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 eye s of 15 subjects was measured with the high (90%) and low (10%) contrast lo gMAR letter charts, while they wore prisms of varying power. The results sh owed that when the power of the conventional CR39 prism and the Fresnel pri sm reached 10 prism dioptres and 5 prism dioptres, respectively, significan t reduction (1 tail t-test, p <0.05) of the high and low contrast acuity oc curred. The Fresnel prism caused a significantly greater acuity reduction t han the conventional CR39 prism for powers ranging from 5 to 30 prism diopt res 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 in creasing prism power was greater with the Fresnel prism than with the conve ntional 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. (C) 2001 The College of Opt ometrists. Published by Elsevier Science Ltd. All rights reserved.