sentences:  The reference flat is slightly beveled (only a fraction of a degree of beveling is necessary) to prevent the rear surface of the flat from producing interference fringes.A collimated beam of monochromatic light illuminates the two flats, and a beam splitter allows the fringes to be viewed on-axis.The reference piece is sometimes realized by a diffractive optical element (computer-generated hologram or CGH), as this can be manufactured by high accuracy lithographic methods.