A method and apparatus are disclosed for providing image data for constructing an image of a region of a three dimensional target object. The method includes the steps of providing incident radiation, via at least one detector detecting an intensity of radiation scattered by the target object, repositioning incident radiation relative to the target object, subsequently detecting the intensity of radiation scattered by the target object, determining a probe function indicating an estimate of at least one characteristic of the incident radiation at one or more depths of the 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 the probe function.
