Spiral grain occurs naturally more or less in all trees. Instead of running parallel to the pith, the grain runs spiralled around the trunk like a helix. Since wood is an orthotropic material with higher shrinkage perpendicular to than parallel with the grain, the tree will twist when dried, and so will a plank or board cut from it. The tracheid effect utilizes the light-conducting properties of the softwood tracheids to measure the direction of spiral grain. A small laser point was projected on the wood surface. The light transmitted in the wood and scattered back forms an elliptic shape extended in the direction of the fibres. The ellipse of light was registered with a CMOS camera and the orientation of the ellipse's major axis was computed by image processing in real time. A camera system for measuring spiral grain on the debarked surface of a log was installed at a sawmill. In normal production, centre planks of Norway spruce were marked with colour in six different groups according to the size and direction of spiral grain measured by the camera system. All marked planks were manually measured for twist after drying. Measurements on 843 pieces show a coefficient of determination of 0.76 between spiral grain under bark and twist after drying.