File:Optical flat interference fringes.jpg

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.