1. A method of reconstructing an approximate structure of an object in an inspection apparatus, the method comprising: determining, using a processing device, a model of an electromagnetic scattering property of an estimated structure of the object, the estimated structure being periodic in a direction and comprising materials of differing properties that causes a discontinuity in an electromagnetic field at a material boundary of the estimated structure, wherein the determining comprises: numerically solving, using the processing device, a volume integral equation for a vector field to determine an approximate solution of the vector field, the vector field being related to the electromagnetic field by a change of basis and constructed to be continuous at the material boundary, and determining, using the processing device, the electromagnetic field from an equation comprising the approximate solution of the vector field; detecting an electromagnetic scattering property of the object; and comparing, using the processing device, the detected electromagnetic scattering property to the model of the electromagnetic scattering property.