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Terahertz imaging spectroscopy for quality inspection in the food industry

Figure. 1. (a) Schematic setup of a pulsed and a continuous wave THz system. (b) Photograph of a fiber-coupled, pulsed THz spectrometer used for industrial applications [2].
Figure 2 (a) THz image of a chocolate bar contaminated with a buried glass, stone and metal splinter. Existing inspection systems such as ultrasound or X-ray may fail to detect such contaminants (b) Representation of the practical application of a THz scanner in a food production process.

Terahertz (THz) scanners are taking the stage as a novel tool for the quality inspection of food products. The non-destructive and contact-free nature of the THz technology  could offer substantial advantages over conventional X-ray or ultrasonic testing. This article provides an introduction to THz systems and shows how the food industry could benefit from this innovative technology.

by C. Jansen, B. Scherger, Dr C. Jördens, Dr I. A. Ibraheem Al-Naib and Dr M. Koch


Sandwiched between microwaves and the infrared, the THz region of the spectrum bridges electronics and photonics. While this particular spectral position has lead to a tremendous theoretical scientific interest, the lack of THz sources has for a long time hindered practical progress in this field. In the late 1980s, the advent of femtosecond lasers together with the appearance of related THz emission and detection schemes [1] ended this era of niche existence, transforming THz technology into a dynamic and commercially driven field of research. This progress was further assisted by the development of continuous wave (cw) THz systems, e.g. based on photomixing or quantum cascade lasers. Today, THz systems have reached a mature state and are challenging established techniques, especially in the sector of non-destructive testing. If the expected increase in measurement speed is realized and system costs are further reduced, large scale market introduction of THz systems will occur in the near future.
Many applications for THz systems have been reviewed in the literature [2], ranging from biological imaging and security scanning to next-generation wireless communication systems. Currently, the most promising field seems, however, to be the non-destructive testing and process control. In this area, THz measurements have a high potential for providing potentially important information about the samples being investigated. In particular, the high signal-to-noise ratio of up to 70 dB as well as phase information due to the coherent detection scheme set THz technology apart from competitor techniques.

The first section of this article provides a basic introduction to THz systems and the underlying principles of operation, discussing both pulsed and cw operation. On the basis of this technical background, we will then describe the potential of using THz systems for the non-destructive and contact-free quality inspection of food products, providing examples from ongoing projects.

THz spectroscopy systems   
While a large number of differing individual THz systems exist, there are two main categories: pulsed and continuous wavw (cw) systems. While the former category makes use of electromagnetic picosecond pulses, which have spectral components ranging from 100 GHz to several THz, the latter category employs waves which only have a single frequency component at a time. Depending on the applications,  both kinds of systems offer advantages. On the one hand, the frequency resolution which can be achieved with cw systems is considerably higher than that of pulsed systems so cw systems are the first choice for sensing sharp absorption lines as are encountered in gases. On the other hand, pulsed systems deliver broad spectral information with a single measurement and enable the easy determination of the sample thickness [3] so that they are particularly suited for most non-destructive testing applications. For a long time, the purchase costs of cw systems have been considerably lower than pulsed systems as they use diode lasers while pulsed systems in general require more expensive femtosecond laser sources. This situation has however changed with the advent of low-cost Er+ doped femtosecond fibre lasers which can replace the previous expensive Ti:Sa sources, so that costs are now similar.

Underlying principles Broadband pulsed THz systems
Among the huge variety of available THz sources and detectors, photoconductive antennas are one of the most popular choices [4]. As shown in the upper part of Fig. 1(a), beams of ultra short, sub-100-fs pulses, emitted from a femtosecond laser are split into an emitter and a receiver arm. The beam in the emitter arm is focused onto a biased photoconductive antenna. The bias field causes an acceleration of the optically excited electron hole pairs, which leads to the generation of a THz pulse with spectral components from approximately 0.1 THz to 3 THz. This pulse is guided through the sample under test by polymeric lenses and is finally focused onto the receiver antenna. By inverting the principle of the generation process, the THz pulse can be detected: as before, an optical pulse generates free carriers between the electrodes of the antenna. However, instead of applying an external bias field, the incoming THz field is used to accelerate the carriers, driving them into the electrodes, resulting in a photocurrent. While the optical gating pulse is in the order of 100 fs, the THz pulse has a temporal length of picoseconds, so that the resulting photo current directly corresponds to a single point of the THz pulse shape. Introducing a variable time delay in one of the arms, which can be done either by a mechanical delay line or a fibre stretcher for fast scanning applications, the complete THz pulse is sampled step by step. Using a fibre stretcher, current THz systems are able to capture up to 200 full THz pulses per second, which correspond to 200 individual pixels in a THz image. Figure 1(b), shows an actual fibre-coupled pulsed THz system suitable for industrial applications

CW systems
When studying sharp absorption lines e.g. of gases, cw THz systems deliver an outstanding performance in terms of frequency resolution without the need for a femtosecond laser source. The basic system concept is visualized in the lower part of Figure 1(a). The beams of two frequency-stabilized laser diodes are spatially-overlapped in a beam combiner and then focused onto a photoconductive antenna with a similar geometry as is used in the case of pulsed systems. Due to difference frequency generation, a continuous THz wave is emitted. Detuning one of the laser diodes enables sweeping of the emission frequency. Using a second photoconductive antenna, the THz wave can be coherently detected [5].

