Subjective Refraction Test Using a Smartphone for Vision Screening

Summary

This paper presents the protocols and clinical validation data for using a smartphone app to subjectively measure refractive error.

Abstract

To improve access to vision care and to enable mass vision screening, a smartphone app has been developed to measure refractive errors. Without needing any external attachment, the app running on a standalone phone can be used by lay personnel to measure subjective refraction. Its validity has been pilot-tested in clinical settings and underserved communities. The app estimates the refractive error by measuring the distances of far points for discerning visual stimuli. Spherical equivalent refraction and astigmatism can be measured using Tumbling E letters and grating patterns, respectively. The purpose of this paper is to describe the measurement protocols for performing subjective refraction using the app. Experimental results with 34 subjects (30 eyes for spherical equivalent and 38 eyes for astigmatism assessment) are presented. Measurement with the app was compared with standard clinical methods. The average absolute error of spherical equivalent refraction was 0.63D, and the average absolute error of astigmatism measurement was 0.28D. In addition, 22 subjects were enrolled to evaluate inter-pupillary distance (IPD) measurement with the app. The average absolute error in IPD measurement with the app was 1.2 mm. The protocol for measuring IPD with the app is also described.

Introduction

Uncorrected refractive error (URE) is a major cause of blindness and vision impairment in the world, affecting 86¹ to 116 million individuals², although it is treatable with glasses. Studies have shown that the prevalence of URE in remote areas is primarily driven by the low number of eye care professionals and the lack of an adequate health infrastructure to dispense glasses³. For instance, the prevalence of vision impairment due to URE among adults above 50 years of age in Sub-Saharan Africa is 10 times higher than that in high-income countries¹.

With the current advancements in the industry, the cost of eyeglasses has dropped to just a few US dollars. However, training eye care professionals is costly and time-consuming -- requiring years of training⁴. A recent study concluded that low per capita spending on health continues to limit meaningful integration of eye care within the broader healthcare systems, particularly in remote areas⁵. These grim realities point out the great need to make the diagnosis of URE accessible.

Thanks to their affordability, ubiquity, and validity, smartphone-based vision screening tools can play a pivotal role in vision screening efforts^{6,7,8,9,10,11,12,13}. These innovative tools may impact eye health care by providing a cost-effective and convenient solution for screening, identifying, and addressing vision issues, especially in underserved communities.

One example of such technology is the Peek Acuity app, which has made significant strides in the field of mobile vision screening. This app has been deployed to screen tens of thousands of individuals in some studies in Africa^14,15,16. By offering an efficient way to measure visual acuity, the Peek Acuity app has empowered healthcare providers to reach more people, making it a useful tool for addressing visual impairment. In addition to visual acuity, smartphone-based technologies for measuring refractive error have also been proposed and evaluated^17,18. Salmerón-Campillo et al. used a smartphone screen to present blue visual stimuli in a Badal optometer for visual acuity and refraction measurement¹⁷. Tousignant et al. tested the Netra smartphone refractor, which consists of a handheld binocular viewer with a smartphone inserted in it¹⁸. Compared with general smartphone apps, dedicated components or attachments other than smartphones in these systems may limit the accessibility of the technology because users have to purchase specially made devices.

To address the accessibility issue of mass URE screening, we have developed a smartphone-based refraction app (Figure 1), which employs computer vision and psychophysical methods to measure refractive error¹⁹. The app measures subjective refraction by finding the far points for given stimuli (tumbling E for spherical equivalent and grating for astigmatism) in myopic patients. A key feature of the app is that no specially made attachment is needed. All the processing needed to perform a measurement is carried out on-device within the app, and no cloud computing is involved. Thus, refraction can be measured without needing to connect the app to the network. With minimum training, laypersons can use the app to measure the refraction of patients if their smartphones are compatible. The accuracy of the app has been evaluated previously against standard clinical testing methods⁸. While the app may not be directly used for prescribing glasses, it has the potential to be used in myopia screening. Recently, it was successfully used in a vision screening among school students in a rural area⁹. This paper presents the protocols for using the app to measure subjective refraction.

