A new instrument allows accurate measurement of retinal visual acuity behind mild to moderate cataracts. Mounted on a slit lamp, the Potential Acuity Meter projects a Snellen visual acuity chart into the eye via a narrow beam of light converging to a minute aerial aperture only 0.15 mm in diameter. The examiner aims the narrow beam through "windows" in the cataract, avoiding blockage or scattering of the light that would otherwise occur. In 47 cataractous eyes having best preoperative visual acuity of 20/200 and better, the postoperative visual acuity was predicted to within three lines in 100% of cases, and to within two lines in 91%. With a successful result from cataract surgery defined as postoperative vision of 20/40 or better, the prediction of success with the Potential Acuity Meter was correct in 95% of cases.