The effect of conventional CR39 and Fresnel prisms on high and low contrast acuity

The effect of conventional CR39 and Fresnel prisms on high and low contrast letter acuity was studied. Visual acuity of the fully corrected better eyes of 15 subjects was measured with the high (90%) and low (10%) contrast logMAR letter charts, while they wore prisms of varying power. The results showed that when the power of the conventional CR39 prism and the Fresnel prism reached 10 prism dioptres and 5 prism dioptres, respectively, significant reduction (1 tail t-test, p < 0.05) of the high and low contrast acuity occurred. The Fresnel prism caused a significantly greater acuity reduction than the conventional CR39 prism for powers ranging from 5 to 30 prism dioptres for both contrasts. The rate of acuity reduction with increasing prism power was greater with the low contrast targets than with the high contrast targets for both prisms. In addition, the rate of acuity reduction with increasing prism power was greater with the Fresnel prism than with the conventional CR39 prism for both contrasts. The conventional CR39 prism reduced acuity by a ratio of about 0.8-0.9 of that of the Fresnel prism for powers ranging from 5 to 20 prism dioptres and by about 0.7 for 30 prism dioptres. These ratios applied for both high and low contrast acuity, and therefore were independent of the level of contrast used.