Illumination Aspects in Active Terahertz Imaging

2008 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 58, NO. 7, JULY 2010

Illumination Aspects in Active Terahertz ImagingWolff von Spiegel, Christian am Weg, Ralf Henneberger, Ralph Zimmermann, and Hartmut G. Roskos

Abstract—Our paper focuses on illumination aspects in ac-tive terahertz imaging. First, we introduce our fast 620-GHzcamera and discuss imaging with focused illumination. Afterthat, we switch to global illumination and discuss its benefits anddrawbacks. The loss in intensity—compared to focused illumina-tion—does not fully come to bear on the dynamic range becauselosses at the beam-splitter and back-reflection by the optics intothe detection path can be avoided. The adaptation to the angularrange accepted by the detector optics is more difficult, but flexi-bility is gained with separated illumination and detection paths.An outlook is given on a 812-GHz camera currently being devel-oped and exhibiting global illumination and a 32-pixel detector forimproved frame rates. Furthermore, a concept for high-accuracytopography reconstruction from multiple terahertz images takenwith several angles of illumination is discussed.

Index Terms—Active terahertz imaging, global illumination,spurious reflections, topography reconstruction.

I. INTRODUCTION

I LLUMINATION is a very important aspect of imaging. Inthe terahertz regime, we distinguish two kinds of imaging

systems: passive and active ones. The passive systems acquireradiation, which is either emitted by the object itself, or by someother natural source and reflected by the object. Active systemsutilize terahertz sources for illumination, and acquire the signaltransferred to the detector by reflection or scattering from theobject. Several materials even provide spectroscopic features inthe terahertz range [1]. In this paper, we only address coherentactive systems, which can not only measure the power of thesignal, but also its phase [2], [3].

II. TERAHERTZ-IMAGING SYSTEM

First, we introduce our fully electronic imaging system. Fig. 1shows the corresponding diagram. The radiation (with a powerof about 1 mW) emitted by the 620-GHz source S1 is collimatedby the lens L1. The part transmitted by the mylar beam-splitterBS passes the center hole in the primary mirror (M1,

cm, cm) and is expanded by the secondary mirror(M2, cm, cm) onto the aperture of the pri-mary mirror, which performs the focusing into the object plane.

Manuscript received November 04, 2009; accepted March 12, 2010. Date ofpublication June 03, 2010; date of current version July 14, 2010. This workwas supported by the Bundesministerium für Bildung und Forschung (BMBF),Project LYNKEUS and Project TERA-CAM, under Contract 16SV2308 andContract 13N9300–13N9304.

W. von Spiegel., C. am Weg, and H. G. Roskos are with the PhysikalischesInstitut, Johann Wolfgang Goethe-Universität, 60438 Frankfurt am Main, Ger-many (e-mail: [email protected]).

R. Henneberger and R. Zimmermann are with RPG Radiometer PhysicsGmbH, 53340 Meckenheim, Germany.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMTT.2010.2050247

Fig. 1. Diagram of the fast active 620-GHz camera configured for focused il-lumination. The illumination and the detection beams share most of the path.

The planar scanning mirror (M3, cm), rotating with660 r/min, moves the focus spot within the object plane. Its tiltby 5 with respect to its fast spinning axis leads to an ellipticallyshaped scanning path in the object plane. An additional slow(10 in 9 s) rotation about a perpendicular axis translates the el-lipse horizontally. The combination of both movements allowsfor a field of several dm (depending on the working distance,typically 0.5–1 m), covered by the scanning process. The radia-tion scattered or reflected by the object point in the focus returnsvia the mirrors M3, M1, and M2 through the hole in M1 and isreflected by the beam-splitter to the lens L2, which focuses itinto the detector . The effective diameter of the PTFE-lensesis 4.5 cm, they—as well as the mirrors—are in-house-manu-factured using a common CNC turning machine. Their surfacequalities are not suitable for the visible, but fine for the 0.5-mmwavelength. M1 and M2 are slightly polished by hand, to reducethe scattering of the red laser beam we use for some adjustments.

