 The Fizeau interferometer is shown as it might be set up to test an optical flat . A precisely figured reference flat is placed on top of the flat being tested , separated by narrow spacers . 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 . Separating the test and reference flats allows the two flats to be tilted with respect to each other . By adjusting the tilt , which adds a controlled phase gradient to the fringe pattern , one can control the spacing and direction of the fringes , so that one may obtain an easily interpreted series of nearly parallel fringes rather than a complex swirl of contour lines . Separating the plates , however , necessitates that the illuminating light be collimated . Fig 6 shows a collimated beam of monochromatic light illuminating the two flats and a beam splitter allowing the fringes to be viewed on @-@ axis . 