Protocol

The study was conducted in accordance with the tenets of the Declaration of Helsinki at Mass Eye and Ear Infirmary (Boston, MA). Informed consent was obtained from all the participants. The study was approved by the local institutional review boards of Mass Eye and Ear (Boston, MA). Subject inclusion criteria were diagnosis of myopia and no other eye conditions, such as cataracts and retinal disease, according to an optometrist.

1. Measuring spherical equivalent

1. Launch the app and tap the Refraction button on the home page (Figure 1). Place the phone at least 2 m away from the patient.
2. Select button E for Tumbling E stimuli (Figure 2). Select the Eye to be measured (Left or Right) and ask the patient to cover the other eye.
3. Hold the phone with the screen facing the patient and tap the Start button. Ask the patient if he/she can tell the orientation of all the letters displayed on the phone screen.
4. If the patient cannot tell the orientation of the letters, gradually move the phone towards the patient.
5. While gradually approaching the patient, keep checking whether they can identify the letters. Stop as soon as the patient can tell the letter orientations.
6. Tap the Verify button. The true letter orientations will be shown in text on the screen. Compare them with the patient's report. If all 3 answers match, tap the Correct button to conclude the testing. If the orientation of any of the letters is not correctly reported, tap the Wrong button to redo the testing.

2. Measuring astigmatism

1. Launch the app and tap the Refraction button on the home page (Figure 1). Start from at least 2 m from the patient.
2. Select Astigm 1 stimulus, which is a clock dial pattern (upper right inset in Figure 3). The stimulus consists of a series of line groups from one spot pointing toward different directions, like a clock. Each line group includes 3 parallel fine lines.
3. Select the eye to be measured and ask the patient to cover the other eye. Hold the phone with the screen facing the patient and tap the Start button.
4. Ask the patient whether the line group in any of the directions appears as 3 separate lines. If the patient cannot see separate lines in any of the directions, gradually move the phone towards the patient.
5. Keep checking with the patient during the approaching movement. Stop as soon as the patient can see separated lines in at least one direction.
6. Select Astigm 2 stimuli, which are two grating patches in red and green colors. These patches are wider on one end than the other (lower right inset in Figure 3).
7. Rotate the phone around the axis perpendicular to the phone screen to a position where the grating is roughly the direction of the clearest line group of Astigm 1.
8. Fine-tune the phone rotation to find the best spot where the patient can see the red and green grating patches equally clearly. Once the best spot is found, tap the Point 1 button to record the first far point.
9. After Point 1 is recorded, the Astigm 2 stimulus will rotate 90˚ automatically. From the first far point (Point 1 position), keep the phone orientation and move the phone closer until the patient can see the red and green grating patches equally clearly and tell the wider end.
10. Tap the Point 2 button to record the second far point. With the two far points recorded, the app can calculate spherical refractive error and astigmatism and show the results on the screen.
11. On the result message box, tap the Save button to save the result or, if needed, the Retake button to redo the measurement.

3. Measuring inter-pupillary distance (IPD)

1. Launch the app and tap the IPD button on the home page (Figure 1). Place the phone approximately 40 cm away from the patient at eye level and ask the patient to look at the flashlight.
2. Tap the round button on the right to take a picture of the patient's face. The app will then start processing the image.
3. When two green crosshairs are plotted on the screen (Figure 4), examine if their locations are aligned with the center of the eyes. If yes, tap the Save button.
4. If either of the cross hairs is apparently not at the center of the eye, reject the measurement by tapping the Retake button to go back to step 3.2 and redo the measurement.

Results

For this study, the interface of the refraction test is shown in Figure 2. Depending on the selected stimuli, the app performs a spherical equivalent or full refraction test. When Tumbling E is selected, the app measures the spherical equivalent (Figure 2). When grating stimuli Astigma 1 or 2 is selected, the app measures refractive error, including astigmatism (Figure 3).

To demonstrate the app's eff...

Discussion

Using the app, it is feasible for a person without professional optometry training to perform subjective refractive error tests. Its application in vision screening has been demonstrated in a recent eye screening study among school-aged students in a rural area⁹. Compared to the other mass vision screening methods that are solely based on visual acuity testing²¹, this app can provide measurement of refraction in terms of spherical and cylindrical error values typically used...

Materials

Name	Company	Catalog Number	Comments
Smartphone	Samsung	Galaxy	commercially available smartphone