- Email: [email protected]
Broadband terahertz imaging of documents written with lead pencils
Optics Communications 282 (2009) 3104–3107
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Broadband terahertz imaging of documents written with lead pencils E. Abraham *, A. Younus, A. El Fatimy, J.C. Delagnes, E. Nguéma, P. Mounaix Centre de Physique Moléculaire Optique et Hertzienne, Université de Bordeaux 1, UMR 5798, 351 Cours de la Libération, 33405 Talence Cedex, France
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 16 April 2009 Accepted 16 April 2009
a b s t r a c t Far infrared transmission spectra of several graphite pencil leads on paper have been measured up to 2 THz using time-domain spectroscopy. The observation of the gradual absorption depending on the graphite proportion has been assessed for different pencils from hard to soft black-marking graphite leads. The resulting graphite transmittance is used to perform two-dimensional transmission terahertz imaging of written documents. Ó 2009 Elsevier B.V. All rights reserved.
Between the microwave and infrared frequencies lie now the well-known terahertz frequency radiation which have been successfully employed for both spectroscopy and imaging. Especially, terahertz time-domain spectroscopic (THz-TDS) imaging has become an interesting new tool for non-invasive and non-destructive material testing [1]. For instance, for security analysis or biomedical applications, it is mandatory not only to visualize the samples but also to explore and identify the chemical composition of the test objects [2,3]. For this purpose, THz waves exhibit attractive features such as good penetration depth in certain materials (e.g. paper, wood, plastics, etc.), low scattering, free-space propagation, low photon energy (non-ionizing), good beam coherence and broad spectral bandwidth. Since many substances exhibit spectral fingerprints in the THz region [4], spectroscopic analysis in this region has received much attention as a new tool for material characterization. Furthermore, the availability of coherent optical imaging has made THz imaging an attractive non-contact, non-ionizing method for a variety of applications [5]. Since the first demonstration of THz transmission imaging [6], several promising THz imaging techniques have emerged such as reflection tomography [7], computed tomography [8], near-field imaging [9], cw imaging [10,11], real-time imaging [12] and spectroscopic imaging [13]. In particular, the latter allows discrimination and spatial imaging of the chemical components in a sample on the basis of the THz spectral fingerprints. Consequently, this system offers new opportunities in detection of illicit drugs [14] and explosive [15], biomedical applications [3] and more recently art conservation [16]. For this latter purpose, X-ray fluorescence, UV and near-infrared imaging are commonly used [17,18]. However, in terms of conservation, it is essential to employ non-destructive and non-invasive techniques to investigate art objects including paintings, murals, colored sculptures or furniture. Therefore, THz radiation has been * Corresponding author. E-mail address: [email protected] (E. Abraham). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.04.039
used to reveal murals hidden beneath coats of plaster or paint in century-old buildings [16,19]. As a result, THz imaging makes possible the evaluation of historical artifacts such as frescoes, mural paintings or underdrawings. It could also be an efficient tool to prevent counterfeit art products due to chemical diagnostic capabilities of THz spectroscopic analysis. The present paper reports measurements, performed in the 0.1– 2 THz frequency range, on a set of lead pencils ranging from a hard light-marking pencil to a very soft black-marking one. The THz absorption has been analyzed on paper in order to demonstrate the feasibility of 2D THz imaging of pencil lead with high contrast, sensitivity and good resolution. The broadband THz spectrometer is based on a standard transmission THz-TDS setup. We used an 80 fs Ti–Sapphire laser with a 76 MHz repetition rate. The laser output is split into a pump and a sampling beam. The pump beam is focused onto a bare InAs layer for surface field emission of the THz pulse. After emission, the THz beam is collimated, focused and transmitted through the sample location. The transmitted THz pulse is measured using the current generated in a LT–GaAs semiconductor (photoswitch) receiver triggered by the sampling pulse. This current is amplified and processed with a lock-in digital amplifier. The measured current is directly proportional to the time-varying electric field of the THz radiation. This setup provides a frequency response up to 3.5 THz [20]. In fact, the major advantage of this technique is the direct access to the electric field amplitude of the THz pulse thus avoiding any Kramers–Krönig analysis [21]. Thus the time-domain THz spectrometer allows a direct determination of the complex dielectric response e(x). Before performing transmission THz imaging with this abovementioned experimental apparatus, we evaluated the transverse resolution dmin. Since the imaging system consists of raster scanning the sample through the THz beam, the transverse resolution is directly connected to the diameter of the THz beam at the sample position. Consequently, a tightly focused THz beam at the sample position will provide a high transverse resolution. Taking
