Contrast transfer ratio in normal, cataractous, and intraocular implant lenses

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Abstract
In order to clinically evaluate the optical performance of the crystalline lens as well as that of intraocular implant lenses, measurements were made of the brightness of two markedly different and easily sited landmarks on the retina (the papilla and the macula) using the photopapillometer. The contrast transfer ratio (CTR) thus generated permitted investigation of three groups of subjects: 1. Normal persons (n = 53) with visual acuity of 1.0 (20/20) from each mature decade of life provided values indicating the age-related changes in eyes not necessarily tending toward lens opacification. Relatively constant CTR values (approximately 2:1) were observed up to 60 years, after which the ratio fell significantly to about 1.5:1 (ranging from 1.41:1 to 1.68:1, depending on the site measured on the papilla). Such a finding in subjects with normal vision was considered to demonstrate the tendency for older lenses to become yellow, which would reduce the CTR. 2. Patients (n = 33) who were being followed clinically for developing lens opacification had their CTRs measured for correlation with their visual acuities. Here, strong correlation (P < 0.001) was observed. 3. Evaluation of the improvement wrought by the implantation of an intraocular lens in yet another group of patients (n = 38) revealed an increase in the CTR to a virtually normal level (1.97:1). This method provides information about the actual optical efficiency of the lens when the other ocular media are clear, supplementing other methods for evaluating lens density and clarity, and thus further facilitating critical assessment of the progress of lens opacification.

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