Recognition of tangential knot area by light reflection and image processing.
JAS (Japan Agricultural Standard) adopts the knot ratio system for visual grading for strength. This is defined as the ratio of the knot diameter to the board width. It is important to recognize the knot area in the tangential direction. Furthermore, the knot area in the tangential direction which is recognized from a monochrome photograph almost coincides with the area which has a large cutting resistance due to a knot. In this paper, the irradiation directions and incident angles needed to recognize a knot area from monochrome photographs more easily are discussed. Monochrome photographs of hinoki (Chamaecyparis obtusa) lumber surfaces containing single knots were taken under sunlight irradiation. The irradiation directions were from the upper side of the knot in the longitudinal direction to 315° at 45° intervals. Incident angles were 15°, 45°, and 75°. The grey level profiles, scanned along the tangential lines of the photographs, were analysed by means of an image analysing apparatus. The difference of the grey level at the knot area boundary is at a minimum, and it is difficult to recognize the knot area, when the incident angle is 15°. It is at a maximum and easy to recognize the knot area, when the irradiation direction is from the down side of the knot. In addition, when the irradiation direction is from the down side of the knot, the grey level profile shows a small peak next to the dark part of the knot due to heartwood. This peak may indicate the light part of the knot due to sapwood. Gloss was also measured of a board surface irradiated from upper to down, down to upper, left to right, and right to left side of the knot.