page_title: Optical flat
section_title: Flatness testing
hierarchical_section_title: Optical flat / Flatness testing
caption_attribution_description: English: Photos of two glass optical flats testing the flatness of surfaces, showing the different patterns of interference fringes. The flats are resting with their bottom surfaces in contact with the surface to be tested (toolmaker's flats) and they are illuminated by a monochromatic light source. When there is a slight gap between the two surfaces, the light waves reflected from both surfaces interfere, resulting in a pattern of bright and dark bands called an interference pattern. The surface in the left photo is nearly flat, indicated by a pattern of almost straight parallel interference fringes at equal intervals. The surface in the right photo is uneven, resulting in a pattern of curved fringes. Each pair of adjacent fringes represents a difference in surface elevation of half a wavelength of the light used, so differences in elevation can be measured by counting the fringes. The flatness of surfaces can be measured to millionths of an inch by this method.
context_page_description: An optical flat is an optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few tens of nanometres. They are used with a monochromatic light to determine the flatness of other surfaces, whether optical, metallic, ceramic, or otherwise, by interference. When an optical flat is placed on another surface and illuminated, the light waves reflect off both the bottom surface of the flat and the surface it is resting on. This causes a phenomenon similar to thin-film interference. The reflected waves interfere, creating a pattern of interference fringes visible as light and dark bands. The spacing between the fringes is smaller where the gap is changing more rapidly, indicating a departure from flatness in one of the two surfaces. This is comparable to the contour lines one would find on a map. A flat surface is indicated by a pattern of straight, parallel fringes with equal spacing, while other patterns indicate uneven surfaces.
context_section_description: An optical flat is usually placed upon a flat surface to be tested. If the surface is clean and reflective enough, rainbow colored bands of interference fringes will form when the test piece is illuminated with white light. However, if a monochromatic light is used to illuminate the work piece, such as helium, low-pressure sodium, or a laser, then a series of dark and light interference fringes will form. These interference fringes determine the flatness of the work piece, relative to the optical flat, to within a fraction of the wavelength of the light. If both surfaces are perfectly the same flatness and parallel to each other, no interference fringes will form. However, there is usually some air trapped between the surfaces. If the surfaces are flat, but a tiny optical wedge of air exists between them, then straight, parallel interference fringes will form, indicating the angle of the wedge (i.e.: more, thinner fringes indicate a steeper wedge while fewer but wider fringes indicate less of a wedge). The shape of the fringes also indicate the shape of the test surface, because fringes with a bend, a contour, or rings indicate high and low points on the surface, such as rounded edges, hills or valleys, or convex and concave surfaces.
caption: Testing the flatness of surfaces with optical flats. The lefthand surface is flat; the righthand surface is astigmatic, with curvatures in two orthogonal directions.
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