Fresnel diffraction patterns of single and double disks. Parameters: Ω = 4 mm, ω = 3 mm, and z − d = d = 0.1 m. Top panel: diffraction of disks Ω (top left quadrant A) and ω (bottom left quadrant B) alone. The right two quadrants (C) show the diffraction of the system of two disks. Vertical and horizontal lines correspond to ±Ω/2 (external black lines), ±ω/2 (blue dashed lines) and the geometrical limit of the dark zone (red lines) AB of Fig. 1 and Eq. (7). Bottom panel: zoom on the central region around the Arago spot. In this example, the gain in darkness is about 8.