Apparatus for inspecting a surface, including a plurality of pump sources having respective pump optical output ends and providing respective pump beams through the pump optical output ends, and a plurality of probe sources having respective probe optical output ends and providing respective probe beams through the probe optical output ends. There is an alignment mounting which holds the respective pump optical output ends and probe optical output ends in equal respective effective spatial offsets, and optics which convey the respective pump beams and probe beams to the surface, so as to generate returning radiation from a plurality of respective locations thereon, and which convey the returning radiation from the respective locations. The apparatus includes a receiving unit which is adapted to receive the returning radiation and which is adapted to determine a characteristic of the respective locations in response thereto.