Terahertz measures paper dryness

24 Jun 2008

Could the paper industry be next in line to benefit from terahertz radiation? Researchers in Germany think so, and tell optics.org why.

Terahertz radiation is often cited as being good for security applications because it can penetrate many materials, allowing hidden objects like weapons to be "seen". Now, researchers in Germany have shown that T-rays can be employed for a new type of application altogether – accurately measuring the water content in paper. If perfected, the technique might become an important tool in the paper industry (Optics Express 16 9060).

Watching paper dry

Drying is an important part of paper production because it directly affects final paper quality and overall production costs. An uneven moisture distribution in paper can, for example, lead to curling or cockling and misfeed in a printer. Overdrying paper also consumes a lot of energy and is therefore expensive.

Currently, paper dryness is measured using microwave or near-infrared radiation. Although microwaves penetrate deep into paper, they exhibit poor spatial resolution because of their large wavelength. The disadvantages of near-infrared frequencies are that they are significantly scattered by the paper's texture and do not penetrate heavier samples very well.

In contrast, terahertz (THz) radiation (which lies in the far-infrared part of the electromagnetic spectrum) allows real-time analysis of the water content in a wider range of paper thicknesses than traditional methods. This is because its wavelength is small enough to permit a resolution down to the sub-millimetre range, but large enough to avoid severe scattering – inhomogeneous structures in the texture of paper are much smaller in size than the wavelength of the THz radiation itself. Indeed, THz waves have already been used to measure the moisture content in plant leaves for this reason.

The technique, developed by Wolff von Spiegel of Johann Wolfgang Goethe Universität in Frankfurt, Debasish Banerjee of Technische Universität in Darmstadt and colleagues, works because paper weakly absorbs THz waves while water strongly absorbs them. The water content in paper can therefore be determined by measuring the amplitude and phase of THz waves after they pass through a paper sample.

Von Spiegel and Banerjee used radiation at 0.6 THz with a power of 0.5 mW from a frequency-multiplied Gunn source. The researchers positioned samples of moist paper on a translation stage and moved them through the focus of the THz beam. A series of optical-density and phase images were acquired while the paper dried in air.

The researchers demonstrated that the amplitude of the THz waveform increases as the paper dries (see image). They also showed that the THz wave arrived earlier at the detector when there was less water in the sample.

The team says that the technique might be used to optimize paper drying during manufacture. "Online monitoring and diagnostics in paper drying are important issues and moisture mobility and its distribution are very crucial aspect as far as certain paper qualities are concerned," Banerjee told optics.org.

The group would now like to try out its technique on heavier paper. It eventually hopes to develop a real-world online moisture sensor based on THz radiation suitable for the paper industry, probably based on a semiconductor detector array.

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