English: An optical flat test performed on an optical window. The flat is fused silica measuring 2 inches (5.1 cm) x 1/2 inch (13 mm) thick, with a flatness of λ/10. The optical window it is resting on is 2 inches square x 1/8 inch (3 mm) thick, made of borosilicate float-glass and coated with a multilayer antireflection coating. By drawing a straight line across the flat, along the edge of one fringe, and counting how many fringes cross that line, the surface of the window can be measured for flatness to within 1/10 of the wavelength of the 532 nm laser light that illuminates it. If a bright fringe is used as a reference, then a difference of one dark fringe crossing that line equals a deviation on the surface of λ/2 (1/2 of the wavelength, or in this case 266 nm). A bright fringe crossing that line equals a deviation of λ (one wavelength). The test shows the window is flat to 4--6λ (~2000--3000 nm) per inch. The AR coating makes a white-light test nearly impossible, but simply changing the viewing angle reveals that the surface is slightly concave, even though it looks convex