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E. Abraham et al. / Optics Communications 282 (2009) 3104–3107
k NA
ð1Þ
where k is the wavelength and NA the numerical aperture of the focusing lens. To enhance this parameter, we inserted in the THz pathway two homemade High Density Poly Ethylene (HDPE) lenses, both with a 10 mm focal length and a 10 mm diameter. For imaging, the object is inserted between these two lenses in a confocal configuration. To measure the corresponding transverse resolution of the imaging system, we used a calibrated object. Here, we chose a 1951 USAF Resolution Test Chart printed on regular white sheet paper. The pattern consists of vertical lines with 2.5 lp/mm corresponding to a transverse resolution of 200 lm. With the THz-TDS imaging system, THz images can be reconstructed for all frequencies up to 2 THz. For higher frequencies, the paper absorption starts to limit the signal-to-noise ratio of the detection. At 2 THz, the Fig. 1a shows the corresponding THz image obtained with a 50 lm step size. The three lines (white color) are easily visible indicating that the transverse resolution of the THz imaging system is at least 200 lm at 2 THz. Taking into account Eq. (1), this result is in perfect agreement with the theoretical value of 210 lm assuming a NA of 0.44 for the focusing lens with a 10 mm diameter. Fig. 1b shows the intensity profile of the THz image along the horizontal line of Fig. 1a. Along this line, the contrast of the THz image can be estimated. At 2 THz, for the 200 lm width pattern, we obtained an average contrast of 70%. For lower frequencies, we also calculated the spatial resolution of the imaging system (data not shown). In agreement with Eq. (1), we observed a linear variation of the spatial resolution with the wavelength. For instance, at 1 THz, the lateral resolution naturally decreases to 400 lm. Finally, it can be concluded that the THz imaging system described in this paper is able to provide diffraction limited THz images with an excellent transverse resolution and contrast ratio. Further, we investigated the THz imaging of pencil lead on paper in transmission mode. The writing characteristics of graphite based pencils (black color, hardness, etc.) arise from the ratio of graphite to clay. These pencils are usually graded by manufactures using the letter H for hardness (more clay, less graphite) and the letter B for blackness (less clay, more graphite). Especially, by altering the proportion of graphite to clay, graphite pencils are usually graded by manufactures using the letter H for hardness (more clay, less graphite) and the letter B for blackness In this study, we tested several pencil leads with increasing level of graphite in the lead from the standard writing pencil HB to softer black-marking pencils indicated as 2B, 4B, 6B and 8B. The main purpose of this research work is the assessment and identification of the graphite
reference : white paper
(a)
2B 4B
1
6B 8B
0
-1
-2
2
3
4
5
6
7
1.0
(b)
0.8
0.6
0.4
2B 4B
0.2
6B 8B
0.0 0.1
0.5
1.0
1.5
2.0
frequency (THz) Fig. 2. Terahertz transmission properties of several graphite pencil grades. (a) THz waveforms obtained after transmission through white paper without writing (reference) and white paper recovered by graphite pencils from 2B to 8B; (b) corresponding transmission spectra in the THz region.
100
(b)
80 60 40 20 0
200µm
8
delay time (ps)
intensity (a.u.)
(a) (a)
2
Electric field (a.u.)
dmin ¼ 0:61
pencil lead grade using THz spectroscopic imaging for the future investigation of underdrawings beneath paintings. First, we investigated the THz transmission through a square graphite area (1 1 cm2) drawn with different pencil leads on a regular 80 lm thick white sheet paper. Although it is difficult to evaluate the graphite quantity deposited onto the paper, owing to the hand drawing process, we can estimate that the sample thickness is less than a few micrometers. Moreover, the exact quantity of graphite deposited onto the paper is difficult to evaluate because the pencil lead is made of graphite powder and also various kind of binders. For different pencil grades, Fig. 2a shows the temporal transmitted data through the sample. Although the amplitude of the THz wave can vary depending on the pen pressure
Transmission
into account the well-know Rayleigh criterion, the theoretical transverse resolution dmin can be expressed as:
0
200
400
600
800
1000
1200
distance (µm)
Fig. 1. Image of USAF test pattern, 2.5 lp/mm. (a) THz image at 2 THz; (b) intensity profile along the horizontal line indicated in (a).
