 Newton ( test plate ) interferometry is frequently used in the optical industry for testing the quality of surfaces as they are being shaped and figured . Fig . 13 shows photos of reference flats being used to check two test flats at different stages of completion , showing the different patterns of interference fringes . The reference flats are resting with their bottom surfaces in contact with the test flats , and they are illuminated by a monochromatic light source . The light waves reflected from both surfaces interfere , resulting in a pattern of bright and dark bands . The surface in the left photo is nearly flat , indicated by a pattern of 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 the surfaces can be measured to millionths of an inch by this method . To determine whether the surface being tested is concave or convex with respect to the reference optical flat , any of several procedures may be adopted . One can observe how the fringes are displaced when one presses gently on the top flat . If one observes the fringes in white light , the sequence of colors becomes familiar with experience and aids in interpretation . Finally one may compare the appearance of the fringes as one moves ones head from a normal to an oblique viewing position . These sorts of maneuvers , while common in the optical shop , are not suitable in a formal testing environment . When the flats are ready for sale , they will typically be mounted in a Fizeau interferometer for formal testing and certification . 
