In the wood and pulp industry it is often desirable to be able to detect an anomaly in a biological material. The anomaly may be an organic anomaly, such as rot, insect attacks, knots, sap stain, or annual rings in wood. The anomaly may also be an inorganic object, such as stone, metal, or glass, present in the biological material.
Detection of organic anomalies may, for example, be useful to be able to sort out biological material of inferior quality to ensure that an end product meets a specified standard. Detection of inorganic objects present in the biological material, on the other hand, may be required in order to remove the inorganic object as it may otherwise damage machines for chopping, grinding or sorting the biological material and cause costly stops.
For the purpose of detecting metal or stone in a stream of wood-like material, it is previously known to allow electromagnetic radiation to penetrate the stream of wood-like material and thereafter to measure the intensity and calculate the amount of the original intensity that has been absorbed. Since the absorption coefficient typically is higher for metal or stone than for wood, an increased intensity absorption may indicate presence of stone or metal. An example of such a method and device is found in EP 1 484 612. This known method and device use radiation of one single wavelength, and the results are unpredictable. Typically, an intensity absorption threshold used to indicate the presence of stone or metal is either too high such that some inorganic objects are not detected which may result in damage to the machines and costly stops due to breakdowns, or, if the intensity absorption threshold is lowered in an attempt to avoid undetected inorganic objects, there tend to be a significant number of false alarms resulting in a large amount of short but unnecessary stops. Moreover, the sensitivity of this method is insufficient for detection of organic anomalies.
WO 97/35175 discloses a method for using radiation of several energy levels in order to distinguish between e.g. different types of material in wood, etc. However, this method is primarily intended to determine the quality of tree trunks and the like, and is neither intended nor suitable for monitoring e.g. pre-processed material in an industrial process.
There is therefore a need for a method and apparatus that may be conveniently used in an industrial process for reliably detecting anomalies in a biological material, such as pre-processed biological material.