where l is the wavelength of the X-ray, b and d are the imaginary and real parts of the refractive index, respectively. After propagating a sufficient distance, the phase shifts in the downstream beams are transformed into measurable intensity variations by means of Fresnel diffraction. An image detector records them as the phase contrast image. According to Fresnel diffraction theory and the wave front function, the Fourier transform of the recorded intensity can be approximated bỹ