Fig. 23. Frequency doubling technology perimetry. A. The stimulus is a low spatial frequency sinusoidal grating (0.25 cpd) subtending 10°, presented at 1 of 17 test locations. The grating alternates at a high temporal rate (25 Hz) between two phases, shown at left. The frequency doubling illusion is the subjective perception that the grating has twice the number of dark and light bars (i.e., its spatial frequency appears to be 0.50 cpd), as shown in the diagram. B. Array of 17 stimulus locations, 1 central 10° circle and four squares in each quadrant. C. Sample FDTP printout from OS in a patient with an inferior nasal deficit, showing numerical full threshold values (top), probability plots of the total and pattern deviations (center and bottom, respectively), and global and reliability indices (bottom). D: Humphrey full threshold 30-2 grayscale printout from the same patient, for comparison.