When choosing wood for applications such as load-bearing beams in houses, it's important not to use pieces that contain cracks or other defects that could affect their structural integrity. While not quite as crucial, it's also nice to avoid flaws when building things like wooden furniture, piano soundboards, or window frames. Typically, people have been limited to visually checking the wood for such defects. Now, however, researchers at Germany's Fraunhofer Institute for Wood Research have developed a system that highlights faults invisible to the human eye, using a process called high-power ultrasound thermography.

Using an ultrasound agitator (also known as a sonotrode), solid or composite wood is vibrated at a frequency of 20 kilohertz. At that frequency, any flaws - such as cracks, delaminations, gluing errors or knots - causes the different parts of the wood to vibrate against one another. This in turn produces heat, which is picked up and displayed on a monitor by a thermal imaging camera.

In the case of finished wood products, the system can even "see" underneath surface coatings, to reveal defects that have been hidden.

The technology is said to work with all types and grades of wood, and with damp wood. The depth to which the wood can be analyzed depends on its thermal conductivity, although up to 20 millimeters is reportedly possible. Although non-destructive, the process does leave small pressure marks in the wood.

In laboratory tests, the Fraunhofer system has also been used to detect cracks in ceramics and glass.