This semiconductor inspection and metrology system includes a knife-edge mirror configured to receive light reflected from a wafer. The knife-edge mirror is positioned at a focal point of the light reflected from the wafer such that the reflective film on the knife-edge mirror is configured to block at least some of the light reflected from the wafer. The portion of blocked light changes when the light reflected from the wafer is under-focused or over-focused. At least one sensor receives the light reflected from the wafer. Whether the light is under-focused or over-focused can be determined using a reading from the at least one sensor. A height of an illuminated region on the surface of the wafer can be determined using such a reading from the at least one sensor.