实际光源不是绝对的单色，包含一定的光谱宽度△λ。△λ范围内的每一种波长的光都生成各自一组干涉条纹，除零干涉级外，相互间均有位移，使条纹对比度下降。-The actual light source is not absolutely monochromatic, contain a certain spectral width △ λ. Each wavelength of light within the range △ λ generates a set of interference fringes each, in addition to the zero level outside interference, mutual displacement are so fringe contrast decreases.