A comparison of the effect of 3M Fresnel prisms and Trusetal prism foils on visual function

Aim: To determine any differences in visual acuity (VA) and contrast sensitivity (CS) between 3M Fresnel prisms and Trusetal prisms foils.

Methods: Sixteen participants (mean age 20 years) with normal VA and CS participated. The effect of 5 ^Δ, 10 ^Δ, 20 ^Δ and 30 ^Δ prism strengths on monocular VA and CS was assessed using a Bailey-Lovie logMAR chart and Pelli-Robson chart, respectively. Measurements were carried out for both 3M Fresnel prisms and Trusetal prism foils. A comparison of the physical properties of the two prism types was made.

Results: Deterioration in VA and CS was evident with increasing prism strength with both prism types. VA was more significantly reduced with 3M Fresnel prisms than with Trusetal prism foils [F1,15 = 19.63, p < 0.001]. There was significant interaction between prism types and prism strength [F3,45 = 10.36, p < 0.0001], due to 30^Δ 3M Fresnel prisms reducing VA by 0.13 logMAR more than the Trusetal prism foils. There was no statistically significant difference between the prism types for CS [F1,15 = 2.21, p = 0.1582]. 3M Fresnel prisms were thinner than the Trusetal prism foils. The thicker Trusetal prism foils consist of fewer bases per centimetre but were more difficult to fit to spectacle lenses.

Conclusions: There is no difference in VA or CS regardless of whether a 3M Fresnel prism or Trusetal prism foil is used until 30^Δ, when Trusetal prism foils give better VA. For large deviations it may therefore be more beneficial for the patient to use a Trusetal prism as part of their treatment plan.