A system for measuring a coating thickness on a target surface includes a terahertz spectroscopy device and a reference image projector. The terahertz spectroscopy device includes a radiation head that is operable to project a terahertz radiation beam onto the target surface and receive a reflected beam. The reference image projector includes a visible light device and is operable to project a reference image using the visible light device onto the target surface. A visual characteristic of the reference image indicates at least one of distance, rotational alignment, and angular alignment of the radiation head relative to the target surface.