Review on the use of Fresnel prism in low vision

Fresnel prisms are often prescribed for visual field defects in low vision patients. These prisms are made of optical polyvinyl chloride (PVC) and this material increases chromatic dispersion and produces a loss of contrast. In this presentation, the effect of chromatic dispersion on contrast sensitivity is determined. It has been verified that loss of contrast sensitivity is greater at higher spatial frequencies with Fresnel prisms than with glass prisms of the same power. Above 10 prism diopters, Fresnel prisms reduce both contrast sensitivity and visual acuity substantially. However, low vision patients with visual field losses appear not to be affected due to their sometimes very low remaining contrast sensitivity and visual acuity. In the presence of both blurred and diplopic vision, a case report indicating the use of prisms in alleviating diplopia caused by trauma is presented. In spite of a reduction in visual acuity and contrast sensitivity, the patient preferred to have the Fresnel prism on his spectacle lenses for distance viewing. Diplopia appeared to cause more annoyance than the reduction of acuity and contrast sensitivity due to the Fresnel prism.