The document discusses the Amsler grid, a diagnostic tool used to screen for and monitor macular diseases. It consists of a grid with small squares that patients look at to detect any distortions, gaps, or blurred areas that could indicate conditions like macular degeneration or edema. The grid evaluates the central 20 degrees of vision. Amsler first developed the grid in 1945 and variations include different color patterns and line configurations to test for specific visual abnormalities. Patients hold the grid at 16 inches and report any anomalies in the grid lines with each eye separately. Abnormal findings can then be marked and referred to an ophthalmologist for further evaluation.