Effect of Illumination on Near Positive Fusional Vergence in Young Adults

Abstract

Purpose

Positive fusional vergence (PFV) is important for binocular fusion, yet the influence of illumination on near PFV remains underexplored. This study investigated the effect of different illumination levels on PFV parameters (blur, break, recovery) to provide insights that could help eye care professionals optimize clinical assessments and make recommendations for near-vision activities.

Methods

Using a within-subjects design, thirty young adults (mean age: 21.87 ± 2.79 years; range 18–30) with normal binocular vision were evaluated at the University of Benin Optometry Teaching Clinic. PFV was measured under three controlled lighting conditions (50, 100, and 150 lux) using a phoropter, photometer, and LED lighting system. Preliminary tests (visual acuity, cover test and Von Graefe phoria) preceded PFV measurement, with participants reporting blur, break, and recovery points. Data were analysed using Friedman and Wilcoxon Signed Ranks tests (SPSS v25.0; p < 0.05), with effect sizes calculated via rank-biserial correlation.

Results

PFV values significantly decreased with increasing illumination (all p < 0.001). At 50 lux, mean PFV measures were highest (blur: 12.93 ± 1.62^Δ, break: 22.87 ± 3.06^Δ, recovery: 15.67 ± 2.78^Δ). These declined progressively at 100 lux (blur: 11.40 ± 1.28^Δ; break: 20.80 ± 2.71^Δ; recovery: 14.07 ± 2.72^Δ) and 150 lux (blur: 10.27 ± 1.72^Δ; break: 18.83 ± 2.09^Δ; recovery: 12.70 ± 2.74^Δ). The steepest reductions occurred between 50 and 150 lux (blur: −20.5%; recovery: −19.0%), and rank-biserial correlations indicated large effect sizes (−0.798 to −1.000).

Conclusion

Higher illumination significantly reduces PFV, which suggests that lower-to-moderate lighting optimises binocular coordination during near tasks. These findings support adjusting ambient lighting in educational, occupational, and clinical settings to reduce visual fatigue during near tasks.

Plain Language Summary

During close-up tasks like reading, our eyes rely on positive fusional vergence (how the eyes turn inward to focus on close objects within the visual field). Lighting is one factor influencing this eye coordination, yet its effect is understudied. We tested 30 adults with normal vision under three lighting conditions: dim (50 lux), moderate (100 lux), and bright (150 lux). Participants read close-up text, we noted when the text became blurry, when they saw double, and when their eyes recovered to a single vision. We found that the eyes worked best in dim light (50 lux), where participants could focus longer without blur or double vision. Brighter lighting made it harder for the eyes to stay coordinated: at 150 lux, vision broke down faster, and eyes took longer to recover. These results suggest that for tasks requiring prolonged focus on near tasks, lower to moderate lighting may help your eyes work more comfortably. Eye care professionals can use this information to advise patients and possibly adjust lighting conditions during eye exams. This study shows how everyday environments, like desk lighting, influence visual comfort and performance.

Introduction

Binocular vision depends on the accurate coordination of both eyes to fuse retinal images into a single three-dimensional (3D) perception, which enhances depth perception and spatial awareness.¹ This enables effective navigation and safe interaction with one’s environment. One of the most important aspects of such eye coordination is positive fusional vergence (PFV), the ability to converge eyes beyond habitual positions without disrupting binocular vision.² This is particularly relevant for comfortable and sustained near-vision tasks, such as digital screen use, prolonged desk work, reading and clinical assessments.³

Appropriate illumination improves visual stability, while improper lighting causes ocular discomfort, convergence disruption, and reduced near task efficiency.⁴ Low light can deteriorate accommodative precision and vergence amplitudes, which results in eye fatigue and discomfort, while excessive light introduces glare that compromises correct binocular coordination.⁵ Physiologically, illumination affects PFV with changes in pupil size such that an increase in illumination results in miosis and thus enhances depth of focus and reduces accommodative demand. This pupil constriction also improves retinal image contrast to enable more precise binocular alignment and vergence control. This can facilitate PFV response efficiency in terms of clearer and better vision and less accommodative noise under higher illumination.⁵

Current workplace lighting standards recommend 200–700 lux, depending on tasks.⁶ While illumination affects vergence control, its specific impact on near PFV remains underexplored.⁷ However, various studies have examined the impact of illumination on PFV, but results have also varied. Major fluctuations in near PFV (blur, break, and recovery) over varying light conditions were observed by Majumder & Sinathamby using a visual display unit (VDU) as a target of focus, with special reference to the impact of illumination on vergence dynamics.⁸ Likewise, Azam et al observed that heightened illumination (50 lux–100 lux and 150 lux) resulted in significantly lower PFV blur, break, and recovery values, with considerable alterations in pupil diameter, but with no effect on contrast sensitivity.⁹

