| With the FRT WLI PL, a camera takes pictures of interference patterns in almost the same manner as in classical interferometry. The interference patterns are generated by the coherent superposition of light reflected by the object with light reflected by a reference mirror. For that purpose, the FRT WLI PL uses an interferometer set-up with a special illumination beam and light source. The specific coherence length of the lightsource results in spatially restricted interference patterns where the magnitude decreases from the center to the left and right. The big advantage of this specific pattern is that its center of gravity can be directly and precisely assigned to a z-height within the object. Hence, exact topography measurements are possible. A detailed analysis of the interference pattern enables resolutions of a few nanometers. The height information is obtained by vertical scanning,
i.e., stepwise moving of the sensor head and taking an image at each step. The images then are combined to the final image of the object.
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