Terahertz imaging of objects in powders

Terahertz imaging of objects in powders

M.Herrmann, M.Tani, M.Watanabe and K.Sakai

Abstract: The use of terahertz radiation for-detecting and imaging objects hidden in powders is demonstrated. Glass, eggshell and various plastics have been imaged in powders such as wheat flour, talc and sugar. The method may have applications in quality control. The images havc been compiled by making use of various features of the terahertz time-domain waveforms. Details arising from refraction and interference close to the resolution limit are discussed. A simple method is suggested for distinguishing between objects of different materials even if their thickness is unknown. The potcntial of THz radiation for measuring the local density profile of powders is also demonstrated

1 Introduction

Terahertz (THz) imaging [1-6] is one of the more recent applications of THz time-domain spectroscopy. Since its development in 1995 [I], many applications for THz imaging have been suggested [ 3 , 61, for example. the detection of bubbles in plastics or replacing X-rays by THz radiation (‘T-rays’) at airport security checks. Such suggestions reflect that THz radiation penetrates many nonmetallic materials that arc opaque to visible light or other ranges in the electromagnetic spectrum. One class of substances that are nontransparent for a very wide spectral range are powders because they consist of a huge number of surfaces all of which can give rise to scattering. Only radiation at the high-frequency end (X-rays) and the low- frequency end (radio waves) of the electromagnetic spectrum penetrate powders. A condition at the low-frequency end is that a powder can become transparent when the wavelength is larger than the grain size of the powder. The grain size of many powders for example in the food industry is in or just below the mm range, and THz radiation is then thc radiation with the shortest available wavelength for an investigation, and it is thus the radiation that can yield the best image resolution. Alternatively, X-rays can be used for imaging in powders and this should work well with metal objects, hut with X-rays it can be difficult to dctect or image objects made of other low-index materials, particularly plastics, in organic powders. For the purpose of quality control in the food industry, the detection of any objects would he desirable. We investigate the potential of THz radiation for detecting and imaging objccts in powders.

2 Experimental

Thc THz imaging system (Fig. 1) is based on a pump-and- probe sctup with a Ti:sapphire fcmtosecond laser, THz radiation with a centre frequency of about 0.5 THz being generated when a laser pulse hits a dipole antenna on LT- GaAs. The radiation is focused onto the sample which is mechanically scanned in the x- and y-directions. The radiation transmitted through the sample is again focused onto the receiver, another dipole antenna which is gated by the probe pulses. With a delay loop in the probe beam, a time-domain waveform is recorded at every image position. Rearrangement of the data results in a set of images, onc image for each position of the time-delay loop, a ‘THz movie’ [7]. These THz movies (functions of three variables) are mapped to images (functions of two variables) using various ‘display modes’ each of which takes advan- tage of a specific feature of the time-domain waveform or its Fourier transform, for example thc amplitude, arrival time or the width of the main peak (the corresponding modes here called amplitude mode, delay-time mode or pulse-width mode, respectively).

The powder samples were produced by filling powder into a plastic box. The box was then closed and shaken thoroughly to reduce the size of cavities in the powder and to achieve a more homogeneous distribution. The powders were taken ‘off the shelf’ as commercially distributed and can thus be expected to be fairly dry. The boxes wei-e I O min thick, the scan area was 20 x 20mm2. We used an average pump lascr beam power of 25 mW with a pulse width of 80 fs and thus produccd a THz spectrum with a centre frequency of about 0.5 THz. The image acquisition time varied between 9 and 11 h. (Much faster image acquisition is possible with ‘rcal-time imaging systems’ [SI.) The transmission spectra in Fig. 2 were measured with an FT-IR spectrometer, the samples were 7 mm thick.

0 IEE, 2002 1E.E PmceediHgs online no. 20020185 Dol: 10. 1049iip-opt:20020185 Paper first received 2nd November 2001 and in revised form 3rd January 2002 The authors are with the Communications Research Laboratory, Kansai Advanced Rescarch Center, 588-2 Iwaoka, Nishi-ku. Kobc 651-2492. Japan

I I6

3 Powder spectroscopy

Fig. 2 shows the THz spectra of sclected powders: wheat flour, sugar and talc. While talc still transmits a considerable amount of radiation at 1 THz, wheat flour and sugar

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are opaque at this frequency. Lower-frequency radiation, however, is transmitted and can be used for imaging.

The transmission spectra are certain to depend strongly on the optical properties of the grain material, the size of the grains and the size of the cavities between the grains. Other factors include the grain shape and possible contami- nants, of which water is of particular interest here because of its high absorptance of THz radiation and because of its abundance in the atmosphere.

