New technique can detect insects in wood

Materials World magazine
29 Aug 2019

Detecting insect damage in wood has become easier with a new imaging method. Idha Valeur reports.

A new terahertz (THz) imaging technique could be used to detect insect infestation in wood at early stages, preventing widespread invasion and saving costs for the forestry industry.

HAWK University of Applied Science and Arts, Germany, Researcher, Kirsti Krügener, told Materials World that their robot-based THz tomography system is built upon four steps. ‘In a first step, the three-dimensional surface of the object to be examined is recorded. For this purpose, a fringe projection method is used. Next, the measurement range, including the number and resolution of the individual measurement points, is defined by the user. Afterwards, the travel path for the robot arm is determined by an algorithm, which calculates the coordinates of each measurement point and the associated normal vector in order to be able to align the THz sensor perpendicular to the respective measurement point. After these three steps have been completed, the robot-based THz measurement of a three-dimensional object can start in a fourth step.’

Krügener said the data gathered with the system is then analysed using a deconvolution algorithm, but that problems can occur when detecting signals of organic samples, such as wood, due to the overlaying of noise from scattering effects of inhomogeneities in the material. Or, if the sample layers are thin, this could lead to single THz pulses becoming superimposed over each other.

‘By transforming the measurement signal into the frequency-domain, the impulse response function can be determined,’ she said. ‘The impulse response function transformed back into the time-domain contains very sharp peaks representing an interface of the sample. These sharp peaks can be detected easily. If the average refractive index of the sample is known, the spatial distance between the interfaces can be reconstructed based on the according time difference.’

The THz tomography system is based on time-of-flight-measurements, and works similarly to ultrasonic measurement, as each interface in a wood sample reflects a fixed amount of radiation that is analysed in accordance with its time-dependent order. According to the researchers, an infested wood sample will reflect more radiation than an uncontaminated one and will have fewer interfaces.

‘Given the fact that most wood-destroying insects can be found near the surface and the wood layers and burrows made by the insects are very thin, the method can be used in many cases of near-surface detection in order to visualise the very small burrows of insects below the bark,’ Krügener explained.

By integrating the technique into a portable instrument, it would be possible to bring the instrument into forests for individual inspections, thus prohibiting the removal of healthy trees.

The researchers said they have demonstrated that, in general, the robot system is suitable for detecting the presence of harmful insects. But certain elements would need to be improved before it is ready for commercial use, including the speed of measurement, speed of the data analysis and minimising both size and weight of the components.

Advantages and applications

The benefit of this type of testing, according to Krügener, is that the depth resolution of the THz measurements exceeds that of conventional computerised tomography (CT) images. Neither does it require any industrial safety measure, as with X-ray based techniques.

‘In contrast to measurements with micro-CT, which are of comparable resolution, the sample sizes that can be measured with the THz robot system are not limited. The use of THz radiation is not ionising, which eliminates occupational health and safety when dealing with the radiation,’ Krügener said.

The researchers believe the technique could be used for other applications, including art and cultural heritage investigations. ‘Given the value and uniqueness of art pieces, non-invasive diagnostic methods are highly appreciated by conservators. Especially as this technique is relatively new to cultural heritage, an active community is making tremendous progress towards making this technology more economical and user-friendly. Terahertz spectroscopy, in the near future, will be a useful tool in the field of artificial restoration because of its non-contact, non-destructive nature,’ Krügener explained.

She said if the speed of measurement could be improved, another potential application could be in product quality control, including detecting cavities in products.