Fingerprint detection using full-field swept-source optical coherence tomographySatish Kumar Dubey, Tulsi Anna, Chandra Shakher, and Dalip Singh Mehta Citation: Applied Physics Letters 91, 181106 (2007); doi: 10.1063/1.2800823 View online: http://dx.doi.org/10.1063/1.2800823 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-resolution full-field spatial coherence gated optical tomography using monochromatic light source Appl. Phys. Lett. 103, 103702 (2013); 10.1063/1.4820350 Measurement of surface acoustic wave in soft material using swept-source optical coherence tomography POMA 19, 075054 (2013); 10.1121/1.4800920 Measurement of surface acoustic wave in soft material using swept-source optical coherence tomography J. Acoust. Soc. Am. 133, 3358 (2013); 10.1121/1.4805723 Simultaneous Topography and Tomography of Microstructures using Fullfield Sweptsource Optical CoherenceTomography AIP Conf. Proc. 1236, 155 (2010); 10.1063/1.3426103 Phase-sensitive swept source optical coherence tomography for imaging and quantifying of microbubbles in clearand scattering media J. Appl. Phys. 105, 102040 (2009); 10.1063/1.3116614

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Fingerprint detection using full-field swept-source optical coherencetomography

Satish Kumar Dubey, Tulsi Anna, Chandra Shakher, and Dalip Singh Mehtaa�

Laser Applications and Holography Laboratory, Instrument Design Development Centre, Indian Instituteof Technology Delhi, Hauz Khas, New Delhi 110 016, India

�Received 10 August 2007; accepted 1 October 2007; published online 30 October 2007�

We report the application of full-field swept-source optical coherence tomography for fingerprintdetection. This system consists of a superluminescent diode as broadband light source and anacousto-optic tunable filter as wavelength-tuning device. The conventional optical coherencetomographic system was modified by coating aluminum oxide on one side of the beam splitterwhich is used as reference mirror and fingerprints on the glass slide as object. Low-coherenceinterferometry, nonmechanical scanning, and compactness are the main advantages of the proposedsystem over conventional fingerprint detection techniques. The present technique is noninvasive innature and does not require any physical or chemical processing. © 2007 American Institute ofPhysics. �DOI: 10.1063/1.2800823�

Optical coherence tomography �OCT� is a noninvasive,cross-sectional, three-dimensional imaging modality basedon the principle of low-coherence interferometry.1 Broadly,OCT has been classified into two categories: time-domain�TD�-OCT and Fourier-domain �FD�-OCT.2,3 Mechanicaldepth scan �A scan� of TD-OCT is replaced either by a spec-trograph or by a swept-source system in FD-OCT.4,5 Sampledepth information encoded in fringe frequencies is obtainedby Fourier transform of spectral interferograms. Dependingupon the combination of source and detectors, FD-OCT hasbeen further classified into two categories; spectral-domain�SD�-OCT and swept-source �SS�-OCT.3 In SD-OCT, abroadband source is used and output of interferometer ismeasured with a spectrometer and line-scan camera.4 In SS-OCT, a broadband source is tuned rapidly using tunable fil-ters facilitating longer depth range scan and bettersensitivity.5–8 Recently, SS-OCTs with varying combinationsof wavelength-tuning mechanisms have been reported.7–11

Some of these include intercavity spectral filter consisting ofa diffraction grating and polygon mirror,7 superluminescentdiode �SLD� and a tunable fiber Fabry-Perot filter,8 external-cavity diode laser,9 a Ti:sapphire laser driven by a cw steppermotor,10 and a specially designed acousto-optic tunableelement.11 However, all these SS-OCTs use mechanical scan-ning to tune the wavelength, and their optical setups arecomplicated, bulky, and expensive and hence not feasible forfield studies.

Though having been used in biomedical imaging anddiagnosis, and other disciplines for a decade,2,3 OCT has stillnot been able to catch the attention of researchers in the fieldof forensic science. Fingerprinting is one of the most widelyused methods by forensic scientists for identifying and au-thenticating individuals.12–16 There are two types of finger-prints; exemplar and latent. The former is acquired directlyfrom human fingers using specific fluids or scanners.12 Thelatent fingerprints are left at crime scenes and require greateffort to render them visible. When a finger comes into con-tact with any surface, the impression gets imprinted on it dueto dielectric residue corresponding to the ridges offingerprint.13 In most of the detection techniques, surface

suspected to bear fingerprint undergoes physical or chemicalprocessing to enhance the contrast of impression.14 However,such treatment may change the composition of impressionitself leading to incorrect results. Few optical techniqueshave been developed, which do not require treatment of for-eign materials for lifting fingerprints.15,16 However, in mostof the field conditions, they had to either compromise withpoor quality of fingerprint or reintroduce fluorescent materialas quality enhancer. Fingerprint detection was done recentlyby making use of differences in magnitude of polarizationand reflection from the residue laden fingerprint impressionand the rest of surface bearing it.16 It is purely noninvasiveand does not require any physical or chemical treatment.However, in poorly reflecting conditions, sophisticated andsensitive detection unit is needed. A TD-OCT was used forartificial fingerprint recognition using a low-coherence SLDat 1300 nm.17 However, TD-OCT requires rigorous mechani-cal A and B scans to obtain full-field image.