The diagram in Fig. 2 illustrates the RF electronics. Thesource, as well as the detector, are each driven by a synthesizerwith 1/36 of their terahertz frequency. The detector synthesizeris tuned to a frequency that is 5 MHz higher than that of thesource synthesizer. Due to the effective frequency multipli-cation factor of 36 in both components, the resulting IF forheterodyne detection is 180 MHz. In order to provide a refer-ence signal for demodulation of the IF signal using a lock-inamplifier, the 5-MHz difference frequency of the synthesizersis generated with a mixer, and fed into a 36-times frequencymultiplier. After demodulation, the amplitude and phase dataare acquired together with the angular data from the motioncontrollers, and processed for image generation. The source,as well as the detector, are products of Radiometer Physics.

0018-9480/$26.00 © 2010 IEEE

VON SPIEGEL et al.: ILLUMINATION ASPECTS IN ACTIVE TERAHERTZ IMAGING 2009

Fig. 2. Diagram of the RF-electronics of the 620-GHz camera.

Fig. 3. 620-GHz single-shot image of a revolver. The detected IF-power (indBm) is encoded in the grey scale shown on the right side. The same scale isalso valid for Figs. 5 and 6.

The output power of the source reaches up to 1.1 mW, theconversion loss of the sub-harmonic mixing detector is nearlyequal to the amplification of the IF signal (both about 40 dB).Therefore the IF power detected by the lock-in amplifier isclose to the detected terahertz power. If the full source poweris directly coupled into the detector, a dynamic range of morethan 100 dB can be reached with an integration time of 10 ms.A more detailed description of the camera system is given in[4].

III. RESULTS WITH FOCUSED ILLUMINATION

This setup allows us to acquire a terahertz image in a com-puter-controlled scanning process with a measurement time of9 s. The lock-in integration time is 100 s. Fig. 3 shows the tera-hertz image of a handgun. One can distinguish several regions inthe image: The brightest region (A) shows specular reflectionsfrom the metal part, which was angle adjusted for maximumsignal from a selected spot. Region B corresponds to the signalscattered by the grip. It is orders of magnitude weaker than thespecular signal, but is extraordinarily important for the identifi-cation of the shape of the object under test. As we have shownin [5], the observation of scattered signals usually requires a dy-namic range of the measurement system of at least 30 dB. Re-gion C shows artifacts that probably come from spurious mul-tiple reflections returning from somewhere in the room, respec-tively, the handgun or the optical components, through the op-tical system into the detector. The artifacts are related to a ghostimage or lens flare in photography. The signal in region D cor-responds to the background signal from the residual reflection

Fig. 4. Illustration of geometrical aspects in specular imaging.

at the center of the secondary mirror (although the noise limitdetermined by the dynamic range of the lock-in amplifier in theselected sensitivity range is not far away). The dynamic range isfound to be about 40 dB. It is limited by reflection of radiationfrom mirror M2 directly back into the detector. We spent con-siderable effort to minimize the degree of reflection by drilling ahole into the center of the mirror, by adding conical metal pieces,etc. The best solution (and the one applied in this system) turnedout the be to cover the central part of the mirror with fibers froma coco mat [6].

While looking at Fig. 3, one notices that the barrel of thegun is not visible in this image. The radiation reflected fromit misses the cone of acceptance of the imaging system. Thereason is illustrated in Fig. 4. If the focused terahertz beam hitsa specularly reflecting spot of the object with a surface orienta-tion, which reflects the power back into the optics [such as inFig. 4(a)], a bright signal is received. In Fig. 4(b), the angle ofthe scanning mirror has changed, another point of the object isilluminated, and now the reflected radiation fails to return intothe optics because the surface orientation does not fit to the angleof incidence. This is the situation for the barrel and the reasonfor it being invisible. If we change the orientation of the object,as shown in Fig. 4(c), the formerly dark object point becomesbright because now the surface orientation fits to the angle ofincidence.

Fig. 5 shows a terahertz image of the handgun, which wasrotated by a few degrees. Now the barrel is clearly visible, butparts of the frame are not. The signal scattered from the grippanel is visible for both object orientations because the power isdistributed over a wider angular range in this case. Fig. 6 showsa composition of the two terahertz images with the full revolverbeing visible.


Fig. 5. 620-GHz single-shot image of a revolver taken at a slightly differentangle than in the image of Fig. 3 (upper panel) and photograph of the revolver(lower panel).

Fig. 6. 620-GHz 2-image composite of the left part of Fig. 3 and the right partof Fig. 5.

Rotating the object is not a matter of choice for many applica-tions. Focused illumination does not allow for changing the il-lumination angle independently from the detection angle easily.

