Effect of Illumination on Near Positive Fusional Vergence in Young Adults

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.

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.

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).

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.