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and the graphite thickness, we measured the transmission of the THz wave through the graphite and the paper. To ensure the validity of the method, we repeated several times the measurement taking into account the variations of the pen pressure on the paper during the hand writing process. Finally, the THz signal transmission can be directly related to the graphite absorption since the scattering of the THz radiation is reduced because its wavelength is larger than the typical spatial dimensions of the surface texture. Obviously, it would have been useful to measure the absorption spectra of graphite pellet to avoid the problem of the unknown quantity of graphite deposited onto the paper. However, for future practical applications in art conservation, we also found it interesting to determine at least qualitatively the absorption of graphite on paper. As a reference, we plotted the data corresponding to the blank paper sheet. For the HB pencil (or even harder leads from 2H to 9H), the THz absorption was negligible (data not shown) and the data are similar to the reference. However, from 2B to 8B, we clearly observed a significant attenuation of the transmitted THz beam due to the absorption of the THz radiation. This absorption is mainly due to the presence of graphite in the pencil lead since the concentration of this mineral increase regularly from HB to 8B. It is well known that graphite can absorb or reflect THz radiation due to its high electronic conductivity [22]. A slight temporal shift is also observed which originates from the variations of the optical path due to increasing sample thickness. This behaviour can be directly used as a pertinent parameter for THz imaging as illustrated later in the paper. Fig. 2b represents the corresponding transfer functions of the various pencil leads on paper with respect to frequency up to 2 THz. This corresponds to the transmission spectra of the different graphite leads on paper from 2B to 8B. First, we can notice that no strong spectral difference is visible in the spectra. This may be explain by the fact that the chemical constitution of the different pencil leads is almost the same for all the samples since only the proportion of graphite changes from 2B to 8B. However, the amplitudes of the spectra are clearly different which offers the possibility to identify the lead at least qualitatively. For the different pencils from hard to soft black-marking graphite leads, we identified the gradual absorption of graphite proportion. The transmission spectra extend from 0.1 to 2 THz since the paper starts to absorb strongly above this limit. For example, at 2 THz, the transmission of 2B pencil is 65% and decreases down to 25% for the 8B pencil. These results emphasize that a thin graphite layer deposited by a pencil lead can provide sufficient amplitude and/ or phase contrast in the THz domain. Consequently, through the analysis of the transmission spectra, we can assume that THzTDS imaging allows identifying the quantity of graphite, i.e. the label of the pencil lead, deposited onto the paper. To this aim, we imaged the CNRS logo where each letter has been written with a different lead grade (2B, 4B, 6B and 8B, respectively) and the five oblique lines were made with the harder HB grade. The corresponding visible and THz pictures of the logo are shown in Fig. 3a and b, respectively. The sample area is 20 20 mm2. In the visible image, it is almost impossible to discriminate which pencils have been used to write the individual letters. It means that the visible image does not allow determining the chemical nature of the different letters. To obtain the THz image of the logo, we used a raster scanning system. The image consists of 41 41 pixels with 0.5 mm spacing. For each pixel, it takes about 2 s to obtain the transmitted time-domain waveform with a lock-in time constant of 10 ms. Therefore the essential measurement time takes less than one hour and could be strongly reduced by using a faster optical delay line. After Fourier-transform of the temporal data, we computed images at various frequencies. Fig. 3b represents the image of the CNRS logo at 2 THz. Using a false color grayscale, we can clearly visualize that the letters
Fig. 3. Digital photograph (a) and THz image at 2 THz (b) of the CNRS logo drawn with five different graphite pencil grades. Oblique lines around S: HB, C: 2B, N: 4B, R: 6B, S: 8B.
appear darker and darker due to the increase of graphite content from the C letter to the S letter. However, around the S letter, the five oblique lines written with the HB pencil are hardly visible in Fig. 3b, in accordance with the low absorption of the HB lead in the THz region as already mentioned. Finally, with the THz image at 2 THz, various letters can be separated from each others due to the selective THz absorption of the graphite. The discrimination is easy due to the very high signal-to-noise ratio associated with a coherent detection scheme compared with a cw THz imaging system. As already mentioned, it is also possible to compute THz images not only using the spectral amplitude but also directly using the arrival time of the THz pulse (i.e. temporal phase shift). Such image contains information about the optical path of the sample in the THz range. Fig. 4 represents the 3D THz map of the CNRS logo where the THz main pulse time delay has been plotted. This phase delay map appears different to the previous spectral picture at 2 THz (Fig. 3b). To explain this, we have to take into account that the grayscale variations of the map do not reflect the variations of the sample chemical properties but either the variations of the sample thickness (in micrometer). The accuracy of the depth measurement is related to the measurement of the arrival time of the THz pulse and the acquisition step of the delay line. With a micrometer scan of the delay line, we can estimate that the accuracy of the depth measurement is about 5 lm. This precision is not as good as a previous work of Hills et al. who reported an ultra-high depth resolution of 0.5 lm. However, the authors used a completely different system using a heterodyne detection with a hybrid cw THz source [23]. In Fig. 4, it appears that, similarly for
Fig. 4. THz map of the CNRS logo obtained using the THz main pulse time delay. The scale of the 3D map surface is in lm and represents the paper thickness variations due to the pencil grade writing.
