Interferometer assessment of potential visual acuity before YAG capsulotomy: relative performance of three instruments.

The accuracy of white light and laser interferometers in predicting visual acuity after YAG laser capsulotomy was compared. 42 eyes of 41 patients were tested with both a Haag-Streit (Lotmar) white light interferometer and a Rodenstock laser interferometer, and 14 were also tested with a Site white light machine. The laser interferometer predicted a final visual acuity to within one line of that actually achieved in 93%, and to within two lines in 98%, whereas for the Haag-Streit these figures were 64% and 81%, and for the Site 77% and 92%. In patients with poor initial visual acuity, the difference in the relative performance of the two instruments was increased further. When interferometry was repeated after capsulotomy, the values obtained with all instruments agreed closely with Snellen acuity. This difference in predictive accuracy shows that capsular thickening causes a greater degree of optical degradation of the image produced by a white light interferometer than occurs when a laser interferometer is employed.