Thickness measurement of thin wood material by differential laser triangulation method

Improving end product quality, minimizing the manufacturing costs and maximizing the yield in wood industry, an accurate, high speed and non-contacting measurement method for measuring thin wood thickness variations is required. Optically homogeneous materials have been measured successfully with laser triangulation for decades; however, non-homogeneous and porous target materials, such as wood, are more complicated to measure with high accuracy. The light scattering in wood is strongest in the parallel direction of wood grain cells shaping the originally round laser spot into an elongated form (tracheid effect). This study is focused on discovering an optimal sensor head orientation with respect to the elongated spot, comparing two different sensor technologies, testing how surface roughness affects on thickness measurements and determining the optimal laser wavelength range for measurement of wood thickness. The main sources of measurement uncertainty for laser triangulation in the measurement of thin wood thickness are also discussed. The results suggest that the laser triangulation plane should be aligned perpendicular to the grain direction and the wavelength of light around 550 nm might be optimal choice in order to minimize the measurement error.