E. Abraham et al. / Optics Communications 282 (2009) 3104–3107
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the drawing with bare eyes (Fig. 5b). Fig. 5c shows the THz image of the sample for 2 THz. In spite of the presence of the black ink, the butterfly is clearly visible with a good contrast and high resolution. However, the fine details at the top of the butterfly wings are hardly visible. However, in Fig. 5d which represents the THz image using the arrival time of the THz pulse (temporal phase), the image quality is excellent and the finest details of the sample can be resolved. As previously, we attribute the high quality of the phase image to the fact that the arrival time of the THz pulse is very sensitive to any thickness variations of the sample. This simple demonstration offers the opportunity to visualize sketches of original paintings beneath layers. Here, in this THz imaging application, we investigated drawings under a layer of ink, in the future we will study drawings under media such as paint. For a better efficiency and utility, a reflection configuration with a larger imaging depth-of-field can be employed to investigate thicker samples. This technique is a promising tool in fields such as art conservation and archaeology where non-invasive imaging and chemical analyses are often required. Acknowledgment
Fig. 5. Butterfly drawn on paper with a 4B graphite pencil. (a) Digital photograph of the sketch; (b) digital photograph of the sketch painted with a black ink felt-tip pen; (c) THz image at 2 THz; (d) THz image using the THz main pulse time delay.
This work is supported by the ‘‘Conseil Régional d’Aquitaine”, in collaboration with the Labri (UMR 5800) and Innovative Imaging Solutions (www.i2s-corp.com I2S).
References all letters, the hand writing process decreases the paper sheet thickness from 80 lm (no inscription on the paper) to about 50 lm (in the region corresponding to an inscription). This 30 lm decrease is simply due to the pressure of the lead on the soft sheet paper. It appears also that the width of the letters seems smaller than in Fig. 3. This may be due to the fact that the phase delay picture in Fig. 4 is very sensitive to any variation in thickness which improves the image resolution. It can also be noticed in Fig. 4 that the five HB oblique lines around the S letter are well visible in the image since the contrast does not depend anymore on the chemical properties of the pencil leads but only on the paper thickness variations. In short, for the study of writing documents on soft materials such as sheet, the phase THz imaging seems to be a very interesting tool to determine the variation of the sample thickness. In that sense, the imaging system works as a kind of non-contact THz profilometer. At least, the results described in Figs. 3 and 4 show that it is possible, from a single THz imaging experiment, to combine spectral and temporal THz analysis in order to improve the effective contrast and reinforce the efficiency of the THz imaging. Finally, we emphasize on the real advantage of the apparatus which could be powerful for the investigation of underdrawings beneath ink layers. For that purpose, it was proposed to use the transmission TDS-THz imaging system to recover buried layer information. The object is a handmade butterfly drawn on standard paper with a 4B pencil lead (size is 15 15 mm2). Fig. 5a represents a standard photograph of the object under study. Before performing THz imaging, the butterfly sketch has been fully recovered with a black ink felt-tip pen so that it is impossible to distinguish
[1] W.L. Chan, J. Deibel, D.M. Mittleman, Rep. Prog. Phys. 70 (2007) 1325. [2] D. Creeden, J.C. McCarthy, P.A. Ketteridge, P.G. Schunemann, T. Southward, J.J. Komiak, E.P. Chicklis, Opt. Express 15 (2007) 6478. [3] S.Y. Huang, Y.X.J. Wang, D.K.W. Yeung, A.T. Ahuja, Y.T. Zhang, E. PickwellMacPherson, Phys. Med. Biol. 54 (2009) 149. [4] O. Graydon, Nature Photon. 2 (2008) 68. [5] V.P. Wallace, E. MacPherson, J.A. Zeitler, C. Reid, JOSA A25 (2008) 3120. [6] B.B. Hu, M.C. Nuss, Opt. Lett. 20 (1995) 1716. [7] D.M. Mittleman, S. Hunsche, L. Boivin, M.C. Nuss, Opt. Lett. 22 (1997) 904. [8] B. Ferguson, S. Wang, D. Gray, D. Abbot, X.-C. Zhang, Opt. Lett. 27 (2002) 1312. [9] S. Hunsche, M. Koch, I. Brener, M.C. Nuss, Opt. Commun. 150 (1998) 22. [10] K.J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H.G. Roskos, S. Czasch, Appl. Phys. Lett. 80 (2002). [11] A. El Fatimy, J.C. Delagnes, E. Abraham, A. Younus, E. Nguéma, P. Mounaix, F. Teppe, W. Knap, Opt. Commun., in press. [12] T. Yasui, K.I. