Participants

In total, 28 healthy volunteers (mean ± standard deviation, 21.2 ± 1.5 years old; range: 18–25 years) were included in this prospective study. All subjects underwent complete ophthalmological examinations, including measures of best-corrected distance visual acuity at 5.0 m, stereoacuity at 40 cm (Titmus Stereotest; Stereo Optical Co., Inc., Chicago, USA), close (33 cm) and distant (5.0 m) alternating prism cover tests to identify heterophoria, and fundus examinations. Patients with ocular disease and a history of ocular surgery were excluded from the study. The individuals with large phorias were not excluded in this study.

After explaining the nature of the study and possible complications, all subjects provided informed consent for inclusion in the identification of information/images in an online open-access publication. This study adhered to the tenets of the 2013 revised Declaration of Helsinki by the World Medical Association. The Institutional Review Board of Teikyo University approved the experimental protocol and consent procedures (approval no. 21–067).

ApparatusChronos

This study used the Chronos binocular/monocular refraction system (Fig. 1). It was equipped with two auto refractometers based on the KR-800 automated refractometer (Topcon Corp, Tokyo, Japan) to measure the objective ocular refraction from − 25.00 to + 22.00 diopters (D) for spherical refractive error and up to 10.00 D of cylinder refractive error. The eye trackers detected each eye from the pupil and corneal reflections, positioned the auto-refractometers in front of each eye, and measured the ocular refraction in both eyes. The target fixation image was a house with a red roof.

The Chronos binocular/monocular refraction system uses several optical display systems for subjective refraction. The two optical systems project liquid-crystal images onto the subject’s right and left eyes and use the following four mechanisms to achieve a subjective refraction function:

1.

Control of the convergence stimulus: The angle of measurement is adjusted by the three motors to realise a convergence stimulus at any distance from 25 cm to 6 m.

2.

Control of accommodation stimulus: The spherical lens position is adjusted using one motor to realise accommodation stimuli at any distance from 25 cm to 6 m.

3.

Control of spherical power: Spherical lens position is adjusted by one motor to realise any spherical power setting from + 18D to -18D.

4.

Control of cylindrical power and cylindrical axis: The rotation angle of the two cylindrical lenses is adjusted by two motors to realise any cylindrical power and cylindrical axis setting from 0D to -8D [13, 14].

Chronos binocular/monocular refraction system can display optotypes on the two liquid crystal displays and occlude a non-tested eye using the console; however, the display for that eye does not turn off completely. The target (e.g. the Landolt ring) vanishes (Fig. 2A), whereas the peripheral frame remains to maintain binocular fusion (Fig. 2B).

The subjects fixated on the Landolt ring at a 5.0 m distance, which was adjusted optically, and the first ophthalmic lenses were selected automatically, based on the objective ocular refraction in both eyes. The examiner determined the subjective ocular refraction while examining the participant’s response using a Bluetooth tablet linked to Chronos binocular/monocular refraction system. The tablet device can change the status of the Chronos binocular/monocular refraction system, such as the measuring distance, spherical and cylindrical lens power, and target (Fig. 3).

Fig. 1

Experimental Chronos binocular/monocular refraction system set up. (B) Enhanced image of the yellow square in (A). (A’) and (B’) show the images acquired during the actual measurement. A headrest and cheek rest to fix the participant’s head are visible

Fig. 2

Participant’s image displays during a subjective refraction test using Chronos binocular/monocular refraction system. The Chronos binocular/monocular refraction system displays the target in one eye. However, the peripheral black frame remains (A) to maintain binocular fusion (B). LE, left eye; RE, right eye

Fig. 3

Control screen in the examination using Chronos binocular/monocular refraction system. Pressing the start panel initiated the objective refractive test in both eyes (A). This was followed by the subjective refraction test (B). The cylindrical lens was adjusted using a Jackson cross cylinder (C). In (A), S, C, A, PD, and VD indicate spherical power, cylindrical power, cylindrical axis, pupillary distance, and vertex distance, respectively. AVE indicates the average values of three measurements for the spherical lens power, cylindrical lens power, and cylindrical axis. (B) BINO, R, and L indicate binocular and monocular vision in the right and left eyes, respectively. The initial lens power in the subjective refraction test corresponds to the autorefractometer values in the AR Data. SPH is the spherical lens power. In (C), CYL and CC indicate the cylindrical lens and the cross-cylinder, respectively

Conventional auto refractometer

A KR-800 refractometer was used as the conventional autorefractometer. It can measure monocular objective ocular refraction over the same range as that measured by Chronos binocular/monocular refraction system.

