Form profiler based on imaging with spatially partially coherent light

We present a fast and robust method for shape measurements of technical objects. It is based on imaging with light of limited spatial coherence by employing a 4f-optical filter with a modulator placed at its common Fourier plane. The modulator is used to enable fast depth scanning without mechanically moving parts. To determine the shape of the investigated object, the limited spatial coherence of the light in the object plane and the limited aperture in the Fourier plane play an important role. Directly behind the object, light diffracted from any connected area with a diameter equal to the spatial coherence length can mutually interfere, but it cannot interfere with any light from outside of it. Since the contrast is rapidly decreasing with increasing propagation distance, a high contrast at the image plane is only obtained at positions where the corresponding part of the object is in focus. Keeping the spatial coherence smaller than or equal to the point spread function of the imaging system, depth discrimination during the depth scan process is realized. The method is demonstrated by measuring the form of a technical object.