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, T. Araki, Opt. Express 16 (2008) 1208. [13] C.D. Stoik, M.J. Bohn, J.L. Blackshire, Opt. Express 16 (2008) 17039. [14] K. Kawase, Y. Ogawa, Y. Watanabe, H. Inoue, Opt. Express 11 (2003) 2549. [15] Y.C. Shen, T. Lo, P.F. Taday, B.E. Cole, W.R. Tribe, M.C. Kemp, Appl. Phys. Lett. 86 (2005) 241116. [16] K. Fukunaga, Y. Ogawa, S. Hayashi, I. Hosako, IEICE Electron. Express 4 (2007) 258. [17] W. Köhler, M. Panzer, U. Klotzach, S. Winner, M. Helm, F. Rutz, C. Jördens, M. Koch, H. Leitner in: European Conference of Non-Destructive Testing 2006 Proceedings, 2006, Poster 181. [18] D. Barilaro, V. Crupi, D. Majolio, G. Barone, R. Ponterio, J. Appl. Phys. 97 (2005) 044907. [19] J.B. Jackson, M. Mourou, J.F. Whitaker, I.N. Duling III, S.L. Williamson, M. Menu, G.A. Mourou, Opt. Commun. 281 (2008) 527. [20] E. Nguéma, V. Vigneras, J.L. Miane, P. Mounaix, Eur. Polym. J. 44 (2008) 124. [21] P. Mounaix, M. Moustakim, S. Le Boiteux, J.P. Delville, R. Wunenburger, L. Sarger, Appl. Phys. Lett. 83 (2003). [22] T. Ji, M. Ge, W. Wang, Z. Zhang, X. Yu, H. Xu, He Jishu/Nuclear Techniques 29 (2006) 561. [23] B. Hills, M.D. Thomson, T. Löffler, W. Von Spiegel, C. Am Weg, H.G. Roskos, P. De Maagt, D. Doyle, R.D. Geckeler, Opt. Express 16 (2008) 11289.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Broadband terahertz imaging of documents written with lead pencils E. Abraham *, A. Younus, A. El Fatimy, J.C. Delagnes, E. Nguéma, P. Mounaix Centre de Physique Moléculaire Optique et Hertzienne, Université de Bordeaux 1, UMR 5798, 351 Cours de la Libération, 33405 Talence Cedex, France
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 16 April 2009 Accepted 16 April 2009
a b s t r a c t Far infrared transmission spectra of several graphite pencil leads on paper have been measured up to 2 THz using time-domain spectroscopy. The observation of the gradual absorption depending on the graphite proportion has been assessed for different pencils from hard to soft black-marking graphite leads. The resulting graphite transmittance is used to perform two-dimensional transmission terahertz imaging of written documents. Ó 2009 Elsevier B.V. All rights reserved.
Between the microwave and infrared frequencies lie now the well-known terahertz frequency radiation which have been successfully employed for both spectroscopy and imaging. Especially, terahertz time-domain spectroscopic (THz-TDS) imaging has become an interesting new tool for non-invasive and non-destructive material testing [1]. For instance, for security analysis or biomedical applications, it is mandatory not only to visualize the samples but also to explore and identify the chemical composition of the test objects [2,3]. For this purpose, THz waves exhibit attractive features such as good penetration depth in certain materials (e.g. paper, wood, plastics, etc.), low scattering, free-space propagation, low photon energy (non-ionizing), good beam coherence and broad spectral bandwidth. Since many substances exhibit spectral fingerprints in the THz region [4], spectroscopic analysis in this region has received much attention as a new tool for material characterization. Furthermore, the availability of coherent optical imaging has made THz imaging an attractive non-contact, non-ionizing method for a variety of applications [5]. Since the first demonstration of THz transmission imaging [6], several promising THz imaging techniques have emerged such as reflection tomography [7], computed tomography [8], near-field imaging [9], cw imaging [10,11], real-time imaging [12] and spectroscopic imaging [13]. In particular, the latter allows discrimination and spatial imaging of the chemical components in a sample on the basis of the THz spectral fingerprints. Consequently, this system offers new opportunities in detection of illicit drugs [14] and explosive [15], biomedical applications [3] and more recently art conservation [16]. For this latter purpose, X-ray fluorescence, UV and near-infrared imaging are commonly used [17,18]. However, in terms of conservation, it is essential to employ non-destructive and non-invasive techniques to investigate art objects including paintings, murals, colored sculptures or furniture. Therefore, THz radiation has been * Corresponding author. E-mail address: [email protected] (E. Abraham). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.04.039
