The Effect of Illumination on Positive Fusional Vergence

BACKGROUND: Positive fusional vergence (PFV) is vital in maintaining fusion in critical and continuous near tasks such as reading or performing digital screen tasks. This study investigated how PFV changed under various lighting conditions. METHODS: This cross-sectional study recruited 34 participants aged between 21 and 25 years, with best corrected visual acuity (BCVA) 0.0 logMAR and insignificant refractive error. Three different illuminations—low illumination (50 lux), medium lighting (100 lux), and high illumination (150 lux)—were used to examine the ocular parameters PFV (blur, break, and recovery points), contrast sensitivity and pupil diameter. RESULTS: Pupil diameter changed significantly in different room illuminations (p = 0.00). There was no significant difference in contrast sensitivity across the three levels of room illumination (p = 0.368). Mean PFV (SD) (blur) was 14.5 (2.5) in 50 lux, 10.2 (2.2) in 100 lux, and 8.2 (2.1) in 150 lux. Under 50, 100 and 150 lux, respectively, the mean PFV (SD) (break) values were 16.7 (2.4), 13.4 (1.8), and 10.8 (2.2), and the mean PFV (SD) (recovery) values were 13.3 (2.1), 10.7 (2.1), and 7.5 (2.7). With increased illumination levels, PFV blur, break, and recovery values were significantly lower (p < 0.001). CONCLUSIONS: PFV values were significantly higher in lower illumination. Clinicians should be aware that room illumination affected the PFV values measured.