1. A method of providing image data for constructing an image of a region of a three dimensional (3D) target object, comprising the steps of: providing incident radiation, from a radiation source, at a 3D target object; via at least one detector, detecting the intensity of radiation scattered by said target object with the incident radiation at a first position with respect to the target object; re-positioning the incident radiation relative to the target object; subsequently detecting the intensity of radiation scattered by said target object with the incident radiation at a second position with respect to the target object; determining a probe function. indicating an estimate of at least one characteristic of the incident radiation, at one or more depths in the 3D object; and providing image data, from which an image of one or more regions of the object may be constructed via an iterative process using said probe function.