Real-time inspection using Chemical Sensing.

Real-time inspection using Chemical Sensing for heartwood, sapwood, knots and wood defects etc.

Due to their different properties, pine heartwood and pine sapwood can almost be seen as two different timber products. Sapwood is more vulnerable to moisture, air and solvents because of its open structure. This on the other hand also makes it more susceptible to treatments such as painting or impregnating. Heartwood is more stable and durable because of its resistance to moisture fluctuations. This leads to the preferred use of heartwood when sturdiness is needed and sapwood when a treatment needs to be applied. In order to optimally use pine wood logs, sorting by its heartwood content should be done, as suggested for spruce or pine by Leisse(1). This allows the use of its optimal properties and thus increases revenue.

In this example, Chemical Sensing is used to detect the heartwood in six pieces of pine timber. The left most image shows the timber pieces as they are. The middle image is used as a reference, for it pieces were cut off beforehand and chemically treated using Diazobenzenesulfonic acid to colour the phenol in the heartwood. And finally the right image shows the Chemical Sensing result.

Chemical Sensing uses hyperspectral imaging technology and spectroscopic analysis and modelling algorithms to portrait chemical/molecular properties as colours in an image (see Wikipedia(2) for a brief introduction to hyperspectral imaging or Boldrini(3) for a more extensive review of hyperspectral imaging). The more open structure and the susceptibility to moisture of sapwood allow for a differentiation between heartwood and sapwood in the infrared spectral range between 1000 nm and 1700 nm (Gjerdrum(4)). The images were recorded using the measurement system Perception HEAD Model 2 and analysis and modelling were done using Perception STUDIO.

In the Chemical Sensing image the heartwood is depicted in green, the sapwood is depicted in red and resin and knots are depicted in blue and cyan respectively. This clearly shows the non-destructive detection of pine heartwood and pine sapwood offered by Chemical Sensing without the need for using expensive and unhealthy chemicals.

(1) Leisse B (1996) Sortierung von Schnittholz nach den Splintanteil. Holz-Zentralblatt 30:478–480

(2) https://en.wikipedia.org/wiki/Hyperspectral_imaging

(3) Boldrini, Barbara & Kessler, Waltraud & Rebner, Karsten & Kessler, Rudolf. (2012). Hyperspectral Imaging: A Review of Best Practice, Performance and Pitfalls for in-line and on-line Applications. Journal of Near Infrared Spectroscopy. 20. 438-. 10.1255/jnirs.1003

(4) Gjerdrum, Peder & Høibø, Olav. (2004). Heartwood detection in Scots pine by means of heat-sensitive infrared images. Holz als Roh- und Werkstoff. 62. 131-136. 10.1007/s00107-004-0467-0

Article written by: Christoph Miksits, Senior Application Engineer, March 2022.