Open-Field Binocular Refraction Approaching Natural Viewing Conditions

We spend most of our daily lives with both eyes open, that is, under open-field binocular conditions. Activities such as walking, reading, driving, and using smartphones rely on integrated binocular visual information supported by coordinated eye movements, fusion, and a balance between convergence and accommodation. Thus, it would be preferable to assess and prescribe corrective lenses, such as eyeglasses and contact lenses, under conditions that mimic real-world environments. However, conventional refraction involves testing under monocular conditions, due to structural and methodological limitations of the instruments. Although monocular testing is useful for isolating each eye’s refractive state, it differs from everyday vision; therefore, binocular balancing has traditionally been used to adjust monocular findings to better approximate binocular conditions.^1,2 Although binocular balancing can help equalize the accommodation between eyes and determine the final prescription, it has practical challenges, such as increased testing steps, dependence on the examiner’s skill, and extended examination time in outpatient settings. Although the ideal binocular approach has always been recognized, conventional methods have operated within technical and instrumental constraints.

Natural Viewing Conditions and the Importance of Pupil Size

Given that daily activities generally require visual binocularity, pupil size is a critical factor. Pupil size is affected by monocular occlusion versus binocularity, ambient illumination, and fixation distance. Under binocular viewing conditions, the pupil tends to remain relatively small, resulting in an increased depth of focus³ that affects both the refractive evaluation and the quality of vision. Notably, studies have reported that larger pupils can amplify the impact of astigmatism on visual function, leading to the conclusion that pupil size should be considered in astigmatism assessment.⁴ This is relevant for multifocal/EDOF IOLs and contact lenses, which are highly sensitive to pupil size. Clinically, even with the same refractive power, differences in pupil conditions can significantly affect visual quality and patient satisfaction.^5,6 Therefore, understanding the pupil’s size and shape in binocular viewing conditions is essential when considering postoperative or corrective vision quality. Ideally, refraction testing should be performed under binocular viewing conditions while maintaining a natural pupil state, but device limitations have made this difficult until now.

PHANTOM: the Open-field Binocular Refraction Device

One solution to these challenges is an all-in-one open-field binocular refraction system, PHANTOM (NIDEK), which enables a seamless workflow—from objective to subjective refraction and visual acuity testing—while maintaining unobstructed binocular vision (Figure 1). For objective refraction, the system uses an optical system with a concave mirror to achieve fogging for distance targets and simultaneous binocular measurements. In subjective refraction, it simulates the effect of conventional lenses by controlling the light rays entering the eyes, enabling the correction of refractive errors. Additionally, by controlling the convergence angle, the device facilitates testing at various distances under natural viewing conditions.

This system introduces a new perspective to the concept of refraction testing. Unlike conventional methods that require binocular balancing to approximate binocular vision, the PHANTOM performs the entire examination under true open-field binocular conditions, thereby simplifying the process and enabling refraction testing in a setting closer to daily life.

Clinical and Research Experience

The PHANTOM is being used in routine clinical practice and evaluated for research purposes. Although detailed quantitative analysis is ongoing, preliminary data indicate good agreement with conventional refraction. In individual cases (Table 1), open-field binocular measurements sometimes appear slightly more hyperopic than conventional monocular refractions (objective and subjective), which may reflect physiological differences between testing conditions, particularly pupil size. Because pupils are typically smaller under binocular viewing,³ depth of focus increases, which can reduce perceived blur and influence the refractive endpoint. Therefore, the pupil’s state and the viewing conditions under which refraction is assessed are important considerations; PHANTOM’s ability to evaluate refraction under conditions closer to everyday vision may be clinically advantageous.

Significance of Workflow Optimization and Reduced Examination Time

This device has significantly transformed the workflow associated with refraction testing, because it can successively perform objective refraction, subjective refraction, and visual acuity assessment. The steps often required with conventional refraction systems—moving between instruments, changing lenses, and binocular balancing—are no longer necessary. Hence, the total examination time has significantly decreased (Table 1), mitigating the physical and mental burden on clinicians while improving patient throughput. In facilities with high patient volumes or in settings with limited qualified staff, the ability to maintain consistent testing quality within a shorter timeframe is a major advantage. Additionally, reduced examination time also shortens patients’ waiting periods, ultimately enhancing their overall satisfaction with clinical care.

Conclusion

Traditionally, refraction and visual acuity testing have been performed primarily under monocular conditions, with prescriptions determined from those results. Although techniques such as binocular balancing can approximate binocular vision, the exam itself remains monocular, and binocular viewing is only indirectly estimated. In contrast, the PHANTOM enables a complete sequence—from refraction testing to visual acuity assessment—under open-field binocular conditions, offering an opportunity to rethink the conventional approach. Because real-world vision is binocular, evaluating refractive status and visual performance under such conditions is ideal. It simplifies the workflow by eliminating binocular balancing while allowing testing in an environment closer to natural vision, which is clinically significant. Furthermore, open-field binocular refraction supports patient communication, addressing common concerns in conventional testing (e.g., “I could see well during the test, but my glasses feel uncomfortable in real life”) by letting patients experience vision closer to everyday conditions and feel more confident and satisfied with their prescription.

Although research is still ongoing, preliminary experience suggests similar refractive endpoints with the PHANTOM compared with conventional testing. Additionally, reduced examination time may lessen the examiner’s workload, improve clinical efficiency, and shorten patient wait times, all of which may enhance the overall quality of outpatient care. Looking ahead, the role of refraction and vision testing will likely expand beyond simply determining lens power to evaluating visual quality under conditions that approximate everyday vision, thereby improving patient satisfaction. In this context, a device that enables comprehensive testing under open-field binocular conditions represents a promising direction for next-generation refraction and vision assessment, with broad potential for future clinical application.

Supplied by NIDEK. The views and opinions expressed here may not reflect those of Bryn Mawr Communications or Modern Optometry.