While lighting conditions are understood to influence aspects of vision, their influence on PFV under conditions of near work remains underexplored. With the increase in the use of digital devices and variable ambient lighting, there is a need for an investigation into the influence of lighting on the function of binocular vision. Therefore, this study investigated how different illumination levels can influence near PFV parameters of blur, break, and recovery to enable clinicians to make useful recommendations for effective and comfortable near-vision tasks.

Materials and Methods

Research Setting and Population

A within-subjects design was used in this study to compare the effects of different levels of illumination on the performance of PFV. Using each participant as their control under different lighting conditions ensured that confounding variables such as personal variations in initial PFV functioning and visual comfort were held constant, whereas repeated measures helped provide strong comparisons and ensured internal validity.

It was performed using the University of Benin Optometry Teaching Clinic, in Benin City, Edo State, Nigeria, with standardised equipment and controlled environmental conditions for effective data collection. Convenience sampling was the method of choice for practicality and time-saving, enabling the timely accrual of participants that fit the inclusion criterion of the study.

Adults between 18 and 30 years of age with normal vision or corrected-to-normal vision (≤20/20 Snellen equivalent), willingness to adhere to the data-gathering procedures and ability to perform PFV tasks correctly were included in the study population. People with a history of eye disease (eg, strabismus, convergence deficiency), vision-affecting systemic conditions (eg, diabetes), and medication intake that influences visual function (eg, antidepressants, antihistamines) were excluded.

During the study, which lasted for six weeks from January through February 2025, four weeks were spent recruiting participants and collecting data and two weeks on analysing and preparing the manuscript.

Determination of sample size employed a 95% confidence level (1.96 Z score),¹⁰ 5% margin of error,¹⁰ with a hypothesised proportion of 2% (0.02) used to optimise sample size needs and derived from a related past study.⁹ Using these parameters, the calculated sample size was 30.14, which was rounded up to 30 participants. A post hoc power analysis (GPower 3.1) indicated 95% power to detect large effects (f = 0.35, α = 0.05) for within-subject comparisons across three conditions, confirming adequacy of n = 30.

Study Instruments

We used the following instruments: a phoropter (Reichert Phoropter, Reichert Technologies, USA) with rotary prisms to measure baseline fusional vergence (FV), a photometer (Testo 540, Testo SE & Co., Germany) for accurate lux measurement, an LED illumination source (Kaiser RB 5007 LED, Kaiser Fototechnik, Germany) to provide controlled lighting at 50, 100, and 150 lux intensity, a near point card (N-notation near point card, Bernell Corporation, USA), a Snellen visual acuity chart (Naugramedical, India), and a near point rod (Keeler RAF Rule, Keeler Ltd., USA) that held the accommodative objects at 40 centimetres (cm) during the assessment.

Data Collection

Mode of Illumination

This process was developed from Azam et al.⁹ Three 15-watt fluorescent lights that could be adjusted independently were installed in the testing room. Three widely used lighting modes were tested by the experiment, with light intensity determined by a lux meter. Mode A utilized one LED light source with 50 lux light intensity at the position of the viewer. Mode B had two LED light sources for 100 lux, while Mode C had three LED light sources with 150 lux light intensity at the position of the viewer.

Preliminary Measurements

At this point, several visual tasks were tested, both distance and near monocular visual acuity, and cover tests over a range of distances to measure binocular vision, according to the methodologies of earlier work.^11–13 For measurement of distance visual acuity, subjects were tested with their habitual correction placed in a phoropter in a uniformly lit, distraction-free room, with a Snellen visual acuity chart held 6m (20 feet) away. The eyes were tested one by one by covering the fellow eye, and subjects were asked to read letters out loud from the top line down through the smallest readable line, with acuity expressed as a fraction of the testing distance by the smallest readable line. Participants with a visual acuity of 6/6 or higher were included.

