An optical flat is a precision tool made from high-quality glass or quartz with surfaces that are polished to an extremely high degree of flatness. It is primarily used to test the flatness of other surfaces through the process of optical interference.
The principle behind an optical flat is based on the interference patterns that occur when light reflects off both the optical flat and the surface being tested. When the optical flat is placed on or above a surface, and monochromatic light (such as from a sodium or helium lamp) is shone on it, light waves reflecting off the two surfaces interfere with each other. This interference produces a pattern of light and dark bands, known as fringes.
The shape, spacing, and alignment of these fringes reveal the flatness of the surface. If the surface is perfectly flat, the fringes will be straight, parallel lines. Any deviations from flatness will cause the fringes to curve or become irregular. The number and shape of these fringes indicate the degree of flatness or curvature of the surface being tested.
Optical flats are commonly used in metrology labs and precision engineering environments where the flatness of surfaces is critical, such as in the manufacturing of precision components, lenses, and other optical elements. They are especially useful because they provide a non-contact, highly accurate method for measuring flatness, with deviations often measurable in fractions of a wavelength of light (on the order of nanometers).
The quality of an optical flat is determined by its surface accuracy, typically specified in terms of “fringes” or “waves,” which refer to the wavelength of light used in the test. A high-quality optical flat might have a surface accuracy of a fraction of a wavelength, ensuring it can detect even the smallest imperfections in flatness.