used to reveal murals hidden beneath coats of plaster or paint in century-old buildings [16,19]. As a result, THz imaging makes possible the evaluation of historical artifacts such as frescoes, mural paintings or underdrawings. It could also be an efficient tool to prevent counterfeit art products due to chemical diagnostic capabilities of THz spectroscopic analysis. The present paper reports measurements, performed in the 0.1– 2 THz frequency range, on a set of lead pencils ranging from a hard light-marking pencil to a very soft black-marking one. The THz absorption has been analyzed on paper in order to demonstrate the feasibility of 2D THz imaging of pencil lead with high contrast, sensitivity and good resolution. The broadband THz spectrometer is based on a standard transmission THz-TDS setup. We used an 80 fs Ti–Sapphire laser with a 76 MHz repetition rate. The laser output is split into a pump and a sampling beam. The pump beam is focused onto a bare InAs layer for surface field emission of the THz pulse. After emission, the THz beam is collimated, focused and transmitted through the sample location. The transmitted THz pulse is measured using the current generated in a LT–GaAs semiconductor (photoswitch) receiver triggered by the sampling pulse. This current is amplified and processed with a lock-in digital amplifier. The measured current is directly proportional to the time-varying electric field of the THz radiation. This setup provides a frequency response up to 3.5 THz [20]. In fact, the major advantage of this technique is the direct access to the electric field amplitude of the THz pulse thus avoiding any Kramers–Krönig analysis [21]. Thus the time-domain THz spectrometer allows a direct determination of the complex dielectric response e(x). Before performing transmission THz imaging with this abovementioned experimental apparatus, we evaluated the transverse resolution dmin. Since the imaging system consists of raster scanning the sample through the THz beam, the transverse resolution is directly connected to the diameter of the THz beam at the sample position. Consequently, a tightly focused THz beam at the sample position will provide a high transverse resolution. Taking
3105
E. Abraham et al. / Optics Communications 282 (2009) 3104–3107
k NA
ð1Þ
where k is the wavelength and NA the numerical aperture of the focusing lens. To enhance this parameter, we inserted in the THz pathway two homemade High Density Poly Ethylene (HDPE) lenses, both with a 10 mm focal length and a 10 mm diameter. For imaging, the object is inserted between these two lenses in a confocal configuration. To measure the corresponding transverse resolution of the imaging system, we used a calibrated object. Here, we chose a 1951 USAF Resolution Test Chart printed on regular white sheet paper. The pattern consists of vertical lines with 2.5 lp/mm corresponding to a transverse resolution of 200 lm. With the THz-TDS imaging system, THz images can be reconstructed for all frequencies up to 2 THz. For higher frequencies, the paper absorption starts to limit the signal-to-noise ratio of the detection. At 2 THz, the Fig. 1a shows the corresponding THz image obtained with a 50 lm step size. The three lines (white color) are easily visible indicating that the transverse resolution of the THz imaging system is at least 200 lm at 2 THz. Taking into account Eq. (1), this result is in perfect agreement with the theoretical value of 210 lm assuming a NA of 0.44 for the focusing lens with a 10 mm diameter. Fig. 1b shows the intensity profile of the THz image along the horizontal line of Fig. 1a. Along this line, the contrast of the THz image can be estimated. At 2 THz, for the 200 lm width pattern, we obtained an average contrast of 70%. For lower frequencies, we also calculated the spatial resolution of the imaging system (data not shown). In agreement with Eq. (1), we observed a linear variation of the spatial resolution with the wavelength. For instance, at 1 THz, the lateral resolution naturally decreases to 400 lm. Finally, it can be concluded that the THz imaging system described in this paper is able to provide diffraction limited THz images with an excellent transverse resolution and contrast ratio. Further, we investigated the THz imaging of pencil lead on paper in transmission mode. The writing characteristics of graphite based pencils (black color, hardness, etc.) arise from the ratio of graphite to clay. These pencils are usually graded by manufactures using the letter H for hardness (more clay, less graphite) and the letter B for blackness (less clay, more graphite). Especially, by altering the proportion of graphite to clay, graphite pencils are usually graded by manufactures using the letter H for hardness (more clay, less graphite) and the letter B for blackness In this study, we tested several pencil leads with increasing level of graphite in the lead from the standard writing pencil HB to softer black-marking pencils indicated as 2B, 4B, 6B and 8B. The main purpose of this research work is the assessment and identification of the graphite
reference : white paper
(a)
2B 4B
1
6B 8B
0
-1
-2
2
3
4
5
6
7
1.0
(b)
0.8
0.6
0.4
2B 4B
0.2
6B 8B
0.0 0.1
0.5
1.0
1.5
2.0
frequency (THz) Fig. 2. Terahertz transmission properties of several graphite pencil grades. (a) THz waveforms obtained after transmission through white paper without writing (reference) and white paper recovered by graphite pencils from 2B to 8B; (b) corresponding transmission spectra in the THz region.
