Identification of 3D wood grain angle by directional reflection measurement

Abstract

Wood grain direction is an important quality-controlling feature of wood. It strongly affects wood strength, so much so that a 15-degree grain angle deviation can cause a 50% strength loss. Thus, grain angle measurement can provide a powerful means of wood strength grading in sawmills. The measurement method used here is a further development of an optical technique investigated previously but not much implemented in practice because of the complex sensor setup required. The method involves measuring the directional distribution of reflected light from the wood surface when illuminated by a laser spot. The grain structure of the wood causes the light to reflect preferentially in the cross-grain direction. The technique is of particular interest because it measures the 3D angle of the wood grain, comprising both the surface (in-plane) and dive (out-of-plane) grain angles. The measurement procedure developed here uses a digital camera to measure the reflected light distribution and thereby to identify wood grain direction. Measurements with a prototype system on straight-grain lodgepole pine specimens with dive angles in a range 0–10° gave dive angle measurements with an accuracy of approximately one degree, with similarly accurate surface angle measurements up to a 6° dive angle.