1 THz corresponds to a wavelength of 1. = 300 pm in vacuum or air. The grain size d of the powders varies from -20 pm in talc to -50 pm in flour and -300 pm in sugar. The size parameter is d / i < 1/10, and thus in the range of Rayleigh scattering which predicts that the mean free path depends on the wavelength as 1 * c c i m with m = 4 . However, the underlying model assumes that the Scattering centres are weak or diluted. In particular, it neglects interference between different scattering centres. This approach need not work for a powder where scattering centres are closerto each other than the wavelength. So it is not surprising that the transmission in Fig. 2 does not follow the Rayleigh formula. We observe instead a propor- tionality with the exponents ni = 1, 1.5 and 2 for flour, talc and sugar, respectively.

A more elaborate model for scattering in random media was recently presented by Kawato [8] and shown to work

IEE Pmc.-Opioeleclron.. Yo/. 149, No, 3, June ZOO2

well for a porous glass with porc sizes of 0.15 to 0.5 pm under visiblc and near-infrared light. We expect that i t should work as well for our powders under THz radiation, but since our work by now has focused on the imaging aspects ofobjects in powders, we have not tested the model on our powders yet.

As a rule, powders with smalle? grains transmit high frequencies to a larger extent and can thus result in greater

-image resolution if the focus is adjusted precisely.

4 Imaging results and discussion

Fig. 3 displays two objects in powdered sugar in various display modes. The left object is a piece of egg shell, the right one a piece of plastics. The top left image (Fig. 3u) is a constant-time mode image as taken with the delay stage fixed. Theie is an inherent problem with this display mode when imaging objects in powders because powders tend to be more dense at the bottom than at the top, which results in a considerable shift in pulse arrival time. In the bottom half of Fig. 3u the image is mostly white which means that at the time of measurement the main pulse arrived and the THz electric field was strongly positive. The top of the image is black because at the time of measurement the main THz pulse had already passed and the first strong minimum arrived. The phase or time difference thus amounts to approximately half a cycle of 0.5 THz radiation, or l ps. This means that pulsed THz radiation should be an efficient tool in studying the vertical density profile and local density fluctuations in powders. But it also means that for the purpose of imaging objects in powders constant-time mode images are difficult to interpret.

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Fig. 3 a n d plusticr pight) hidden in powdered sugar (THz beam usas blocked by sample holder in the comcrsj il Constant-time mode h Amplitude mode c Delay-time mode dPulse-width mode

Image.< in various display modes showing epgchell (leli.,

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117

Images calculated from time-domain waveforms, as those following, can display objects much more clearly.

Fig. 36 is an amplitude-mode display of the same measurement. I t maps the peak signal of the waveform at each sample position. Both objects can be seen with much better contrast than in Fig. 3a. The piece of eggshell appears darker than the plastic piece or than the surround- ing powder. This means decreased transmittance due to stronger absorption or scattering. The dark edges around the objects are caused by diffraction due to the same phenomenon that causes a dark minimum and interference lines when visible light passes along a sharp edge [7]. The black areas in the comers are shadows from the sample holder.

The bottom-left image (Fig. 3c) displays the arrival time of the pulse maximum. An offset plane was used on this image (and also on all subsequent images) to compensate for the vertical variation of powder density described. Both objects produced comparable pulse delay although the plastic piece was considerably thicker than eggshell (about 1.5 against 0.4 mm). This reflects the greater refractive index of eggshell (approximately 2.7 against 1.6 for plastic). The plastic piece in this display mode shows a sharp boundary on the right-hand side and a soft boundary on the left-hand side. The reason for this is the shape ofthe plastic piece which was broken off from a bigger disc. The right-hand edge was parallel to the THz beam while the let"-hand edge was at an angle of 45" so that the thickness on the left edge reduced gradually.

In Fig. 3d the width of the pulse maximum is displayed. Both objects appear with reduced brightness corresponding to reduced pulse width compared with their environment. The most likely cause is that the powder is less transparent for the high-frequency components of the radiation (cf. Fig. 2) and thus broadens the pulse while plastics and eggshell do not broaden the pulse so much. Another possible reason is that the strong dispersion in the powder significantly changes the pulse shape and widens the pulse. The boundaries of the objects appear bright. At these places, both the pulse through the object and the pulse around the object arrived in comparablc intensity but a t different times, and the rcsulting interfering waveform was a double pulse that was interpreted by the evaluation software as a very wide pulse. In the casc of the plastic piece, this effect was sharp on the right-hand side and soft on the left-hand side just as and out of the same reason as the boundary effect in Fig. 3c.