Near acuity was determined at 40cm under conditions of high illumination in an attempt to minimise glare, with a near card and applying the same occlusion procedure. Individuals with the ability to read N5 and higher were included. Cover tests were performed for both distance (6m on a standard fixation target) and near distance (0.4m on an accommodative target). For these tests, participants focused on the target whilst alternately occluding each eye using a handheld occluder. Ocular deviations were checked on occlusion and on uncovering the eye, and any eye movement noted down. Participants were included if they demonstrated phoria within normal clinical ranges (≤2^Δ exophoria to ≤3^Δ esophoria at distance; ≤3^Δ exophoria to ≤5^Δ esophoria at near).³ Those outside these ranges or with symptomatic heterophoria were excluded.

To mitigate potential confounders, testing occurred exclusively between 9:00 AM and 11:00 AM to control for circadian variations in visual performance. Participants refrained from digital screen use for ≥1 hour before testing to minimise transient visual fatigue. The illumination sequence (50, 100 and 150 lux) was randomised across participants to counterbalance order effects and fatigue accumulation. Standardised 5-minute breaks were enforced between lighting conditions to allow vergence system recovery. Room temperature (23°C ± 1°C) and humidity (50% ± 5%) were maintained consistently throughout all sessions. The testing room measured 4m X 5m with a background luminance of ≤5 lux. LED panels were mounted 1.5m above eye level, angled 30° away from the participant’s line of sight to prevent direct glare.

Von Graefe Phoria Test

The room was equipped with standard light, and the participant’s correction for distance was adjusted with the phoropter. One letter, one size higher than the patient’s best corrected visual acuity in the weaker eye, was placed on the distance visual acuity chart. The right eye had the base-in 12^Δ Risley prism placed in front of it for the measurement prism, and a 6^Δ base-up prism was placed before the left eye to be used as the dissociation prism. The base-in prism placed in front of the right eye was decreased gradually by a rate of about 2^Δ per second until the participant noted the alignment of the two targets. When alignment was completed, both the strength of the last prism and the direction of the base of the prism were noted.^13–15

Procedure for PFV Testing

Participants sat comfortably in the examination room, with their usual distance correction placed in the phoropter. A vergence target was placed on a near point rod 40cm from them, lit to 50 lux. At zero position, the Risley prism was placed in front of both eyes. Subjects were asked to sustain the target’s clear vision and report occurrences of blur (blur point), doubling (break point), and re-singling (recovery point). Base-out prisms were presented before both eyes at a rate of about 1^Δ per second. The illumination was subsequently set to 100 lux and, later on, to 150 lux, with each intensity carefully measured with a photometer, and the PFV testing under each illumination condition.^9,16

Data Analysis

Data collection and analysis were performed using the Statistical Program for Social Sciences, version 25.0 (IBM SPSS Inc., Chicago, IL, USA). Descriptive and inferential statistical analyses were performed on the collected data. Descriptive statistics such as means, standard deviations, and frequency distributions were utilised in representing the demographics and phoria distributions of the sample. Inferential analysis entailed the use of the Friedman test, which revealed statistically significant differences in PFV measurements between different lighting conditions (p < 0.001). The Wilcoxon Signed Ranks Test was used for comparison of PFV blur, break, and recovery under the different lighting conditions. Effect sizes were calculated using rank-biserial correlation to quantify the magnitude of changes in PFV parameters across illumination levels.

Ethical Approval

Ethics committee approval was granted from the University of Benin Department of Optometry Research and the Ethics Committee (REC number of EC/UBEN/LSC.OPT/24/105). The study followed the Declaration of Helsinki. Written informed consent was given by the participants; anonymized data were held securely, with destruction after analysis planned.

Results

The minimum age was 18, and the maximum age was 30. The majority of the participants (43.3%) were aged 19–21 years, while the smallest age groups were 18 years (3.3%) and 28–30 years (3.3%). The mean age of the 30 participants was 21.87 years ± 2.79. Male participants represented 63.3% of the sample, and females represented 36.7%. This is presented in Table 1.

Table 1.

Sociodemographic of the Study Participants

Variable		Frequency (n)	Percent (%)
Age range	18.00	1	3.3%
	19.00–21.00	13	43.3%
	22.00–24.00	11	36.7%
	25.00–27.00	4	13.3%
	28.00–30.00	1	3.3%
	Total	30	100.0%
Gender	Male	19	63.3%
	Female	11	36.7%
	Total	30	100.0%

Phoria at 6m was distributed as follows: 6.7% of participants had exophoria, 33.3% had esophoria, and 60% had orthophoria. This is presented in Table 2.

Table 2.

