Spatial Filter Design Techniques with Applications in Optical Velocity Measurement

This investigation was stimulated by the search for a simple and easy-to-apply method for measuring the movement of a car safety belt under a crash. Conventional methods would consist of gluing an incremental barcode-like strip on the belt, observing it with a reflective optical sensor, and counting the periods occurring while the belt is pulled out. However, as the woven structure of the belt constitutes a quite well-defined periodic texture by itself, the question arises if it is possible to determine the velocity from observing this. The method would then require imaging the texture to a suitably designed transmitting reference mask and evaluating the periodical signal observed behind the mask while the belt is moving. Mathematically, the proposed procedure can be called an optical spatial filter (OSF) technique. Of course, when applying OSF technique for velocity measurements of periodic objects it is vital to adapt the spatial filter closely to the moving object. While objects with a random surface or texture are quite easy to deal with, those with a periodic surface need special consideration. This investigation is aimed to explain the necessary considerations and to present techniques for suitably designing optical spatial filters.