IV. ADAPTATION TO GLOBAL ILLUMINATION

With global illumination of the scene under investigation,significant degrees of freedom are gained. Fig. 7 illustratesthe changes in the setup. The terahertz power emitted by thesource is spread over the illuminated area, the intensity on theobject surface, and therefore, the detected power decrease withgrowing illuminated scenes.1 This can also affect the contrastto spurious reflections. On the other hand, the direct reflectionof illumination radiation into the detector by the secondarymirror M2, as well as the losses due to the beam-splitter BS areavoided.

1For convenience in the actual experiment, the source and detector are ex-changed so the illumination is focused and scanned, while the detector receivesits signal from a wider angle. This has no significant influence on the imaging.For easier reading and understanding, the text and figures describe the experi-ment in a scanned-detector configuration (shown in Fig. 7).

Fig. 7. Diagram of the fast active 620-GHz camera configured for global illu-mination. The illumination and the detection paths are now separated.

The freedom to place the source(s) allows to illuminate theobject from various angles. Here, we show an example, whichtakes advantage of this freedom for panning-mode imaging. Asequence of 15 separate images was acquired without changingthe scanner or object position, but each image was taken with adifferent source position (see Fig. 7). The source was mountedat a fixed position on a rail, which was attached to a post closeto the object. The rail was rotated to different angles, while thesource kept on emitting toward the rotation axis. We turned therail by hand step-wise and covered an angle of about 45 . Fig. 8shows several single-shot images from that sequence. The ef-fects of spurious reflections are strongly visible and the fieldof view is rather limited. After the measurement, we took theincoherent average of the single images, simulating the illumi-nation with several independent sources at different positions.The result is shown in the upper panel of Fig. 9. The averagingreduced the influence of the spurious reflections, increased thesize of the field of view, and considerably helps to recognize theobject. The covered field of view is similar, but the suppressionof the spurious-reflection effects is weaker. To illustrate the in-fluence of the number of single shots, a second image generatedfrom the average of only the seven shots with even numbers isshown in the lower panel of Fig. 9.

Fig. 10 allows for a more quantitative analysis. The graphsshow plots of a line of pixels2 marked with a white line in the

2Note, that our measurements do not directly yield a Cartesian array of pixels,but a sequence of detected-power values on the scanning path. Hence, the datashown in Fig. 10 took several steps of processing: At first, the IF-power valuesdetected at points on the spiral-shaped scanning path were binned. About 50 000data points are taken initially, leaving about 10 000 points after binning. Thesewere then connected with a triangular mesh. Each triangle was drawn on a Carte-sian pixel screen and filled with a gradient, which interpolated the gray valuescorresponding to the power values at the triangles’ edges. With this transforma-tion to a pixel image, the values were quantized to 256 steps (8 bit). The pixelvalues from the extracted line were then converted back to power values andplotted.


Fig. 8. Single indiscriminate 620-GHz global-illumination images of the re-volver.

Fig. 9. Average of 15 (upper panel) and seven (lower panel) global-illuminationimages of the revolver, four of which are shown in Fig. 8. The white line in theupper image marks the position, from which the data shown in Fig. 10 was taken.

upper panel of Fig. 9 and the corresponding data from the lowerleft single image in Fig. 8.

The comparison between single- and the averaged-image datashows that the averaging efficiently reduces the power fluctua-tions caused by interference involving the spurious reflections(roughly from 10 to 3 dB). When the bright specular region ofthe averaged image is compared to the dark background, theresult is a dynamic range of slightly more than 15 dB. If wecompare this value with the one from the carefully adjustedimage with focused illumination in Fig. 3 (up to 40-dB dynamic

Fig. 10. Plot of the marked line in Fig. 9 (full) and the corresponding line fromthe lower left single image (dashed).

Fig. 11. 3D-CAD drawing of the 812-GHz camera.

range relative to the background level, or 25 dB relative to thestrongest signal from spurious reflections), we see a loss in dy-namic range of at least 10 dB. In the focused image, we can alsorecognize that the scattered signal from the grip panel is roughly30 dB weaker than the specular one from the metal frame. Underthese conditions, one cannot expect the grip panel to be visible inthe averaged image acquired with global illumination. Althoughthe detected power of the spurious reflections was higher in thefocused case (see region C of Fig. 3), the contrast to the spec-ular signal power is higher there because of the higher intensitydue to focusing onto the object. Hence, the loss of contrast inthe global-illumination case results from the much lower powerreturning specularly from the object.