Phoria of the Study Participants at 6M

Phoria	Frequency (n)	Percent (%)	Valid Percent (%)	Cumulative Percent (%)
Exo	2	6.7	6.7	6.7
Eso	10	33.3	33.3	40.0
Ortho	18	60.0	60.0	100.0
Total	30	100.0	100.0

Phoria at 40cm is distributed as follows: 83.3% of participants had exophoria, 10% had esophoria, and 6.7% had orthophoria. This is presented in Table 3.

Table 3.

Phoria of the Study Participants at 40CM

Phoria	Frequency (n)	Percent (%)	Valid Percent (%)	Cumulative Percent (%)
Exo	25	83.3	83.3	83.3
Eso	3	10.0	10.0	93.3
Ortho	2	6.7	6.7	100.0
Total	30	100.0	100.0

Table 4 shows that PFV blur decreased from 12.93 ± 1.62^Δ at 50 lux, 11.40 ± 1.28^Δ at 100 lux to 10.27 ± 1.72^Δ at 150 lux; PFV break declined from 22.87 ± 3.06^Δ at 50 lux, 20.80 ± 2.71^Δ at 100 lux to 18.83 ± 2.09^Δ at 150 lux; and PFV recovery reduced from 15.67 ± 2.78^Δ at 50 lux, 14.07 ± 2.72^Δ at 100 lux to 12.70 ± 2.74^Δ at 150 lux.

Table 4.

Descriptive Statistics of Positive Fusional Vergence Across Lighting Conditions

Condition	Frequency	Mean	Std. Deviation
PFV Blur
50 lux	30	12.93	1.62
100 lux	30	11.40	1.28
150 lux	30	10.27	1.72
PFV Break
50 lux	30	22.87	3.06
100 lux	30	20.80	2.71
150 lux	30	18.83	2.09
PFV Break
50 lux	30	15.67	2.78
100 lux	30	14.07	2.72
150 lux	30	12.70	2.74

Table 5 shows the Friedman test results, which indicate statistically significant differences across lighting conditions for PFV blur (X² = 48.940, p < 0.001), break (X² = 51.271, p < 0.001), and recovery (X² = 51.094, p < 0.001). The mean ranks show that FV values are highest under 50 lux (blur = 2.90, break = 2.92, recovery = 2.95), moderate under 100 lux (blur = 1.98, break = 2.00, recovery = 1.92), and lowest under 150 lux (blur = 1.12, break = 1.08, recovery = 1.13).

Table 5.

Summary of Friedman Test Results for Positive Fusional Vergence Across Lighting Conditions

Variable	50 Lux	100 Lux	150 Lux	Chi-Square (X2)	df	p-value
PFV Blur	2.90	1.98	1.12	48.940	2	<0.001
PFV Break	2.92	2.00	1.08	51.271	2	<0.001
PFV Recovery	2.95	1.92	1.13	51.094	2	<0.001

The Wilcoxon Signed Ranks Test results showed statistically significant declines in PFV measures (blur, break, and recovery) across pairwise illumination comparisons. For PFV blur, comparisons between 100 vs 50 lux (Z = −4.24, p <0.001; negative ranks: N = 26, sum = 392.00; positive ranks: N = 2, sum = 14.00), 150 vs 50 lux (Z = −4.82, p <0.001; N = 30 negative ranks, sum = 465.00), and 150 vs 100 lux (Z = −3.84, p <0.001; N = 26 negative ranks, sum = 391.00) demonstrated poorer performance at higher lux levels. Similar patterns emerged for PFV break (100 vs 50 lux: Z = −4.31; 150 vs 50 lux: Z = −4.77; 150 vs 100 lux: Z = −4.66; all p <0.001) and PFV recovery (100 vs 50 lux: Z = −4.85; 150 vs 50 lux: Z = −4.67; 150 vs 100 lux: Z = −3.95; all p <0.001), with negative ranks (N = 26–29, sum = 372.50–463.00) consistently outweighing positive ranks (N = 0–3, sum = 0.00–44.00). All tests (two-tailed, N = 30 participants) showed minimal ties (0–2). Negative Z-values and dominant negative ranks indicate lower PFV values at higher lux levels. This is presented in Table 6.

Table 6.

