An apparatus for detecting top scattered light from a substrate. A source directs a light onto a position on the substrate. The light thereby reflects off in a specular beam, scatters off the top surface, and scatters off a bottom surface of the substrate. An objective receives the top and bottom scattered light. The objective has a first focal point focused on the position on the top surface of the substrate, and a second focal point focused on a pinhole field stop. The pinhole field stop passes the top scattered light that is focused on the pinhole field stop, and blocks the bottom scattered light. A sensor receives and quantifies the top scattered light.