Fig. 4 displays a piece of plastics (top) and a piece of glass (bottom) in talc. Glass absorbs THz radiation strongly and correspondingly appears black in amplitude mode (Fig. 4a). Due to its high refractive index it also causes strong contrast in delay-time mode (Fig. 4h). Other materials that produce such strong signals include metals and materials with significant amounts of water (for example, meat). Compared with other, non-granular materials, powders have a relatively low refractive index owing to their high air content, but a high extinction coefficient owing to scattering at the surfaces. This is why objects made of a large variety of materials can be imaged with good contrast using THz radiation.

Special interest was directed to imaging plastic objects in powders. Fig. S shows images of a cable clamp (such as used for fixing cables to walls) in wheat flour. Fig. Sa is a sketch, while Figs. Sh, c and d are THz images in amplitude mode, delay-time mode or pulse-width mode, respectively. The contrast in Figs. Sh and c is strong where the THr beam crosses much plastic material. The cable clamp appears brighiin both modes because the extinction

118

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I Fig. 4 Amplitude and dela)'-firnr di.sp1ay modes of plastics (top) and gluss @onon?) piecer hidden in talc

coefficient of the plastic is lower andi ts refractive index is higher than those of the powder. There is particularly strong contrast at the hook (right part) of the clamp which is made of solid material. The body of the clamp (left part) is hollow, so that strong contrast is produced only by its walls at the sides and the bottom. It is open to the top. In the bottom, a hole for a nail is also clearly identified although with a diameter o f 1.5 mm it is smaller than the optical resolution for most of the radiation transmitted through the powder. The bottom of the clamp

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Fig. 5 Skerch and THz fmages of cable clamp in wheurJloirr a Sketch b Amplitude mode; since THz radiation travels more easily through plastics than through flour, thick parts of sample result in stronger signal and thus displaycd bright c Delay-time made; due to higher refractive index for plastics than for flour. thick parts of clamp result in larger pulsc delay time (bight). Note dark arc8 of reduced pulse delay where body of clamp was not filled densely with powder d Pulse-width mode emphasises sample cdges. Since flour broadens the THz pulse more than plastics, thick parts of sample and arcas with reduced flour density both show darker than background

/€.E Pmc -0proeIei.nun.. Yo/. 149. No. 3, ./uric ZOO2

appears thicker than it actually is; the likely reason being that the clamp was somewhat rotated with respect to the sketch (Fig. sa), especially around an axis horizontally aligned in the plane of the paper. This could have happened during the process of shaking the sample to distribute the powder in the box. A dark area in and on top of the body of the clamp in delay-time mode (Fig. 5c) indicates that the body was probably not densely filled with flour. This is confirmed in pulse-width mode (Fig. 5 4 , where the same region displays reduced pulse width.

5 Distinguishing between materials

The changes in pulse arrival time and amplitude caused by an object in a powder depend on the refractive index and the extinction coefficient of the object, respectively (and on those of the powdcr). When the thickness of an object is known the refractive index and the extinction coefficient can be determined easily from the images. This simple characterisation of a material is not possible, however, when the thickness of the ohjcct is unknown. Yet even in this case it is possible within certain limits to distinguish bctween various materials. For this purpose a quantity can be derived from the waveforms that does not depend on the thickness of an object in the powder.

The difference At in delay time between a pulse through an object as against a pulse that does not cross the object is

where An is the difference in refractive indices of the object and the powder, d is the thickness ofthe object, c the vacuum velocity of light and any quantitics suffixed ‘0’ refer to pulses that do not cross an object while ordinarily those quantities refer to pulses that cross an object. The amplitude reduces as

where Ak is the difference in extinction coefficients of the object and the powder, and tu, for a matter of convenience can be taken as the centre circular frequency of the transmitted radiation though here it serves only as a scaling constant. (Keep in mind that the refractive index or extinction coefficient from a powder can be very different from the refractive index or extinction coefficient of the same substance if it is not a powder.) Eqn. 2 neglects the losses from reflection at the surface of the object, but this loss is ( n - no)2/ (n + no)2 and thus usually small since the refractive indices are often close to each other. For example, typically n = 1.6 for various plastics, and n = I .3 to I .4 for powders as flour, sugar and powdered sugar (depending also on the density of thc powder). The resulting error in amplitude is here less than I %. Eqn. 2 can he rewritten as

