A focus height sensor in an optical system for inspection of semiconductor devices includes a sensor beam source that emits a beam of electromagnetic radiation. A reflector receives the beam of electromagnetic radiation from the sensor beam source and directs the beam toward a surface of a semiconductor device positioned within a field of view of the optical system. The reflector is positioned to receive at least a portion of the beam back from the surface of the semiconductor device to direct the returned beam to a sensor. The sensor receives the returned beam and outputs a signal correlating to a position of the surface within the field of view along an optical axis of the optical system.