A method of reconstructing an approximate structure of an object from a detected electromagnetic scattering property arising from illumination of the object by radiation, the structure (802, 804, 806, 808) comprising materials of differing properties such as to cause a discontinuity in an electromagnetic field at a material boundary, the method comprising the steps: (a) estimating at least one structural parameter; (b) determining at least one model electromagnetic scattering property from the at least one structural parameter; (c) comparing the detected electromagnetic scattering property to the at least one model electromagnetic scattering property; and (d) determining an approximate object structure based on the result of the comparison, characterised in that (e) the model electromagnetic scattering property is determined by performing the steps of: (i) numerically solving a volume integral equation for a contrast current density by determining components of the contrast current density by using a field-material interaction operator to operate on a continuous component of the electromagnetic field and a continuous component of a scaled electromagnetic flux density corresponding to the electromagnetic field, the scaled electromagnetic flux density being formed as a scaled sum of discontinuous components of the electromagnetic field and of the contrast current density; and (ii) calculating electromagnetic scattering properties of the structure using the determined components of the contrast current density, wherein calculating electromagnetic scattering properties further comprises the steps of: (a) using a first continuous-component-extraction operator to extract continuous component of the electromagnetic field; and (b) using a second continuous-component-extraction operator to extract the continuous component of the scaled electromagnetic flux density, wherein the field-material interaction operator operates on the extracted continuous components. (a) estimating at least one structural parameter; (b) determining at least one model electromagnetic scattering property from the at least one structural parameter; (c) comparing the detected electromagnetic scattering property to the at least one model electromagnetic scattering property; and (d) determining an approximate object structure based on the result of the comparison, characterised in that (e) the model electromagnetic scattering property is determined by performing the steps of: (i) numerically solving a volume integral equation for a contrast current density by determining components of the contrast current density by using a field-material interaction operator to operate on a continuous component of the electromagnetic field and a continuous component of a scaled electromagnetic flux density corresponding to the electromagnetic field, the scaled electromagnetic flux density being formed as a scaled sum of discontinuous components of the electromagnetic field and of the contrast current density; and (ii) calculating electromagnetic scattering properties of the structure using the determined components of the contrast current density, wherein calculating electromagnetic scattering properties further comprises the steps of: (a) using a first continuous-component-extraction operator to extract continuous component of the electromagnetic field; and (b) using a second continuous-component-extraction operator to extract the continuous component of the scaled electromagnetic flux density, wherein the field-material interaction operator operates on the extracted continuous components. (i) numerically solving a volume integral equation for a contrast current density by determining components of the contrast current density by using a field-material interaction operator to operate on a continuous component of the electromagnetic field and a continuous component of a scaled electromagnetic flux density corresponding to the electromagnetic field, the scaled electromagnetic flux density being formed as a scaled sum of discontinuous components of the electromagnetic field and of the contrast current density; and (ii) calculating electromagnetic scattering properties of the structure using the determined components of the contrast current density, wherein calculating electromagnetic scattering properties further comprises the steps of: (a) using a first continuous-component-extraction operator to extract continuous component of the electromagnetic field; and (b) using a second continuous-component-extraction operator to extract the continuous component of the scaled electromagnetic flux density, wherein the field-material interaction operator operates on the extracted continuous components. (a) using a first continuous-component-extraction operator to extract continuous component of the electromagnetic field; and (b) using a second continuous-component-extraction operator to extract the continuous component of the scaled electromagnetic flux density,