A method and apparatus are disclosed for providing image data for constructingan image of a region of a three dimensional target object. The method includesthe steps of providing incident radiation, via at least one detector detectingan intensity of radiation scattered by the target object, repositioning incidentradiation relative to the target object, subsequently detecting the intensityof radiation scattered by the target object, determining a probe function indicatingan estimate of at least one characteristic of the incident radiation at one ormore depths of the object and providing image data from which an image of one ormore regions of the object may be constructed via an iterative process using theprobe function.