We propose a noninvasive, nonmechanical scanning,full-field SS-OCT for latent fingerprint detection. The systemincorporates unique combination of SLD and an acousto-optic tunable filter �AOTF� as swept source, a Michelsoninterferometer, and an area detector. To make the systemcompact and handy, conventional OCT interferometer wasmodified by coating aluminum oxide on one side of cubebeam splitter �BS�, which is used as reference mirror �RM�.With the modified version, interference fringes can be ob-tained easily by placing the object on the other side of BS.Detection of very poor quality impressions buried beneathdust layers is also possible using this technique. Since SS-OCT involves interferometry of light reflected from objectand RM, the signal-to-noise ratio �SNR� is high. Main ad-vantages of the proposed system are low-coherence interfer-ometry, nonmechanical scanning, easy for alignment, andcompactness.

In FD-OCT, depth information z, from different layers ofsample, can be retrieved simultaneously from the frequencyof interference fringes without any reference armmodulation.18 If S�r ,k� is spectral density of source, RR andRS are reflectivities of reference and sample arms, respec-tively, then the detected intensity at interferometer output isgiven by19a�Electronic mail: dsmehta@iddc.iitd.ac.in

APPLIED PHYSICS LETTERS 91, 181106 �2007�

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I�r,k� = S�r,k�RR + S�r,k��−�

� �−�

�

��RS�z�RS�z��ei�k��z−z��+��z�−��z����dzdz�

+ 2S�r,k��RR�−�

�

RS�z�cos��kz + ��z���dz , �1�

where r=r�x ,y�, k is wave number, and ��z� is phase shift.The first two terms in Eq. �1� are dc components; one re-flected intensity from RM and second is mutual interferenceof signals from different surfaces within the sample. Thethird term is interference signal between the reference andobject waves. Depth information z of the sample is retrievedfrom the last term by taking the Fourier transform from k toz domain. In the case of complex samples, multiple-peakFourier spectrum is obtained. By selective filtering of peaks,i.e., complete depth information, both amplitude and phaseof desired surface can be determined.

Figure 1 shows the schematic illustration of full-fieldSS-OCT system used for fingerprint detection. Swept-sourcesystem includes a SLD �Superlum Diodes Ltd., Russia� andan AOTF �NEOS Technologies, Inc., USA�. AOTFs aresolid-state electronically tunable optical filters that selectprecise wavelengths by applying appropriate radio frequency�RF� and hence no mechanically moving parts are required.RF applied to AOTF transducer controls the transmittedwavelength. Thus, it provides a fine-tuned quasimonochro-matic light. Further, AOTFs have high speed of operation ofthe order of a few microseconds, large range of tunability�600–1200 nm�, and linear wave-number-RF characteristics.Light emitted by SLD was coupled to the input of AOTFthrough a polarization maintaining single mode optical fiber.Both SLD and AOTF were characterized using a high-resolution spectrometer �Ocean Optics Ltd.�. Figure 2�a�shows the spectral distribution of SLD. It has two spectralcenters at 819.55 and 845.82 nm with spectral full width athalf maximum �FWHM� of 48.38 nm. On applying RF toacousto-optic crystal �TeO2� electronically, output of AOTFwas measured. Figure 2�b� shows tuned spectrum at the out-put of AOTF and Fig. 2�c� reveals a linear relationship be-tween rf and peak wavelength, with experimental pointsshown by solid squares and linear fit by a solid line. Spectrallinewidth �� �FWHM� of frequency-tuned AOTF spectrumremains the same throughout the sweeping width and isfound to be 1.5 nm. Assuming the SLD and AOTF tunedspectrum nearly Gaussian,2 we calculated the coherencelength lc;

lc =4 ln 2

�

�02

��. �2�

Substituting ��=48.38 nm and �0=842.5 nm for SLD inEq. �2�, lc turns out to be 12.954 �m. Axial resolution ofSS-OCT is half of lc and hence is around 6.5 �m in thepresent study. Similarly, lc of tuned spectrum from AOTFwas calculated by putting spectral linewidth ��=1.5 nm and�0=842.5 nm in Eq. �2� which turns out to be 0.418 mm.This is the maximum range of axial direction that can beprobed. The maximum range of axial direction depends uponthe linewidth of tuned spectrum.

