A method of providing image data for constructing an image of a region of a three dimensional target object (31, 903), comprising the steps of: providing incident radiation (30, 34), from a radiation source, at a three dimensional target object (31, 903); via at least one detector (32, 904), detecting the intensity of radiation scattered by said target object (31, 903) with the incident radiation (30, 34) at a first position with respect to the target object (31, 903); re-positioning the incident radiation (30, 34) relative to the target object (31, 903); subsequently detecting the intensity of radiation scattered by said target object (31, 903) with the incident radiation (30, 34) at a second position with respect to the target object (31, 903); characterised by: determining a plurality of depths within said target object (31, 903) in a direction towards the detector corresponding to, for each depth, a respective region of the object (31, 903) which is to be examined; determining a respective probe function, indicating an estimate of at least one characteristic of the incident radiation (30, 34), at each of the plurality of depths in the three dimensional object (31, 903) wherein the probe function forms an illumination function in three dimensional space and the probe function is re-positioned between the first and second positions (36, 37) by a three dimensional vector; and providing image data, as a plurality of sets of data, each data set corresponding to a respective depth, from which an image of one or more regions of the object (31, 903) may be constructed via an iterative process using said probe function. providing incident radiation (30, 34), from a radiation source, at a three dimensional target object (31, 903); via at least one detector (32, 904), detecting the intensity of radiation scattered by said target object (31, 903) with the incident radiation (30, 34) at a first position with respect to the target object (31, 903); re-positioning the incident radiation (30, 34) relative to the target object (31, 903); subsequently detecting the intensity of radiation scattered by said target object (31, 903) with the incident radiation (30, 34) at a second position with respect to the target object (31, 903); characterised by: determining a plurality of depths within said target object (31, 903) in a direction towards the detector corresponding to, for each depth, a respective region of the object (31, 903) which is to be examined; determining a respective probe function, indicating an estimate of at least one characteristic of the incident radiation (30, 34), at each of the plurality of depths in the three dimensional object (31, 903) wherein the probe function forms an illumination function in three dimensional space and the probe function is re-positioned between the first and second positions (36, 37) by a three dimensional vector; and providing image data, as a plurality of sets of data, each data set corresponding to a respective depth, from which an image of one or more regions of the object (31, 903) may be constructed via an iterative process using said probe function. determining a plurality of depths within said target object (31, 903) in a direction towards the detector corresponding to, for each depth, a respective region of the object (31, 903) which is to be examined; determining a respective probe function, indicating an estimate of at least one characteristic of the incident radiation (30, 34), at each of the plurality of depths in the three dimensional object (31, 903) wherein the probe function forms an illumination function in three dimensional space and the probe function is re-positioned between the first and second positions (36, 37) by a three dimensional vector; and providing image data, as a plurality of sets of data, each data set corresponding to a respective depth, from which an image of one or more regions of the object (31, 903) may be constructed via an iterative process using said probe function.