100
(b)
80 60 40 20 0
200µm
8
delay time (ps)
intensity (a.u.)
(a) (a)
2
Electric field (a.u.)
dmin ¼ 0:61
pencil lead grade using THz spectroscopic imaging for the future investigation of underdrawings beneath paintings. First, we investigated the THz transmission through a square graphite area (1 1 cm2) drawn with different pencil leads on a regular 80 lm thick white sheet paper. Although it is difficult to evaluate the graphite quantity deposited onto the paper, owing to the hand drawing process, we can estimate that the sample thickness is less than a few micrometers. Moreover, the exact quantity of graphite deposited onto the paper is difficult to evaluate because the pencil lead is made of graphite powder and also various kind of binders. For different pencil grades, Fig. 2a shows the temporal transmitted data through the sample. Although the amplitude of the THz wave can vary depending on the pen pressure
Transmission
into account the well-know Rayleigh criterion, the theoretical transverse resolution dmin can be expressed as:
0
200
400
600
800
1000
1200
distance (µm)
Fig. 1. Image of USAF test pattern, 2.5 lp/mm. (a) THz image at 2 THz; (b) intensity profile along the horizontal line indicated in (a).
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and the graphite thickness, we measured the transmission of the THz wave through the graphite and the paper. To ensure the validity of the method, we repeated several times the measurement taking into account the variations of the pen pressure on the paper during the hand writing process. Finally, the THz signal transmission can be directly related to the graphite absorption since the scattering of the THz radiation is reduced because its wavelength is larger than the typical spatial dimensions of the surface texture. Obviously, it would have been useful to measure the absorption spectra of graphite pellet to avoid the problem of the unknown quantity of graphite deposited onto the paper. However, for future practical applications in art conservation, we also found it interesting to determine at least qualitatively the absorption of graphite on paper. As a reference, we plotted the data corresponding to the blank paper sheet. For the HB pencil (or even harder leads from 2H to 9H), the THz absorption was negligible (data not shown) and the data are similar to the reference. However, from 2B to 8B, we clearly observed a significant attenuation of the transmitted THz beam due to the absorption of the THz radiation. This absorption is mainly due to the presence of graphite in the pencil lead since the concentration of this mineral increase regularly from HB to 8B. It is well known that graphite can absorb or reflect THz radiation due to its high electronic conductivity [22]. A slight temporal shift is also observed which originates from the variations of the optical path due to increasing sample thickness. This behaviour can be directly used as a pertinent parameter for THz imaging as illustrated later in the paper. Fig. 2b represents the corresponding transfer functions of the various pencil leads on paper with respect to frequency up to 2 THz. This corresponds to the transmission spectra of the different graphite leads on paper from 2B to 8B. First, we can notice that no strong spectral difference is visible in the spectra. This may be explain by the fact that the chemical constitution of the different pencil leads is almost the same for all the samples since only the proportion of graphite changes from 2B to 8B. However, the amplitudes of the spectra are clearly different which offers the possibility to identify the lead at least qualitatively. For the different pencils from hard to soft black-marking graphite leads, we identified the gradual absorption of graphite proportion. The transmission spectra extend from 0.1 to 2 THz since the paper starts to absorb strongly above this limit. For example, at 2 THz, the transmission of 2B pencil is 65% and decreases down to 25% for the 8B pencil. These results emphasize that a thin graphite layer deposited by a pencil lead can provide sufficient amplitude and/ or phase contrast in the THz domain. Consequently, through the analysis of the transmission spectra, we can assume that THzTDS imaging allows identifying the quantity of graphite, i.e. the label of the pencil lead, deposited onto the paper. To this aim, we imaged the CNRS logo where each letter has been written with a different lead grade (2B, 4B, 6B and 8B, respectively) and the five oblique lines were made with the harder HB grade. The corresponding visible and THz pictures of the logo are shown in Fig. 3a and b, respectively. The sample area is 20 20 mm2. In the visible image, it is almost impossible to discriminate which pencils have been used to write the individual letters. It means that the visible image does not allow determining the chemical nature of the different letters. To obtain the THz image of the logo, we used a raster scanning system. The image consists of 41 41 pixels with 0.5 mm spacing. For each pixel, it takes about 2 s to obtain the transmitted time-domain waveform with a lock-in time constant of 10 ms. Therefore the essential measurement time takes less than one hour and could be strongly reduced by using a faster optical delay line. After Fourier-transform of the temporal data, we computed images at various frequencies. Fig. 3b represents the image of the CNRS logo at 2 THz. Using a false color grayscale, we can clearly visualize that the letters
Fig. 3. Digital photograph (a) and THz image at 2 THz (b) of the CNRS logo drawn with five different graphite pencil grades. Oblique lines around S: HB, C: 2B, N: 4B, R: 6B, S: 8B.