Quality inspection of food products
Before describing the potential of applying THz technology to the quality inspection of food products it should first be pointed out THz technologies do have some limitations, which are often not emphasized strongly enough in other publications. Firstly, THz is strongly absorbed by water. While this actually offers an advantage when using THz to determine the hydration state of powders, this makes the transillumination of some food products such as meat or most kinds of cheese impossible. The second limitation is imposed by metal layers in food packages: as soon as the thickness of such a metal layer is more than a few microns, THz waves cannot pass the barrier.

Nevertheless, despite these limitations there are still many scenarios in which food products can benefit from a THz inspection system. As an example, we will show how THz imaging can reveal potentially hazardous inclusions in chocolate bars. While metallic contaminations are easily detected by conventional quality control systems, dielectric contaminations are often hard to find. When the physical contrast between the material and the contamination is slight (e.g. they have similar densities), existing approaches such as ultrasonic or X-ray scans may fail, so that certain types of inclusions, e.g. plastic and wood splinters, etc. remain undetected. However, such contaminations can still present serious health risks for consumers; these security breaches in existing quality control systems should be closed. In contrast to X-ray systems, THz scans do not only consider the amplitude but also the phase information. Thanks to the fact that the dielectric contrast mechanisms are much more pronounced at THz frequencies, hidden inclusions can be reliably revealed in THz images. In Figure 2(a) an image is shown of a chocolate bar contaminated with a buried glass splinter, a stone and a piece of metal, is depicted. It can be seen that the contaminants can be clearly detected. THz phase measurements can even differentiate between intended ingredients such as nuts and hazardous inclusions [6]. Given that, in contrast to X-rays, THz waves are non-ionizing and, in contrast to ultrasound detection, no coupling medium is required, THz scanners could therefore complement existing control systems in the quality inspection of food products. Figure 2(b) shows a future application scenario of a THz scanner operating at a food production line.

Conclusion and outlook
THz technology is taking the stage as a competitor to X-ray and ultrasonic scanners for the non-destructive and contact-free quality inspection of food products. Thanks to strong dielectric contrast, the superior imaging qualities of the trechnology provide a higher safety level for the customer. The non-ionizing nature of the THz waves removes the need for radiation protection systems that are necessary with X-ray systems.
Developmental challenges in the field remain to increase measurement speed and to further reduce system costs. Fortunately, the rapid progress in this highly active field makes it highly likely that,  in the upcoming years, THz systems will succeed in making the move from the research labs into routine production plants.

Acknowledgement
The authors would like to acknowledge the Federal Ministry of Education and Research (BMBF) for funding of the project 13N9466. Benedikt Scherger expresses his gratitude towards the Friedrich Ebert Stiftung for financial support. The authors appreciate the fruitful cooperation and valuable discussion with Dr. Jochen Scholz and Karin Diedrich of Sartorius AG as well as Sven Lübbecke of TEM Messtechnik GmbH.

References
1. Mittleman D (ed.). Sensing With Terahertz Radiation. Springer-Verlag, Berlin 2003.
2. Jansen C, Wietzke S, Peters O, Scheller M, Vieweg N, Salhi M, Krumbholz N, Jördens C, Hochrein T and Koch M. Terahertz imaging: applications and perspectives. Appl Opt. 2010; 49 E48-E57.
3. Scheller M, Jansen C and Koch M. Analyzing sub-100-µm samples with transmission terahertz time domain spectroscopy. Opt. Commun. 2009; 282: 1304.
4. Uhd Jepsen P, Jacobsen RH and Keiding SR. Generation and detection of terahertz pulses from biased semiconductor antennas. JOSA B 1996; 13: 2424.
5. Verghese S, McIntosh KA, Calawa S, Dinatale WF, Duerr E.K. and Molvar KA. Generation and detection of coherent terahertz waves using two photomixers. Appl. Phys. Lett. 1998; 73: 3824.
6. Jördens C and Koch M. Detection of foreign bodies in chocolate with pulsed terahertz spectroscopy. Opt. Eng. 2008; 47: 037003.

The authors
Christian Jansen ,Dipl.-Ing1,2,*, Benedikt Scherger, M.Sc.1,2, Christian Jördens, Ph.D.2, Ibraheem A. Ibraheem Al-Naib, Ph.D.1,2 and Martin Koch, Ph.D.1,2

1Fachbereich Physik
Philipps-Universität Marburg
Renthof 5
35032 Marburg, Germany
2Institut für Hochfrequenztechnik
Technische Universität Braunschweig Schleinitzstrasse 22
38106 Braunschweig, Germany
*Corresponding author:
e-mail: Christian.Jansen@ihf.tu-bs.de


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