Laser Uncovers Cracks in Lumber

Michael D. Wheeler

When wood is dried, its surface layer sometimes shrinks, causing internal stresses that can develop into tiny cracks. And these cracks, invisible to the naked eye, can become much larger cracks that affect the quality and value of the wood. The damage is usually done before humidity, temperature and other variables in the drying process can be adjusted to compensate.

Cracks invisible to the naked eye can expand in lumber to compromise the quality and value of the wood. Analyzing laser speckle patterns from a sample may enable the timber industry to detect these tiny cracks and evaluate the severity of the drying process.

Now a team at the University of Joensuu has come up with a solution by analyzing speckle patterns of laser light, the patterns that form when coherent or partially coherent light reflects from a rough surface or interacts with inhomogeneous material. The pattern is a result of wavelets that are reflected from different parts of the sample at different distances, causing them to interfere.

The researchers use a diode-pumped solid-state laser, emitting 1 mW at 532 nm, as the light source. A series of lenses focus the beam, and a hybrid photodetector records the speckle patterns from the wood samples. This information is sent to a computer for processing and analysis.

Promising for industry

In an experiment that they reported in the May issue of Optical Engineering, the researchers used the technique to test seven samples of wood, each similar in composition, species and sawing pattern, but dried at different temperatures and in varying degrees of humidity. From analysis of the speckle patterns, a clear picture of the number and width of the cracks emerged.

Based on the preliminary findings, the scientists believe that the method is promising for use in the timber industry as a means of measuring the severity of the drying process and of ensuring the quality of the finished product.