A variation of the penlight shadow procedure, using tangential corneal illumination, was compared with the van Herick biomicroscopy technique for estimating the anterior chamber. The purpose was to determine if tangential penlight illumination affords the practitioner an additional, clinically valid means of estimating patency of the anterior chamber angle.Sixty-two subjects (24 males, 38 females) were evaluated with both procedures. For each subject, the temporal anterior chamber angles were estimated on the basis of the degree of nasal brightness via sclerotic scatter after temporal tangential corneal illumination. Comparisons were made with temporal angle estimation using the van Herick biomicroscopy technique.Compared with the van Herick procedure, the tangential corneal illumination technique had a sensitivity of 93.8% in the measurement of open angles, with a specificity of 83.3%. The results were statistically significant at p < 0.001.The results suggest that the tangential corneal illumination technique provides a viable alternative to angle estimation using the biomicroscope. The procedure compared favorably with previous assessments of the standard penlight shadow test. Although limited in certain aspects, the speed and ease of the procedure make it a useful diagnostic tool in situations in which conventional testing is difficult or impossible to accomplish.