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 (billionths of a metre). They are used with a monochromatic light to determine the flatness (surface accuracy) of other surfaces (whether optical, metallic, ceramic, or otherwise), by means of wave 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. Two adjacent fringes indicate a difference in elevation of one-half wavelength of the light used, so by counting the fringes, differences in elevation of the surface can be measured to better than one micrometre. Usually only one of the two surfaces of an optical flat is made flat to the specified tolerance, and this surface is indicated by an arrow on the edge of the glass. Optical flats are sometimes given an optical coating and used as precision mirrors or optical windows for special purposes, such as in a Fabry–Pérot interferometer or laser cavity. Optical flats have uses in spectrophotometry as well.
Flatness testing
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.
Preparation Both the optical flat and the surface to be tested need to be extremely clean. The tiniest bit of dust settling between the surfaces can ruin the results. Even the thickness of a streak or a fingerprint on the surfaces can be enough to change the width of the gap between them. Before the test, the surfaces are usually cleaned very thoroughly. Most commonly, acetone is used as the cleaning agent, because it dissolves most oils and it evaporates completely, leaving no residue. Typically, the surface will be cleaned using the "drag" method, in which a lint-free, scratch-free tissue is wetted, stretched, and dragged across the surface, pulling any impurities along with it. This process is usually performed dozens of times, ensuring that the surface is completely free of impurities. A new tissue will need to be used each time, to prevent recontamination of the surfaces from previously removed dust and oils. Testing is often done in a clean-room or another dust-free environment, keeping the dust from settling on the surfaces between cleaning and assembly. Sometimes, the surfaces may be assembled by sliding them together, helping to scrape off any dust that might happen to land on the flat. The testing is usually done in a temperature-controlled environment to prevent any distortions in the glass, and needs to be performed on a very stable work-surface. After testing, the flats are usually cleaned again and stored in a protective case, and are often kept in a temperature-controlled environment until used again.
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