Recognition of longitudinal knot area by light reflection and image processing.
The variations of light reflections on lumber surfaces of hinoki (Chamaecyparis obtusa) containing knots were studied to recognize areas with sloped grain around the knots. Monochrome photographs of the lumber surfaces containing single knots were taken under sunlight irradiation from three different irradiation directions. The grey-level profiles of photographs scanning along the longitudinal centre lines of the knots and three-bit images of the photographs were analysed by means of image analysis. Irradiation of light from the down side of a knot gave the most valuable information for recognizing a knot area containing both heartwood and sapwood. The areas with sloped grain around a knot were recognized easily from the differences of three-bit images obtained by upper and down side irradiations.