Objective measurement of backscattered light from the anterior and posterior cornea in vivo

PURPOSE. TO develop an objective and repeatable method of measuring corneal backscattered light from different depths of the cornea in vivo. METHODS. A modified slit lamp ("scatterometer"), with a video camera and synchronous white strobe light, was used to capture images of a 0.1-mm-wide slit beam through the cornea. Image analysis software was developed to measure backscatter from digitized high-magnification images of 82 normal corneas of 41 subjects. Forty eyes of 20 of the same subjects were examined again after 1 month. Mean backscatter from the anterior, middle, and posterior thirds of the cornea was compared between repeated measurements, and expressed in arbitrary scatter units (SU). RESULTS. Backscatter in the anterior third of the cornea was 451 ± 42 SU (mean ± SD, n = 82), from the middle third was 274 ± 29 SU (n = 82), and from the posterior third was 242 ± 28 SU (n = 82). The difference in backscatter measured a month apart was 5 ± 27 SU (P = 0.34), 2 ± 17 SU (P = 0.42), and 0 ± 15 SU (P = 0.95) in the anterior, middle, and posterior thirds of the cornea, respectively. Minimum detectable differences between measurements were 12, 8, and 7 SU in the anterior, middle and posterior thirds, respectively (α = 0.05, β = 0.20, n = 40). CONCLUSIONS. Backscatter can be measured at different depths of the cornea from high-magnification digitized images of a narrow slit beam through the cornea. The method is objective and repeatable and can be applied in prospective studies of deep and posterior lamellar keratoplasty.