The fizeau interferometry concept has been extended to use white light generating multiple "rainbow" fringes. The order of rainbow fringes will be reversed when the filism thickness gradient is reversed since the spectral fringes will be in the order of increasing wavelengths (B-G-Y-R) along the direction with a thickening film and decreasing wavelengths (R-Y-G-B) along the direction with a thinning film. This can determine the slope of thin film.