For agricultural produce such as potatoes, size and shape determination is very important in grading lines. If electronic sensor systems are installed, weighing cells and optical sensors such as cameras or simple shadow solutions are predominantly used. These systems are limited with regard to the numbers of `views' of the object, which determines the accuracy for the determination of geometrical parameters. A ring sensor system is described consisting of a large number of optical transmitters and receivers which are arranged alternately. The emitters strobe infrared light in sequence round the ring, creating a large number of `viewpoints'. Optical connections to each receiver are made. As an object passes through the ring, a shadow zone of each transmitter beam is created. As a result, the tangential chords can be found. A modified polar coordinate system, an algorithm and trigonometrical functions are described which allows the contour of the convex hull of three-dimensional objects to be formed. One advantage of this system is that the data from the ring is in the form of components of vectors and can be used to describe the whole object. As a result an enveloping spiral of the object is created.
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