Last year European Union (EU) Customs detained approximately 40 million products suspected of violating intellectual property rights. According to the EU Commission’s annual report, the value of the intercepted goods was close to €1 billion. Of the counterfeit products detained at the EU’s external borders, clothing (16.59%) was the second most intercepted product category.
Counterfeiting goes beyond copycat clothing down to the very fibre of the fabric itself. A manufacturer may say its cloth is 100% wool or a blend of 90% wool/10% mohair, but how can the buyer ever be sure? Scientists from the National Physical Laboratory (NPL) in Teddington, UK, have devised a solution to spot fakes that they believe can help combat textile counterfeiting.
Terahertz time-domain spectroscopy requires the generation of a beam of terahertz radiation, which is a band of electromagnetic radiation that falls between microwaves and infrared light. When a fabric is placed within this beam the terahertz waves pass through and the properties of the cloth are detected.
The composition and structure of different fabrics create different rates of beam scattering and absorption. This means that each fabric has a distinct transmission profile which can be detected to show whether a fabric is counterfeit.
The terahertz research team at NPL is part of the lab’s larger Electromagnetics Technology Group, which conducts research using terahertz (THz) radiation in a variety of fields. NPL has been working on THz spectroscopy for over 30 years. Tests on textiles, a growing area of NPL research, have been ongoing for approximately six months.
“The prevalence of counterfeit and substandard goods is well known,” explains John Molloy, the scientist leading the NPL project. “It negatively affects both the customer and industry, and ultimately costs jobs. I have a family background in the textiles trade and the manufacture of Donegal tweed. Therefore I’m acutely aware of the problems that exist for the industry.
“As some of my colleagues at the lab have had a longstanding interest in examining fibres at THz frequencies, we thought this might be something we could have a real impact on.”
The technique
Each fibre or blend of fibres is associated with a different transmission spectrum, which is linked to the physical properties of the fibre. By changing the fibres you change the spectrum, an alteration which can be measured. Molloy describes the effect as similar to the different colours our eyes perceive under visible light.
The NPL team tested natural fibres such as cotton, linen, wool and cashmere. The THz technology was also used to observe variations in coarse wool versus Merino, natural silks versus synthetic varieties and a number of blends such as wool/silk, wool/cotton/nylon/polyester and cashmere blends. The different blends demonstrate distinct terahertz transmission properties.
Terahertz-Time Domain Spectroscopy (THz-TDS) compares the spectrum of the light that is passed through the test material against the emitted spectrum of the light source. “When you compare the spectra, you will see that for light that passed through the material sample certain wavelengths have been filtered out completely or the energy at those wavelengths has been attenuated,” explains Molloy.
“This loss or reduction in energy is due to light absorption by the material we are testing. The pattern of absorption and the wavelengths at which absorption occurs is an identifying characteristic of each material, not unlike a signature. With this information we can determine what an unknown material placed in the spectrometer is made from.”
The technique can clearly distinguish between fabrics that look and feel similar but have different compositions. Many materials that are visually opaque are transparent to a certain degree at terahertz wavelengths, allowing a wider range of materials to be tested.
Extra information is also recorded about light waves, used to determine the refractive index of the material. This is a further characteristic that can be utilised to help in the identification of a sample.
Database
The NPL team believe it is necessary to create a database of the terahertz transmission properties of different fabrics as a basis for comparison. As each fabric has an identifiable terahertz signature, the quality or authenticity of a fabric is determined by a comparison between the signature and a known reference.
“The consistent accuracy and precision of these measurements is essential if you are to have confidence in the results,” Molloy explains. “In order to ensure this you need look at natural variation that occurs both within and between fabric batches. This needs to happen for each new fabric. To do so you will need to have a database for the range of fabrics.”
Commercial opportunities
According the NPL, the terahertz tests only take a couple of minutes per measurement on the research equipment. “During the initial development stage you are gathering the specific signature for the fabric and fibre,” says Molloy. “Once you get all that background information tied down and you have your database in place, testing would only be a matter of seconds per sample.
“We feel that in many respects THz spectroscopy positively distinguishes itself from other methods available and offers clear advantages. We’d like to see it become an industry standard testing method.”
The development of a precise measurement instrument and the ability to calibrate test devices in the field is crucial. At the moment a model would need to be developed with each partner and tailored to their specific needs.
The future
Molloy sees THz spectroscopy as a reliable and robust test for fibre composition. “We feel that this is good for everyone - the end customer, the retailer and the manufacturer. A test like this builds confidence in the product and confidence in its quality,” he states.
“We had a lot of attention for this work as a means of detecting counterfeit goods and indeed this was the initial inspiration, but it goes further than that. As a non-destructive testing method it can be implemented during production. It can help you reduce wastage; have better efficiency and more control of your manufacturing process. It makes doing better business easier,” Molloy adds.
NPL is looking at new projects with research partners, particularly in the technical textile sector. The team is interested in developments with polymers and coatings, using the THz spectroscopy as an R&D tool for process verification.