a spherical refractive error of -2.0D, this corresponds to the light focusing about 0.8 mm before the retina.
The Personal Vision Tracker is designed to be used by the average person and as such, the refraction values generated as a result of your own testing cannot be considered a prescription – unless you are an eye doctor using the device to write a prescription. EyeQue uses the term Eye Glass Numbers when referring to the three measurements taken. An example set of EyeGlass Numbers (EGNs) is shown here. In this case, the right eye has about -1.5 diopter of spherical error and a cylindrical error of about -1 diopter, oriented at 85 degrees from the horizontal plane of the eye. The blue dots represent the actual refraction tests used in computing the EGN.
Since the tests are self-administered, it’s critical for the user to follow instructions and take at least three refraction tests. Well over 80 percent of our users obtain a level of “good” or “excellent” testing consistency where the results are within 0.25 D of each other. Good consistency suggests they are carrying out the test correctly. Our systems can detect, however, when test results are inconsistent, and this triggers an email to encourage our customers and provide additional resources for improvement.
Paradoxically, accuracy is defined subjectively – as for how close a measurement comes to the perfect working eyeglass as perceived by the eyeglass wearer. With respect to human vision, there is no single true value of sphere, cylinder, and axis that is “best”. To complicate the issue, vision changes with time, in many cases even on the order of hours or days. And to complicate the subject further, young people can change their focus substantially given the flexibility in their eye muscles and lens, putting an uncertainty around any refraction measurement. This accommodation effect is a well-known problem and to that extent, we do not recommend the use of the PVT for persons under 18 years of age.
These complexities are well known and optometrists often define the prescription process they administer as a combination of science and art. Optometrists often give a different value than what the pure refraction results suggest. If you were to take a group of optometrists and they were to all provide a prescription based on the results of the same individual, the range of spherical equivalent (SPHE) could vary by up to 0.5D from each other. In other words, there is no magic set of numbers, and there is no single right answer.
A further complicating factor is that the brain does a fair amount of image processing and correction, so even if an eyeglass prescription is not a good match to the user’s measured refraction it could still be acceptable to a person because their brain adapts.
There is a term called the Spherical Equivalent which is the average refraction correction of the eye, defined as the Sphere plus ½ the magnitude of the Cylinder. So the SPHE is the same, for a range of Spherical and Cylindrical Values.
Therefore, when all is said and done, accuracy is best defined as the refractive correction that the user finds most comfortable.
Many of our EyeQue subscribers have provided previous optometrist prescriptions for comparison to their EyeQue EyeGlass Numbers (EGNs). We’ve also received numerous testimony from persons who purchased eyeglasses using their EyeQue-based refraction numbers (EGNs). The results are most positive. In a recent survey we conducted with users that have carried out the EyeQue test with good or excellent consistency, over 90% said the eyeglasses purchased using their EyeGlassNumbers were as good or better than what their optometrist prescribed – specifically 30% said they were the same while 60% said the new EGN glasses perform better. If a user does well on their testing with the EyeQue device there is a high probability that the results will be close to their recent optometrist’s prescription, and the EGNs will result in a satisfactory eyeglass refraction fit.
One of the advantages of the EyeQue PVT compared to the once a year visit to the optometrist is that you can take refraction tests anywhere, and more importantly, anytime. EyeQue averages the test results over time, discarding results with obvious error and producing a more reliable aggregate set of values. This has certain benefits in assuring a good match.