Simultaneous measurement of lathe check depth and the grain angle of birch (Betula pendula Roth) veneers using laser trans-illumination imaging

Lathe check depth (LCD) and the grain angle (GA) of veneer are important factors because they have an impact on the strength properties of plywood. Several methods to measure the LCD of veneer and the GA of wood or veneer have been developed over the years. In this study, a method to simultaneously measure LCD and GA from veneer is presented using silver birch (Betula pendula Roth) veneer of 3.5 mm nominal thickness as an example. The measurement is based on laser trans-illumination imaging through the veneer. From these images, the projected area of the crack mouth of slightly opened lathe checks was calculated using image analysis techniques. Reference LCD values were measured manually by dyeing the veneer and imaging with an optical microscope. The relationship between the area of lathe checks in the images and the reference LCD values was modelled with regression analysis, and R2 was found to be 0.86 and the root-mean-square error was 9.9 %. GA was determined by recognizing an elliptical illuminated area projected through the veneer from which GA was obtained by measuring the angle of the major axis of the ellipse. The range in the GA of the veneer samples studied was from 1° to 11°. Furthermore, the uncertainty in the reference LCD measurements taken by four operators was estimated with a reproducibility and repeatability test. The reference LCD measurements were found to be dependent on the operator.