Wilcoxon Signed Ranks Test of Positive Fusional Vergence Across All Lighting Conditions

Comparison	Negative Ranks (N)	Mean Rank	Sum of Negative Ranks	Positive Ranks (N)	Mean Rank	Sum of Positive Ranks	Ties	Total	Z-Value	Asymp. Sig. (2-tailed)
PFV Blur
100 lux – 50 lux	26	15.08	392.00	2	7.00	14.00	2	30	−4.2398	0.000
150 lux – 50 lux	30	15.50	465.00	0	0.00	0.00	0	30	−4.821	0.000
150 lux – 100 lux	26	15.04	391.00	3	14.67	44.00	1	30	−3.836	0.000
PFV Break
100 lux – 50 lux	28	14.71	412.00	1	23.00	23.00	1	30	−4.314	0.000
150 lux – 50 lux	29	15.97	463.00	1	2.00	2.00	0	30	−4.766	0.000
150 lux – 100 lux	28	15.36	430.00	1	5.00	5.00	1	30	−4.663	0.000
PFV Recovery
100 lux – 50 lux	29	15.00	435.00	0	0.00	0.00	1	30	−4.851	0.000
150 lux – 50 lux	29	15.78	457.50	1	7.50	7.50	0	30	−4.670	0.000
150 lux – 100 lux	26	14.33	372.50	2	16.75	33.50	2	30	−3.947	0.000

Rank-biserial correlation demonstrated large negative effect sizes (−0.798 to −1.000), with the strongest effects between 50 and 150 lux (blur: r = −1.000; break: r = −0.991; recovery: r = −0.968), followed by 100 vs 50 lux (blur: r = −0.931; break: r = −0.894; recovery: r = −1.000) and 150 vs 100 lux (blur: r = −0.798; break: r = −0.977; recovery: r = −0.835). This is presented in Table 7.

Table 7.

Rank-Biserial Correlations for Wilcoxon Comparisons of PFV Across Illumination Levels

Comparison	Negative Ranks	Sum of Negative Ranks	Positive Ranks	Sum of Positive Ranks	Rank-Biserial Correlation	Effect Size
PFV Blur
100 lux vs 50 lux	26	392.00	2	14.00	−0.931	Large
150 lux vs 50 lux	30	465.00	0	0.00	−1.000	Large
150 lux vs 100 lux	26	391.00	3	44.00	−0.798	Large
PFV Break
100 lux vs 50 lux	28	412.00	1	23.00	−0.894	Large
150 lux vs 50 lux	29	463.00	1	2.00	−0.991	Large
150 lux vs 100 lux	28	430.00	1	5.00	−0.977	Large
PFV Recovery
100 lux vs 50 lux	29	435.00	0	0.00	−1.000	Large
150 lux vs 50 lux	29	457.50	1	7.50	−0.968	Large
150 lux vs 100 lux	26	372.50	2	33.50	−0.835	Large

Discussion

This study was carried out to investigate the relationship between illumination levels and PFV. Specifically, it determined the effect of different levels of illumination on PFV and compared PFV values across illumination levels. By addressing these objectives, the study tested the null hypothesis, which stated that there is no significant difference in PFV values at different illumination levels, against the alternative hypothesis that posits a significant difference, with higher illumination levels leading to increased PFV values.

The socio-demographic data showed that of the 30 participants, the majority were young adults aged 19–21 years (43.3%), male (63.3%), with a mean age of 21.87 ± 2.79 years. At 6m, 60% of participants had orthophoria, 33.3% had esophoria, and 6.7% had exophoria, while at 40cm, 83.3% had exophoria, 10% had esophoria, and 6.7% had orthophoria. This highlights a significant shift toward exophoria at near fixation.

The Effect of Different Levels of Illumination on Positive Fusional Vergence

The results of this study emphatically demonstrated that illumination level has a strong effect on PFV. Table 4, with under 50 lux, saw the highest values of PFV for blur (12.93 ± 1.62^Δ), break (22.87 ± 3.06^Δ), and recovery (15.67 ± 2.78^Δ). The values decreased successively with increasing illumination to 100 lux and 150 lux. For instance, the PFV blur decreased to 10.27 ± 1.72^Δ at 150 lux, a 20.5% decrease from 50 lux. Likewise, PFV break and recovery decreased by 17.7% and 19.0%, respectively. Table 5 had the Friedman test validate statistically significant differences between illumination levels on all conditions of blur, break, and recovery (p < 0.001), with the lowest mean ranks on 50 lux, moderate on 100 lux, and the highest on 150 lux. The findings are in accordance with the hypothesis (H₁) that elevated illumination lowers the values of PFV, and they are in full conflict with the null hypothesis (H₀).

