1. A method of providing image data for constructing an image of a region of a three dimensional (3D) target object, comprising: providing incident radiation, from a radiation source, at a 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; selecting a plurality of depths within said target object corresponding to, for each depth, a respective region of the target object which is to be examined; determining a probe function at each of the plurality of depths in the target object, wherein each probe function indicates an estimate of at least one characteristic of the incident radiation at the respective depth in the target object; and providing image data, as a plurality of sets of data, each data set corresponding to a respective depth in the target object.