Measurement technique and apparatus for examining a region of a patterned surface such as an integrated circuit (IC). Excitation x-ray, neutron, particle-beam or gamma ray radiation is directed toward a two-dimensional sample area of the IC. Emissions (e.g., x-ray fluorescence—XRF) from the sample area are detected. A mask is placed in a planar radiation path formed by the source, detector and the sample area, and in one embodiment moveable relative to the sample area. The mask includes an elongate aperture to substantially confine the excitation radiation directed to the sample area, and the emissions from the sample area, to the planar radiation path when arranged parallel to a first axis of the two-dimensional sample area. The invention allows predictive measurement of feature characteristics in active circuit regions of the IC, using sample areas outside of these regions.