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Abstract

Detection of knots in hinoki and karamatsu lumber by thermography.

Abstract

The detection and size measurements of knots on lumber surfaces were studied by comparing the thermal properties of the knots with those of the surrounding tissues for hinoki (Chamaecyparis obtusa) and karamatsu (Larix leptolepis). First, the densities and thermal conductivities of intergrown round knots, distorted grain around the knots, and clear tissues were measured. Both the density and thermal conductivity of a knot were about twice as large as clear tissue. The values for distorted grain regions were intermediate. Then, a thermographic method was applied to detect the knots and to measure their sizes. The lumber surface was irradiated with incandescent lamps, and the change in surface temperature was measured. As a result, the rate of temperature rise was smallest on the knot, followed by the distorted grain region. From the thermograph, (ie. a pattern showing the temperature distribution 60 s after the start of heating), it was found that intergrown round knots and spike knots could be detected. A binary picture was obtained from the thermograph, and the diameters of knots were calculated in both lumber widths and lengths. The size of a knot measured by the thermographic method was greater than that determined visually because the former included a distorted grain region. This indicates that this method can detect not only knots but also distorted grain regions which affect the strength of the lumber. The method can also detect some of the blind knots.