Double grating interferometer with incoherent illumination

[[abstract]]A double-grating interferometer, illuminated by a spatially incoherent light for testing optics, is described. The cross-spectral density function has been used together with Fresnel diffraction to find the intensity distribution of the interferogram which behaves differently from those obtained either by Lau interferometer or by extended source Ronchi test in wich two coarse gratings were used for optical testing. Especially the effect of finite illumination on the grating has also been included to explain how fringe visibility varies with the distance between the image of the first grating and the second grating. The recorded interferograms seem free of coherent noise and show good contrast[[notice]]補正完