(3)

so that wc arrive at two equations linear in d, and by dividing these through each other we obtain a material parameter

which does not depend on the thickness. Multiplying eqns. 1 and 3 yields a thickness parameter

p2 := w,Atln 2 = AkAn 2 d2 C) (:.I2 which depends strongly on the thickness and less strongly on the materials. - Fig. 6 shows the result of this evaluation for a sample with three plastic objects in wheat floor. The bottom object and the top right object are small plates of polyoxy- methylene (POM). The bottom platc is 2.8“ thick, both top plates are I .O mm thick. The top-left plate is made of acrylnitrile-butadiene-styrole copolymer (ABS). In Figs. 6a and b the amplitudes and delay times of the transmitted waveforms are displayed. The thick POM plate causes the strongest contrast in both images, but the two thin plates also differ in contrast. In Fig. 6c, P2 is displayed on a hrightncss scale while at the same time Q is translated into a colour scale ranging from red to green. The bottom plate still appears brightest according to its thickness. Both POM plates appear green, and the ABS plate red.

We have also tested this method for objects in air (without powder) and it worked even better there although the difference in refractive indices is much larger. Among

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Fig. 6 o Amplitude mode h Delay-time mode c Material parameter Q translated in& colnur scale; POM appcars green while ABS appears rcd; brightness corresponds to objcct thickncrs

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Vertical position dependeuce of delay time of THz pulses

the problems of this method with our THz setup is that powders are not opaque for low-wavelength radiation. The large wavelength causes considerable refraction on the objects which have to he chosen small to fit into our sampling area of 20 x 20 mm'. The thickness of the thin plates, only by a factor two greater than the wavelength, may also cause some problems.

While the formulas presented are simple, a more thorough calculation soon gets very complex because the extinction coefficients of the powders depend strongly on the wavelength, so this dependence must then he taken into account. For this and other reasons it is difficult to find a thickness-independent parameter in a more strict approx- imation.

6 Local variations of powder density

Since many powders have a refractive index for THz radiation significantly different from 1, THz radiation can be used not only for characterising objects embedded in powders hut also for investigating these powders. For example, as mentioned earlier, powders are often denser at the bottom than at the top of the samples. This effect has previously been investigated either with invasive techni- ques or noninvasively with very limited local resolution [9] (four capacitors along a vertical tube). In THz imaging this effect manifests itself in a position-dependent THz amplitude and pulse arrival time and can he investigated nondestructively with a local resolution of approximately 2 mm. For this purpose we prepared a sample by filling wheat flour into a plastics box with an inner thickness of 7 mm. The box was knocked on a table from a height of 3 c m 100 times to achieve a certain level of density relaxation. The powder volume decreased by 30% during this process. The sample was then inserted into the THz imaging chamber and vertical scans were recorded. The resulting dependence of the pulse arrival time on the vertical position is displayed in Fig. 7 . The difference between top and bottom amounts to as much as 1 . 4 ~ s over 16 mm vertical distance while a temporal resolution of I O fs is achievable with the THz imaging setup. The delay time of 1 . 4 ~ ~ corresponds to a difference in refractive indices of 0.060. The average refractive index is -1.4.

Fig. 7 shows that the refractive index near the top surface increases more strongly, and at greater depth increases less strongly. The THz amplitude at the bottom of the sample is 20% less than at the top. With the theoretical model in [8] it should be possible to relate the transmission spectra quantitatively to local variations in the physical constitu- tion of the powder (filling factor, distribution of grain sizes), hut we have not yet attempted this evaluation.

7 Conclusions

Nonmetallic powders with grain sizes below I-mm are often fairly transparent for electromagnetic radiation in and below the THr range. THz imaging is an effective tool for finding objects hidden in such powders, for example in talc, wheat flour, sugar, powdered sugar and pepper. Objects made of glass, ceramics, metal, rubber and various kinds ofplastics can be detected and imaged in this way. The lateral resolution of the images is limited by the wavelength of the transmitted radiation, approximately 2 m m or less depending on the powder grain size. The local distribution of the amplitude, delay time and pulse width provide ample information on details of the objects. They can he translated into a material parameter that does not depend on the thickness of an object. The method can have applications in quality control, particularly in the food industry. In addition, THr time-domain spectroscopy is a powerful tool for studying the local density profile in powders.

8 Acknowledgment

This work was partly supportcd by The New Energy and Industrial Technology Development Organisation (NEDO) through the Proposal-Based New Industry Creative Type Technology Research and Development Promotion Program.

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Terahertz imaging of objects in powders