Subjective ocular refraction

Subjective ocular refraction tests were performed in a well-lit room (600 lx), with both Chronos binocular/monocular refraction system and the trial-frame in real space using VC-60 (Takagi Co., Ltd., Nagano, Japan; Fig. 4), a standard visual acuity chart. The examiner presented the Landolt ring on a liquid-crystal display (resolution of 2560 × 1440 pixels), corresponding to the controller from a distance. The luminance of the backlight was 300 cd/m2.

Subjective ocular refraction was determined by combining the maximum plus or minimum minus spherical and cylindrical lenses necessary to provide a best-corrected visual acuity of − 0.176 logMAR (logarithm of the minimum angle of resolution) at an optical and natural distance of 5.0 m (with decimal acuity of 1.5).

The initial spherical lens was determined by adding + 1.00 D to the objective ocular refraction to prevent overcorrection, and the examiner (M.F.) confirmed that the participant was unable to clearly see the − 0.176 logMAR optotype with this lens. The examiner then added spherical lenses in increments of − 0.25D to obtain the highest visual acuity with spherical lenses alone. Additionally, the examiner used Jackson’s Cross Cylinder of ± 0.50 D and ± 0.25 D to determine the cylindrical lens after the single spherical lens obtained the highest visual acuity. Dot targets were used to determine the power and axes of the cylindrical lenses. The cylindrical lens axis was determined at 5° increments. After identifying the cylindrical lens, the examiner readjusted the spherical lens.

The procedure for subjective refraction test was as follows.

1.

First, the subjective lens was selected by referring to the objective ocular refraction of both eyes. For the spherical lens, a + 1D lens was added to prevent overcorrection.

2.

Showing the Landolt ring on the display, spherical lenses were added so that the subjects could obtain the highest visual acuity.

3.

Showing dot targets, cylindrical lenses and axis were corrected using Jackson’s Cross Cylinder of ± 0.50 D and ± 0.25 D. The cylindrical lenses were added, and the axis was determined using 5° increments.

4.

Showing the Landolt ring, spherical lenses were recorded so that the subjects could obtain their best-corrected visual acuity.

The procedure was the same for the subjective refraction test using Chronos binocular/monocular refraction system and the trial-frame in real space. However, the test using the Chronos binocular/monocular refraction system did not examine the peripheral frame of the eye to maintain binocular fusion. Although binocular balance is important in clinical practice, only the monocular visual acuity was evaluated in this study.

We were concerned that the results of the subjective refractive test would be affected by the objective ocular refraction results. Therefore, the subjects underwent the visual acuity test four times under four random conditions, as follows Fig. 5.

Fig. 4

Exterior of VC-60 VC60 is an optotype on an LCD display. The subjects were required to indicate the direction of the notch in the presented Landolt ring

Fig. 5

Examination flow The solid and dashed lines indicate binocular and monocular conditions, respectively. The subjective refractive test in the trial-frame was performed in real space. Chronos, Chronos binocular/monocular refraction system; Trial-frame, Trial-frame in real space

Additional experiment: Chronos binocular/monocular refraction system under monocular vs. binocular conditions

As the default setting of the Chronos binocular/monocular refraction system is to test with both eyes open, it does not entirely match the conditions of a conventional subjective ocular refraction test conducted with a single eye separately. Thus, we conducted the subjective ocular refraction test on Chronos binocular/monocular refraction system under monocular conditions, such as conventional subjective ocular refraction, with one eye occluded.

