The detection of knots in wood materials using the tracheid effect

Abstract

In the quality inspection of wood materials, it is important to be able to detect knots and measure their dimensions automatically. Knots can be detected on the basis of their colour. A problem arises when the colour of the knot does not differ sufficiently from the surrounding area for a reliable measurement. To overcome this problem the tracheid effect can be used. As a result of the tracheid effect, a circular laser spot that is projected to the surface of wood is extended to an elliptical shape, the major axis of which is oriented in the direction of the wood grain. Since the grains surround knots, they can be detected by evaluating the orientation and eccentricity of individual laser spots. This paper discusses the suitability of the tracheid effect in the quality inspection of thin wood materials. In the course of this work measurements that utilize a laser spot line and an industrial camera are undertaken. An algorithm is developed for detecting and measuring the dimensions of knots. The performance of the algorithm and the measurement setup is evaluated on different knot types and sizes.