V. OUTLOOK

Global illumination also allows for utilizing detector arrays toincrease the acquisition speed in terahertz imaging. Developingthe technology towards real-time operation is an important goalin this field [7], [8] because it could be the key to the successof many promising applications, which have been demonstrated[9], [10]. Our work on a 32-detector-pixel camera operating at812 GHz is in progress (Fig. 11 shows a computer-aided design(CAD) drawing of the focus-adjustable scanning telescope; anapproach for a similar scanning telescope is described in [11]).It will operate with single-axis rotational scanning and cover a


Fig. 12. Illustration of ambiguity reduction by utilizing multiple sources.

field of view of about 50 dm for a working distance of about4 m. The target is to achieve a rate of 10 frames/s.

In this paper, only the power of the detected signal has beentaken into account thus far, although the systems presentedmeasure coherently. In this paragraph, we address the phase ofthe terahertz signal, which can be used for topography recon-struction of objects. Acquiring terahertz images with differentsource positions does not only help to improve the quality ofthe resulting image, but may permit to solve the ambiguityproblem encountered with topography reconstruction fromterahertz-phase images. These provide information on theoptical path length with high resolution [12], but with theambiguity known from interferometry. Fig. 12 illustrates thisaspect: the lateral position of the point under investigation isgiven by 2-D imaging. The -coordinate of the object pointshall be determined by the detected terahertz phase. If we usethe phase data acquired only with source S2, every intersectionof the green arcs (in online version)—representing surfaces ofequal phase—with the -axis (going through the object point)is a possible solution. Similarly, the phase data acquired withsource S3 provides intersections of the -axis with red arcs (inonline version) as possible solutions. For the real position onthe -axis, both measurements have to agree. This is only thecase for two points (A, B) in the figure. Hence, the range ofuniqueness is significantly enlarged (in the illustration by thefactor of 4 in comparison to the S3 measurement or 5 relative tothe S2 measurement). Imagine a third (maybe blue) set of arcsfrom a third source. One arc would intersect with the true pointA again, but it is unlikely that a blue arc would also intersectwith point B so it would help to avoid selecting “wrong” pointssuch as B. Since the unambiguous range resulting from a com-bination of the physical wavelengths varies in space, the rangeof uniqueness changes with the point under investigation.

To demonstrate this concept, we realized an algorithm thatreconstructs the topography of an object from simulated phasedata assuming up to three sources, and demonstrate the suc-cessful reconstruction of an object’s surface. In the simplifiedmodel, the phase information is only calculated from the wave-length modulus of the distance between a source and the pointunder investigation. In the reconstruction, we then treat the in-verse problem, taking the phase data as given and using themto reconstruct the object. The modulo-operation on the phase

Fig. 13. Topography reconstruction from simulated data with three source po-sitions. Precise knowledge of the source positions (left panel), one source dis-placed by 1 mm (right panel). The color-bar encodes the reconstructed �-co-ordinate. The radius of the hemisphere amounts to approximately 100 wave-lengths.

brings in the ambiguity problems. Note that the simplified modeldoes not treat the detection situation displayed in Fig. 12.

The algorithm requires the precise knowledge of the sourcepositions. Fig. 13 shows the topography of a hemisphere, whichwas reconstructed from simulated phase data assuming threedifferent source positions. In the left panel, the exact sourcepositions were put into the algorithm, while in the right panel,the position data of one source was changed by 1 mm. In thatcase, the reconstruction fails for most pixels (every pixel is re-constructed independently without any information from otherpixels). An extended version of the algorithm could be able toself-adjust the information on the sources’ positions from thephase data. In the simulations, we have assumed to detect asignal with a meaningful phase from each source for every ob-ject point. In real measurements, not every object point may pro-vide a useful signal because the specular reflection may miss thedetection optics.

