The effect of Fresnel prisms on dynamic visual acuity

Aim: To investigate the effect of Fresnel prisms on dynamic visual acuity (DVA).

Methods: Sixteen participants with normal visual acuity aged between 18 and 22 years were tested monocularly with four strengths of base-out Fresnel prisms (0^Δ, 5 ^Δ, 15 ^Δ, 30 ^Δ) using a repeated-measures design. DVA was measured as the ability to correctly discriminate the orientation of a Landolt C moving at five different speeds: 0^∘/s, 4^∘/s, 8^∘/s, 12^∘/s and 16^∘/s. Landolt Cs moved horizontally with the gap orientated at either the top, bottom, left or right.

Results: A two-factor analysis of variance (ANOVA) revealed that both increased Fresnel prism strength and target speed significantly reduced DVA. During the experiment participants noted that Landolt Cs with the gap at the top or bottom (vertical gaps) were harder to discriminate. A three-factor ANOVA revealed a significant difference between the Landolt C orientations and showed that both increasing speed and prism size significantly reduced performance for vertical Landolt Cs but not horizontal.

Conclusion: Base-out Fresnel prisms of increasing strength have a significant effect on the DVA of horizontally moving Landolt Cs. Performance decreased as prism strength increased, with speed playing a lesser role. Closer examination of the data showed that the prisms were having a greater effect in impairing performance for vertically orientated Landolt Cs than for horizontal. Performance with horizontal Landolt Cs seemed robust across a range of speeds and prism strengths.