The process of producing the hologram of an incoherently illuminated object and that of obtaining the reconstructed image from the hologram are described mathematically based on the Fresnel-Kirchhoff diffraction integral. Formulas are given which represent the magnification, posi-tion and separation of the reconstructed images. Distortion term does not appear in the case of incoherent holography. The condition for obtaining Fourier transform holograms is outlined. A system consisting of two spherical mirrors and a beam splitter is proposed.