VI. CONCLUSION

We have reported on terahertz imaging with a single-pixeltwo-axis rotational scanner that allows to acquire an image in9 s. In order to achieve frame rates close to or at real time, mul-tipixel detectors are needed and global illumination of the fullscene or parts of it are necessary. To prepare for this imagingmodality, we studied global illumination with the present single-pixel imager. We found that the loss of dynamic range is notas severe as anticipated from the intensity reduction because,in contrast to collinear focused illumination, direct reflectionsfrom the center of the secondary mirror into the detection unitand the losses caused by the beam-splitter are avoided. On theother hand, less tolerance of the imaging system is observedwith respect to the angular position of the reflecting surfacesof the object; multidirectional illumination may be required tocover a full scene. Furthermore, coherent ghost image forma-tion is more significant than for focused illumination. In orderto suppress the ghosts, one may resort to frequency-modulatedcontinuous-wave (FMCW) techniques or imaging with a re-duced coherence length of the terahertz radiation. In this paper,we showed that panning-mode illumination also helps to reducethe effects of spurious reflections. The freedom gained by sepa-rating the illumination from the detection path can also help toincrease the performance of terahertz imaging applications. Theambiguity reduction addressed in the outlook—reminiscent ofsynthetic-aperture methods—could be one example.


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Wolff von Spiegel studied physics at the TechnischeUniversität Darmstadt, Darmstadt, Germany. HisDiploma thesis concerned laser-beam writing forcontinuously profiled diffractive optical elements.He received the Ph.D. degree from the TechnischeUniversität Darmstadt, in 2007. His doctoral researchconcerned holographic projection screens.

In 2007, he joined the Terahertz-Physics Group,Institute of Physics, Johann Wolfgang Goethe-Uni-versität, Frankfurt am Main, Germany. His currentresearch involves terahertz imaging and terahertz

metrology.

Christian am Weg received the Diploma degree inphysics from the Technische Universität Darmstadt,Darmstadt, Germany, in 2006, and is currentlyworking toward the Ph.D. degree at the JohannWolfgang Goethe-Universität, Frankfurt am Main,Germany. His diploma thesis concerned adaptivelens arrays for 3-D display, during which time heinvestigated improvements in picture quality for3-Ddisplay systems (such as integral imaging) due toself-manufactured negative lens arrays filled withliquid crystals.

His current research interests are the simulation, optimization, and realizationof quasi-optical systems for active real-time terahertz cameras.

Ralf Henneberger received the Master degreein physics from the University of Bonn, Bonn,Germany, in 1988. His Master degree researchconcerned avalanche photo diodes for scintillatingcrystal readout.

In 1995, he joined Radiometer Physics GmbH,Meckenheim, Germany, as a System Engineer andQuality Assurance (QA) Manager. He possessesexperience with the 3-D simulation of waveguidemultipliers based on planar Schottky diodes, thedesign, fabrication, and testing of multiplier chains

for the local oscillator (LO) subsystem of the Herschel/Planck satellite, as wellas the design and development of cryogenic transmitter/receiver systems. He isalso involved with QA/product assurance (PA) management for space projects.

Ralph Zimmermann, photograph and biography not available at time of pub-lication.

Hartmut G. Roskos received the Ph.D. degreefrom the Technical University of Munich, Munich,Germany, in 1989, and the Habilitation degree fromRWTH Aachen, Aachen, Germany, in 1996. Hisdoctoral thesis concerned femtosecond spectroscopyin solid-state physics. His Habilitation thesis con-cerned coherent phenomena in solid-state physicsinvestigated by terahertz spectroscopy.

He is currently a Professor of physics with JohannWolfgang Goethe-Universität, Frankfurt am Main,Germany. He studied physics at the Technical Uni-

versity of Karlsruhe and Technical University of Munich. Following earning thedoctoral degree, he spent two-and-a-half years with AT&T Bell Laboratories,Holmdel, NJ. At that time, terahertz phenomena became a new focus of hisresearch. He later started a research group at the Institute of SemiconductorElectronics, RWTH Aachen. In 1997, he joined Johann Wolfgang Goethe-Uni-versität, as a Full Professor. The central themes of his group’s research areterahertz physics and photonics and the time-resolved optical spectroscopyof inorganic semiconductors and organic compounds. In 2005, he spent asabbatical leave with the University of California at Santa Barbara, where hisresearch concerned a Bloch-gain terahertz laser. In 2009, OC Oerlikon AGawarded his group a five-year endowed professorship for terahertz photonics.He was a Guest Professor position with the Osaka University’s Institute ofLaser Engineering (winter semester 2009/2010).

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Illumination Aspects in Active Terahertz Imaging