The subject’s left eye was completely occluded with white gauze (Surgical Pad, Hakujuji Co., Ltd.) to assess the difference between the binocular and monocular conditions in the Chronos binocular/monocular refraction system. Subjective refraction in the additional experiment was initiated by adding + 1.00 D from the subjective spherical equivalents (SE) of the previous subjective ocular refraction test. The experimental environment and procedures for the Chronos binocular/monocular refraction system were the same as those used in the previous experiments.

Data acquisition conditions

In this study, the ocular refraction data were obtained under five conditions　(Fig. 5; Table 1).

Table 1 Conditions in the objective and subjective refraction tests

The objective ocular refraction was measured using two tests: the Chronos binocular/monocular refraction system under binocular conditions and a conventional autorefractometer under monocular conditions.

The subjective ocular refraction was measured using three tests: Chronos binocular/monocular refraction system under binocular and monocular conditions and trial-frame in the real space under monocular conditions.

Data analysis

The objective and subjective ocular refractions were converted to SEs as follows:

where S and C are the spherical and cylindrical lens power, respectively. In this study, a cylindrical lens was used only in the negative format.

Objective and subjective astigmatism were converted from spherocylindrical to power vector notation by applying Fourier transformation using the following equations:

$$ _= -\frac\times cos2a$$

$$ _= -\frac\times sin2a$$

where α is an axis of a cylindrical lens, J0 indicates the cylinder lens power set at 90° and 180°, the positive values of J0 were with the rule (WTR) astigmatism, the negative values of J0 were against the rule astigmatism, and J45 is the cylinder lens power set at 45° and 135°, representing oblique astigmatism.

The simple conversion from the power vector notation to conventional notation is twice the square root of the sum of J02 and J452.

Statistical analysis

Bland–Altman analysis was performed to compare the objective ocular refractions in SE using J0 and J45 of the Chronos binocular/monocular refraction system under binocular conditions and KR-800 under monocular conditions, as well as the subjective ocular refractions in SE using J0 and J45 of the Chronos binocular/monocular refraction system under binocular conditions and the trial-frame in real space under monocular conditions.

After assessing the normality of variable distributions using Shapiro–Wilk tests, the fixed and proportional biases between the two apparatuses and the measurement conditions were analysed using paired t-tests and single linear regression analyses.

The chronosinocular/monocular refraction system maintains the binocular fusion. Therefore, the influence of fusional convergence was considered. The difference in the objective ocular refraction between the Chronos binocular/monocular refraction system under binocular conditions and the KR-800 under monocular conditions was calculated because of accommodation by convergence. The relationship between the degree of heterophoria and the differences in objective ocular refraction was analysed using Spearman’s rank correlation coefficients.

In an additional experiment, the differences between the Chronos binocular/monocular refraction system under monocular and binocular conditions and real space under monocular conditions were analysed using a paired t-test following the assessment of normality distribution using the Shapiro–Wilk test with Bonferroni correction.

IBM SPSS Statistics for Windows version 26 (IBM Corp., Armonk, NY, USA) was used to determine the significance of the differences, and P < 0.05 was considered statistically significant (Fig. 6).

Fig. 6

Statistical Analysis Items

This study achieved five ocular refraction results: two objective ocular refractions of the Chronos binocular/monocular refraction system under binocular conditions and KR-800 under monocular conditions, three subjective ocular refractions of the Chronos binocular/monocular refraction system under binocular conditions, monocular conditions, and a trial-frame in real space under monocular conditions. Bland–Altman analysis was performed to analyse the consistency of the Chronos binocular/monocular refraction system under binocular conditions and the conventional method, which measures monocularly, followed by evaluation of the difference between objective and subjective ocular refraction in the Chronos binocular/monocular refraction system under binocular conditions. The Spearman’s rank correlation coefficient was used to evaluate the correlation between objective ocular refraction by fusional convergence in the Chronos binocular/monocular refraction system under binocular conditions and KR-800 under monocular conditions. Finally, a paired t-test with Bonferroni correction was conducted to assess whether the subjective ocular refraction in the Chronos binocular/monocular refraction system under monocular conditions differed from the Chronos binocular/monocular refraction system under binocular conditions and the real space under monocular conditions.