This parallels the results from Azam et al, which found a 43% reduction in PFV blur (14.5 ± 2.5^Δ to 8.2 ± 2.1^Δ) with increases in illumination from 50 to 150 lux.⁹ Likewise, Majumder & Sinathamby noted higher PFV values under lower light conditions (7 lux) on VDU tasks, although their absolute PFV values (eg, for blur: 10.06^Δ) were lower than those of the present study,⁸ perhaps because of the difference in test conditions (eg, for near work on screens versus controlled laboratory measurements). Jeon also confirms this trend, with peaks in PFV under low light conditions (10 lux).¹⁷ But Kim et al found varying vergence responses under low light conditions with heightened convergence tolerance but decreased divergence capacity,¹⁸ indicating that the effects of illumination could depend on task difficulty or measurement parameters.

Comparison of Positive Fusional Vergence Values at Different Illumination Levels

The Wilcoxon Signed Ranks Test (in Table 6) proved statistically significant declines (p < 0.001) in PFV metrics in all pairwise comparisons of light (50 vs 100 lux, 100 vs 150 lux, and 50 vs 150 lux). The most precipitous declines occurred between 50 and 150 lux, reflected in the largest negative sum ranks (eg, blur: sum = 465.00; break: sum = 463.00; recovery: sum = 457.50) and the most extreme values of Z (eg, blur: Z = −4.82; break: Z = −4.77; recovery: Z = −4.67). Predominant negative ranks (N = 26–30) and minimal positive ranks (N = 0–3) in all comparisons emphasise a strong, light-dependent decrease in vergence ability. The large rank-biserial correlations (in Table 7), from −0.798 to −1.000, confirm that illumination-driven reductions in PFV are not only statistically significant but clinically meaningful. Consistent with our expectations, these findings assert that PFV is extremely light-intensity-sensitive, with higher illumination conditions placing higher demands on the visual system.

However, Chen et al observed no substantial differences in PFV declines across light modes under prolonged 3D television viewing, indicating task-specific effects.¹⁹ Whereas their study observed cumulative declines in PFV post-viewing independent of light intensity, the present findings indicate acute PFV degradation under controlled conditions of acute exposure to brighter light. This difference may be due to methodological reasons: Chen et al assessed PFV post-30 minutes of 3D exposure, while this present study evaluated acute change under controlled, limited time conditions. Also, Jeon indicated that extremely low illumination (10 lux) improved distance and near PFV break points (20.81^Δ and 21.42^Δ, respectively).¹⁷ This present study’s findings (50–150 lux) conform with this trend, indicating a worsening of the rate of progression through fatigue as illumination increases over 50 lux. Additionally, Azam et al and Majumder & Sinathamby are closer in alignment with our findings, as both of these works observed declines in PFV with increases in illumination under conditions of near-vision.^8,9 This study’s values of PFV (eg, break = 22.87^Δ at 50 lux) are higher than in Majumder & Sinathamby’s study (break = 13.69^Δ at 7 lux), a difference likely due to the demographics of participants (eg, age, initial phoria). For example, the aforementioned high rate of 6m esophoria of this present cohort (33.33%) likely increases reserves of convergence, while the participants of Majumder & Sinathamby were tested under conditions of screen usage that inherently stress vergence.

A limitation of this study is that the influence of contrast sensitivity and pupil size under varying illumination was not addressed, despite evidence that pupil constriction in brighter light alters retinal illumination and depth of field, and contrast sensitivity impacts task clarity, both factors that could confound the observed illumination effects. Additionally, while we controlled for time-of-day and prior screen exposure, circadian rhythm variations and unmeasured factors like chronic fatigue or environmental stressors (eg, sleep deprivation, anxiety) could influence PFV outcomes. Future studies should incorporate objective measurements of pupil size, fatigue biomarkers (eg, salivary cortisol), and controlled circadian timing.

Conclusion

In summary, illumination has a strong effect on positive fusional vergence, with increased light intensity causing a measurable decrease in vergence function. Based on the evidence, we consider a moderate level of illumination (50–100 lux) optimum for sustained vergence tasks, where visual performance and comfort are balanced. This study provides evidence-based recommendations for lighting in educational, occupational, and healthcare environments. Our findings highlight the need for adaptive ambient lighting in digital screen ergonomics (eg, reducing glare during prolonged near-work) and occupational optometry (eg, optimising task lighting in offices or clinics to mitigate visual fatigue) and support standardising illumination levels during binocular vision testing.

Abbreviations

PFV, positive fusional vergence; FV, fusional vergence; 3D, three-dimension; VDU, visual display unit.

Data Sharing Statement

The datasets used and/or analyzed in this study are available from the corresponding author on reasonable request.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors declare no financial or personal relationships that could be perceived as influencing the research, findings, or conclusions presented in this article.