Micromirror array based optical spatial filter technique for fast and flexible velocimetry

Spatial filtering technique is an optical method for measurement of the relative velocity between the sensor and the observed surface(s). This technique is well known for tens of years, its benefits are a potentially simple optical setup and low requirements to the data acquisition and analysis. Therefore it is a valuable method for online contactless velocimetry. The classical spatial filtering setup is based on a static grating in the optical detection path with at least one or two photodetectors. Newer developments are using line array or two-dimensional detectors (e.g. CCD or CMOS). Here the grating structure is realized by weighting the pixels in a signal pre-processing. The flexibility of adapting the grating to the application is much higher but the detector speed of the array sensors is much lower than that of the photodetectors in the classical setup. Both properties are important for a spatial filter sensor that can be used in a wide range of application. The novel micromirror array based optical spatial filter design that is presented in this paper combines the advantages of both state of the art systems. It utilizes fast photodetectors in combination with a very precise, flexible and high resolved differential grating that is realized by the micromirror array. The present paper gives a short overview on the spatial measurement technique, explains the micromirror array based sensor approach and shows the proof of concept.