A method of forming a phase image of an object by solving a transport of intensity equation including the steps of: (a) exposing the object (1, 11) to a radiation wave field with a given wavenumber from a source; (b) obtaining two intensity images on respectively two spaced apart surfaces (A, B) extending across the direction of propagation of the radiation wave field; (c) subtracting the two intensity images in order to produce a measure of the rate of change of intensity of the radiation in the direction of propagation of the radiation wave field; (d) obtaining an intensity image on a selected surface extending across the direction of propagation of said radiation wave field on the side of the object (1, 11) remote from the incident radiation; (e) multiplying said measure of rate of change of intensity by the negative of the average wavenumber λ, transforming the result to produce a first integral transform representation, and applying to said first integral transform representation a correspondingly transformed filter corresponding in the untransformed representation to the inversion of a differential operator to produce a first modified integral transform representation; (f) applying an inverse of said first integral transform to said first modified integral transform representation to produce an untransformed representation; (g) applying a correction based on said obtained intensity image on said selected surface to said untransformed representation by dividing said untransformed representation by said obtained intensity image on said selected surface; (h) transforming the corrected untransformed representation to produce a second integral transform representation and applying to said second integral transform representation said correspondingly transformed filter to produce a second modified integral transform representation; (i) applying an inverse of said second integral transform to said second modified integral transform representation to produce a measure of phase of said radiation wave field across said selected surface; and (j) forming a phase image of the object (1, 11) from the measure of the phase of said radiation wave field. (a) exposing the object (1, 11) to a radiation wave field with a given wavenumber from a source; (b) obtaining two intensity images on respectively two spaced apart surfaces (A, B) extending across the direction of propagation of the radiation wave field; (c) subtracting the two intensity images in order to produce a measure of the rate of change of intensity of the radiation in the direction of propagation of the radiation wave field; (d) obtaining an intensity image on a selected surface extending across the direction of propagation of said radiation wave field on the side of the object (1, 11) remote from the incident radiation; (e) multiplying said measure of rate of change of intensity by the negative of the average wavenumber λ, transforming the result to produce a first integral transform representation, and applying to said first integral transform representation a correspondingly transformed filter corresponding in the untransformed representation to the inversion of a differential operator to produce a first modified integral transform representation; (f) applying an inverse of said first integral transform to said first modified integral transform representation to produce an untransformed representation; (g) applying a correction based on said obtained intensity image on said selected surface to said untransformed representation by dividing said untransformed representation by said obtained intensity image on said selected surface; (h) transforming the corrected untransformed representation to produce a second integral transform representation and applying to said second integral transform representation said correspondingly transformed filter to produce a second modified integral transform representation; (i) applying an inverse of said second integral transform to said second modified integral transform representation to produce a measure of phase of said radiation wave field across said selected surface; and (j) forming a phase image of the object (1, 11) from the measure of the phase of said radiation wave field.