appear darker and darker due to the increase of graphite content from the C letter to the S letter. However, around the S letter, the five oblique lines written with the HB pencil are hardly visible in Fig. 3b, in accordance with the low absorption of the HB lead in the THz region as already mentioned. Finally, with the THz image at 2 THz, various letters can be separated from each others due to the selective THz absorption of the graphite. The discrimination is easy due to the very high signal-to-noise ratio associated with a coherent detection scheme compared with a cw THz imaging system. As already mentioned, it is also possible to compute THz images not only using the spectral amplitude but also directly using the arrival time of the THz pulse (i.e. temporal phase shift). Such image contains information about the optical path of the sample in the THz range. Fig. 4 represents the 3D THz map of the CNRS logo where the THz main pulse time delay has been plotted. This phase delay map appears different to the previous spectral picture at 2 THz (Fig. 3b). To explain this, we have to take into account that the grayscale variations of the map do not reflect the variations of the sample chemical properties but either the variations of the sample thickness (in micrometer). The accuracy of the depth measurement is related to the measurement of the arrival time of the THz pulse and the acquisition step of the delay line. With a micrometer scan of the delay line, we can estimate that the accuracy of the depth measurement is about 5 lm. This precision is not as good as a previous work of Hills et al. who reported an ultra-high depth resolution of 0.5 lm. However, the authors used a completely different system using a heterodyne detection with a hybrid cw THz source [23]. In Fig. 4, it appears that, similarly for
Fig. 4. THz map of the CNRS logo obtained using the THz main pulse time delay. The scale of the 3D map surface is in lm and represents the paper thickness variations due to the pencil grade writing.
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the drawing with bare eyes (Fig. 5b). Fig. 5c shows the THz image of the sample for 2 THz. In spite of the presence of the black ink, the butterfly is clearly visible with a good contrast and high resolution. However, the fine details at the top of the butterfly wings are hardly visible. However, in Fig. 5d which represents the THz image using the arrival time of the THz pulse (temporal phase), the image quality is excellent and the finest details of the sample can be resolved. As previously, we attribute the high quality of the phase image to the fact that the arrival time of the THz pulse is very sensitive to any thickness variations of the sample. This simple demonstration offers the opportunity to visualize sketches of original paintings beneath layers. Here, in this THz imaging application, we investigated drawings under a layer of ink, in the future we will study drawings under media such as paint. For a better efficiency and utility, a reflection configuration with a larger imaging depth-of-field can be employed to investigate thicker samples. This technique is a promising tool in fields such as art conservation and archaeology where non-invasive imaging and chemical analyses are often required. Acknowledgment
Fig. 5. Butterfly drawn on paper with a 4B graphite pencil. (a) Digital photograph of the sketch; (b) digital photograph of the sketch painted with a black ink felt-tip pen; (c) THz image at 2 THz; (d) THz image using the THz main pulse time delay.
This work is supported by the ‘‘Conseil Régional d’Aquitaine”, in collaboration with the Labri (UMR 5800) and Innovative Imaging Solutions (www.i2s-corp.com I2S).
References all letters, the hand writing process decreases the paper sheet thickness from 80 lm (no inscription on the paper) to about 50 lm (in the region corresponding to an inscription). This 30 lm decrease is simply due to the pressure of the lead on the soft sheet paper. It appears also that the width of the letters seems smaller than in Fig. 3. This may be due to the fact that the phase delay picture in Fig. 4 is very sensitive to any variation in thickness which improves the image resolution. It can also be noticed in Fig. 4 that the five HB oblique lines around the S letter are well visible in the image since the contrast does not depend anymore on the chemical properties of the pencil leads but only on the paper thickness variations. In short, for the study of writing documents on soft materials such as sheet, the phase THz imaging seems to be a very interesting tool to determine the variation of the sample thickness. In that sense, the imaging system works as a kind of non-contact THz profilometer. At least, the results described in Figs. 3 and 4 show that it is possible, from a single THz imaging experiment, to combine spectral and temporal THz analysis in order to improve the effective contrast and reinforce the efficiency of the THz imaging. Finally, we emphasize on the real advantage of the apparatus which could be powerful for the investigation of underdrawings beneath ink layers. For that purpose, it was proposed to use the transmission TDS-THz imaging system to recover buried layer information. The object is a handmade butterfly drawn on standard paper with a 4B pencil lead (size is 15 15 mm2). Fig. 5a represents a standard photograph of the object under study. Before performing THz imaging, the butterfly sketch has been fully recovered with a black ink felt-tip pen so that it is impossible to distinguish
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