The tuned light emerging out of AOTF was collimatedand launched into the modified OCT interferometric system.Object was placed very close to uncoated side of BS. Area ofillumination on the object was 5�5 mm2 and hence no me-chanical A and B scans are required. Two latent fingerprintsamples were designed for the present study. The first samplewas a fingerprint impression taken on a glass substrate usinga white fluid. Second sample was an impression of finger-print sandwiched between a glass substrate and a cover slip.Placing a cover slip over fingerprint impression reduces itsreflectivity significantly. First sample was placed at the inter-ferometric arm and interference fringes were observed. RF toAOTF was tuned sequentially with a constant step of0.1 MHz. A total of 81 interferograms were recorded using acharged coupled device �CCD� detector �Roper Scientific,Inc.� having 1392�1024 pixels with each pixel size of 6.5�6.5 �m2. This covers the spectral sweep width of 0.75�81 nm2, i.e., around 61 nm. Here, 0.75 nm is the peak shiftof tuned spectrum with a constant linewidth of 1.5 nm. Simi-lar procedure was repeated for the second sample. Figure3�a� shows the example of an interferogram for the firstsample recorded at RF of 91 MHz. Interferograms for theentire tuned spectrum were recorded and stacked togetheralong the wavelength axis. Fast Fourier transform �FFT� ofinterferograms was computed, which provides multiplepeaks corresponding to different layers of the object,20 asshown in Fig. 3�b�. Inverse FFT of the first order peak �peak1� yields the brightness distribution of fingerprint and glasssubstrate interface, as shown in Fig. 3�d�. To reconstruct thefingerprint, second order peak �peak 2� of Fig. 3�b� was se-lected using a rectangular filter and its inverse FFT was com-

FIG. 1. Schematic diagram of full-field swept-source optical coherence to-mographic system for detecting fingerprint impression.

FIG. 2. Source characterizations: �a� spectral distribution of SLD, �b� tunedspectrum of SLD using AOTF at a particular RF, and �c� RF vs peak wave-length of tuned spectrum of AOTF.

181106-2 Dubey et al. Appl. Phys. Lett. 91, 181106 �2007�

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puted. Figure 3�c� shows reconstructed OCT image of finger-print. A photograph of fingerprint sample, as shown in Fig.3�e�, was taken using a high-resolution digital camera�8 megapixels� under high brightness lighting condition.Similarity was observed between latent fingerprints recon-structed using SS-OCT �Fig. 3�c�� and direct image �Fig.3�e�� of the sample.

Interferograms obtained using second sample were alsoanalyzed. Figure 4�a� shows the example of an interfrogramtaken at 91 MHz and Fig. 4�b� its FFT. It can be seen fromFig. 4�a� that the interference fringes are multiplexed due tomultiple reflections from cover slip, fingerprint, and glasssubstrate. Inverse FFT of peak 1 gives brightness distributionof cover slip, i.e., top surface of the sample, as shown in Fig.4�c�. Fingerprint information is retrieved from selective fil-tering of second peak �Peak 2�, as shown in Fig. 4�d�. Figure4�e� is the image directly taken using digital camera. Reso-

lution of the detected fingerprints using present system wasfound to be better than that obtained from a high-resolutiondigital camera. This validates the superiority of present sys-tem over other conventional techniques used for fingerprintdetection. Although, total power of SLD is 7.5 mW, powerof wavelength tuned signal from AOTF onto the sample is ofthe order of a few microwatts. Even in such a low powerconditions, we were able to detect interference fringe signal.This is possible because in OCT, light reflected from RM issignificant and interference does take place even though thereflection from the sample is poor. This is one of the biggestadvantages of the present system over conventional finger-print detection techniques.

In summary, we have demonstrated the application offull-field SS-OCT in forensic science. Latent fingerprintswere detected even from very poorly reflecting samples thatwere unnoticed under ordinary viewing conditions. The OCTimages of latent fingerprints were reconstructed without anyphysical and chemical processing of the sample. Since thetechnique involves low-coherence interferometry, images re-constructed were observed to have better SNR and higheraxial resolution. Unlike the conventional techniques, un-wanted reflections were rejected by means of selective filter-ing of Fourier components.

Authors gratefully acknowledge the financial assistancefrom Department of Science and Technology, Delhi, Govern-ment of India for the Project No. SR/S2/LOP-02/2003.

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FIG. 3. Data for first sample: �a� an example of interferogram at 91 MHz,�b� fast Fourier transform of time-varying interference fringe signal, �c�reconstructed OCT image of the fingerprint filtered at second order peak, �d�reconstructed OCT image of the sample filtered at first order peak, and �e�image of fingerprint impression taken with high-resolution digital camera.

FIG. 4. Data for second sample: �a� an example of interferogram at91 MHz, �b� fast Fourier transform of time-varying interference fringe sig-nal, �c� reconstructed OCT image of the sample filtered at first order peak,�d� reconstructed OCT image of the sample filtered at second order peak,and �e� image of fingerprint impression taken with high-resolution digitalcamera.

181106-3 Dubey et al. Appl. Phys. Lett. 91, 181106 �2007�

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