Aim: To investigate and compare the effect of Fresnel prisms on visual acuity in photopic and mesopic conditions.
Methods: Twelve participants aged between 18 and 26 years were recruited who had good visual acuity and normal binocular single vision. Monocular visual acuity was measured with three strengths of base-out Fresnel prisms (5Δ, 15 Δ, 30 Δ) and without a Fresnel prism. The right eye was tested in all participants. This was assessed at 60 cd/m2 and 1 cd/m2. Participants adapted to 1 cd/m2 for 5 min before being tested at this luminance, and those tested at 1 cd/m2 first were allowed to adapt to normal room lighting (60 cd/m2) for 2 min before being tested at this level of lighting.
Results: A two-factor analysis of variance (ANOVA) revealed that both increased Fresnel prism strength (F11,3 = 204.762, p < 0.0001) and luminance (F11,1 = 343.303, p < 0.0001) significantly reduced visual acuity. There was no interaction between prism strength and lighting condition. The reduction in visual acuity in mesopic conditions was approximately 0.2 logMAR with all prism strengths.
Conclusions: Fresnel prisms reduce visual acuity in photopic conditions and lowering luminance to a mesopic level reduces visual acuity further. When a Fresnel prism is worn in mesopic conditions a cumulative effect is seen on the extent that visual acuity is reduced with all prism strengths. This should be considered when deciding whether to fit the prism monocularly or binocularly, and advice should be given to the patient about changing driving habits accordingly.