A Practical and Complementary Approach to the Gold Standard of Subjective Refraction– Wavefront Refraction

We recently held a webinar with optometrist Dr. Tihomira on how to incorporate telemedicine into your optometry practice. Dr. Tihomira is a leading optometrist who embraced telemedicine and is influencing the technology development. One of the main topics she discussed was how automated refraction and telemedicine technologies are challenging the gold standard of subjective refraction. Subjective refraction is widely accepted as the optimal refraction method in the industry, despite how variable the process can be. So if subjective refraction is not necessarily the most repeatable method, why is it still considered to be the best way to determine someone’s prescription?

The use of a manual phoropter to conduct a subjective refraction continues to be considered the gold standard in the industry. The question is why? This process is influenced by multiple factors including; the ambient lighting of the room, the quality of the projections system, the cognitive awareness of the doctor and the patient during the process, and both the doctor’s and patient’s previous experience with refraction. All these factors play a role in the determination of the final prescription and the accuracy of that prescription.

Doctors try to control as many variables as possible but ultimately refraction remains a very variable process. A clinical study published by Bullimore illustrates the variability of subjective refraction. The study compared refractions done on the same patient by two separate clinicians and found that, for sphere, the 95% limit of agreement varied from -0.9 D to +0.6D. The study also reported that automated refraction is more repeatable than subjective refraction.

Automated Refraction

Wavefront aberrometry is challenging this gold standard of subjective refraction due to the repeatability of its results. When measuring the visual performance the technology assesses two components of the visual system: the optics that form the retinal image and the neural processing that transforms the retinal image into perception. Subjective refraction has historically been unique among methods of refraction. In considering both components in asking the observer to choose between a series of options, the goal is to optimize visual acuity.

However, wavefront aberrometry together with visual image quality metrics are changing the old model. A study published this year by Hastings et al. showed that 72% of patients preferred prescriptions generated using wavefront aberrometry optimized by visual image quality matrix, compared to prescriptions generated by subjective refraction. The SVOne autorefractor will include wavefront aberrometry in September of 2017.

The SVOne Technology

The SVOne autorefractor uses Shack-Hartmann wavefront autorefraction to capture 3 images per eye, taking 3 seconds per eye. Through the analysis of up to 120 points, a Zernike decomposition algorithm extracts the low order aberrations and converts them to sphere, cylinder, and axis. There are several unique features of the instrument, one of them is a least-squares reconstructor algorithm that adapts to pupil size on the fly. The device also features an open field design with machine learning algorithm allowing the system to – as soon as optimal pupil alignment is achieved – autocapture the measurements, thus eliminating proximal cues and limiting accommodation.

The SVOne is capable of measuring a wide range of refractive error, from -14 D to +14 D sphere, up to -7 D cylinder in 0.01D increments and axis measurements in increments of one degree. As you can appreciate from the image shown here, the device is very small, nothing like the autorefractor you are likely to find in most optometric offices. Additionally, it weighs less than 1 pound. To make the transfer of data easy, the device has the capability to store data in a HIPAA compliant cloud platform that can send data to your EMR.

Clinically Proven Accuracy of the SVOne Autorefractor

Clinical study data has been able to highlight the accuracy of the SVOne autorefractor. A study published by Ciufreda et al. in 2015 showed that SVOne refraction is accurate and has higher repeatability than subjective refraction. The study was conducted using 50 normal adults age 18 to 34, all correctable to normal vision. For all subjects retinoscopy, SVOne refraction and subjective refraction were determined and the difference between each of the findings and subjective refraction were quantified using the 95% limit of agreement.

data results from Smart Vision Labs clinical study

The table above shows the values calculated for the 95% LOA in diopters for retinoscopy and SVOne refraction as compared to subjective refraction. The table shows that for sphere, retinoscopy and SVOne show the same limit of agreement. Also, this finding shows that SVOne sphere measurements have variability similar to that of subjective refraction between clinicians as previously reported. The SVOne measurements for cylinder and axis were a bit more variable than retinoscopy, but as the authors concluded that could be due to alignment error, something that has been addressed by the introduction of a stand to which the instrument can be attached to, thus virtually eliminating misalignment errors and contamination of cylinder and axis measurements.


The next table above shows repeatability data of 10 subjects. The data shows that SVOne refraction had higher repeatability than subjective refraction. This finding is again consistent with previous reports by Bullimore et al. showing that automated refraction is more repeatable than subjective.

The adult study was very strong but clinically we also needed to test the accuracy of the device on the majority of the population. A study was conducted to test how well the device performs within the pediatric population. Conducted by Rosenfeld et al., the study showed that SVOne refraction is also accurate in children and has higher repeatability than subjective refraction. The study was conducted using 40 normal children age 5 to 17, all correctable to normal vision. The study followed the same design as the adult study.


The table above shows the values calculated for the 95% LOA in diopters for retinoscopy from SVOne refraction as compared to subjective refraction. The data shows that while sphere measurements using retinoscopy were a little bit more accurate, SVOne refraction varied by only additional +/-0.2 D, less than a quarter of a diopter. And both methods showed the same LOA for cylinder and axis.

The next table above shows repeatability data of 5 subjects. The data shows that the SVOne has the highest repeatability across all components of the refraction, sphere, cylinder and axis. The study confirmed that the SVOne is an accurate and repeatable way to measure refractive error in children.

SVOne Refraction: The New Gold Standard?

Through clinical studies and the feedback and results from doctors using the SVOne, we were able to conclude that automated refraction and the SVOne are certainly challenging the gold standard of subjective refraction due to the accuracy and repeatability of refraction results.

SVOne technology is a practical approach to solve the challenge when subjective refraction is too costly or unavailable. If you are running multiple optical retail stores, and have difficulties to capture walk-in customers due to lack to ability to provide on-site vision exams. SVOne is the perfect solution to provide fast and accurate eyewear prescriptions to your customers. If you are running a mobile clinic, and conducting manual refraction is too cumbersome, SVOne is the solution to provide clinically proven vision care to your customers. If you are running an overseas mission trip, and facing thousands of patients in the duration of a few days, SVOne is the portable solution that can offer efficient vision exams in a big way.

If interested in learning more about the SVOne autorefractor, request a demo to see the refraction process.

Demo the SVOne Autorefractor

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