Optics and Photonics Journal, 2011, 1, 91-96
doi:10.4236/opj.2011.13015 Published Online September 2011 (http://www.SciRP.org/journal/opj)
Anti-Spoof Reliable Biometry of Fingerprints Using
En-Face Optical Coher ence Tomography
Mohammad-Reza Nasiri-Avanaki1,2, Alexander Meadway1, Adria n Bradu1,
Rohollah Mazrae Khoshki3, Ali Hojjatoleslami2, Adrian Gh. Podoleanu1
1Applied Optics Group (AOG), School of Physical Scienc es, University of Kent, Canterbury, UK
2Research and Development Centre, School of Biosciences, University of Kent, Canterbury, UK
3Electronic Engineering Department, RAZI University, Kermanshah, Iran
Received May 22, 2011; revised June 20, 2011; accepted July 5, 2011
Abstract
Optical coherence tomography (OCT) is a relatively new imaging technology which can produce high-reso-
lution images of three-dimensional structures. OCT has been mainly used for medical applications such as
for ophthalmology and dermatology. In this study we demonstrate its capability in providing much more re-
liable biometry identification of fingerprints than conventional methods. We prove that OCT can serve se-
cure control of genuine fingerprints as it can detect if extra layers are placed above the finger. This can pre-
vent with a high probability, intruders to a secure area trying to foul standard systems based on imaging the
finger surface. En-Face OCT method is employed and recommended for its capability of providing not only
the axial succession of layers in depth, but the en-face image that allows the traditional pattern identification.
Another reason for using such OCT technology is that it is compatible with dynamic focus and therefore can
provide enhanced transversal resolution and sensitivity. Two En-Face OCT systems are used to evaluate the
need for high resolution and conclusions are drawn in terms of the most potential commercial route to ex-
ploitation.
Keywords: Optical Coherence Tomography, En-Face OCT, Fingerprints, Biometry, High Resolution
1. Introduction
Optical coherence tomography (OCT) is an advanced
high resolution, non-invasive imaging tool to image the
internal structure of skin. OCT can deliver three-dimen-
sional (3D) images from the microstructure compart-
ments within the skin tissue [1,2]. OCT images are pro-
duced by measuring the backscattered light from differ-
ent depths within the tissue.
A commonly employed modality is time domain OCT
(TD-OCT) where depth is scanned by adjusting the opti-
cal path length in the reference arm of an interferometer
using a movable mirror, as illustrated in Figure 1(a).
When the optical path lengths of the light returned from
the sample and the light in the reference arm match,
modulation of the detected signal takes place. The signal
produced from a reflector is a series of fringes contained
within a Gaussian envelope, the width of which defines
the depth resolution of the system. Cross section images
can be obtained, as shown in Figure 1(b) and hence 3D
volumes can be generated from them.
The amplitude and phase of the back scattered signal
obtained from the photodetector are determined based on
the interaction of skin compartments with light. From the
interference theory, the photodetected signal is:
0
0
2π
2cos 2
sr sr
P
ik x
 



 

 




 (1)
where is the photodetector responsivity,
k
s

the
sample reflectivity, r

the reference mirror reflectivity,
0

the central wavelength of the optical source and 0
the power incident on the object. In terms of the product
of depth resolution and penetration depth, OCT fills the
gap between confocal microscopy and ultrasound imag-
ing. This product is approximately 0.1 µm × 500 µm in
confocal microscopy, 1 µm × 3000 µm in OCT and 50
µm × 5000 µm in high frequency ultrasound [1].
P
Traditional biometrics technologies used for finger-
prints are based on imaging external features of fingertip

M.-R. NASIRI-AVANAKI ET AL. 92
(a)
(b)
Figure 1. (a) Simplest setup of OCT based on a Michelson
interferometer, (b) OCT cross section images (B-scans) from
skin fingertip.
skin [3]. This allows criminals to easily fool and tamper
with devices based on traditional methods, by distorting,
modifying or counterfeiting the superficial features. For
instance, fingerprints can be copied on thin layers at-
tached to the finger or artificial dummies are used. The
images produced with conventional cameras represent a
2D map of the object surface and therefore cannot make
any difference between a genuine finger and a dummy.
Conventional means do not provide any information
from inside the finger. This is the aspect which OCT
addresses uniquely. The extra depth information pro-
vided by OCT has not been translated yet to comer-
cially available fingerprint imaging systems.
OCT has recently been proven as a versatile tool in
biometrics [4-6] that can provide enhanced reliability in
comparison to conventional imaging methods. Conven-
tional biometrics relies on digital cameras to identify su-
perficial features [7]. With OCT the internal and external
features can be extracted from skin, and these can be
used to prevent cheating by those intending to use faked
fingerprints. In this paper we demonstrate that OCT is
capable of producing detailed high resolution cross sec-
tions of unusual layer structures of faked fingerprints. In
this way, OCT can disclose if any additional material has
been added to the skin. Then en-face OCT images can be
produced to read the patterns like any digital camera in
conventional border control imagers. Additional infor-
mation can also be provided such as liveness of the fin-
ger or the stress of the subject. The latter two possibili-
ties are not demonstrated here, but they could be imple-
mented based on the recent progress of OCT technology,
and these will be discussed in the Conclusions.
2. Imaging Technology Developed in the
Applied Optics Group
Recently, spectral domain OCT [8] has been proven as a
fast imaging technique. However, spectral OCT has dis-
advantages in terms of high transversal resolution over
extended axial depths. Therefore, time domain OCT is
better suited for the current application. Different from
the conventional time domain OCT, a special en-face
(eF)-OCT imaging technology is used, developed in the
Applied Optics Group. This allows real time images to
be produced in sequential regimes, en-face (C-scans) and
cross sections (B-scans). Two time domain en-face OCT
systems were used to quantify the parameters of an OCT
reader machine to image fingerprints, as explained below.
These two systems have been further improved for the
task of imaging fingerprints, by using different methods
to enhance the resolution.
The (eF) OCT method creates images by employing
transversal priority scanning (T-scans) [9]. In the eF-OCT,
images are generated from many T-scans. These are
transversal profiles of reflectivity generated by scanning
the optical beam transversally, along different trajecto-
ries (raster, helicoidal, etc). eF-OCT offers certain ad-
vantages. First, T-scans can be used to generate cross
section images (B-scans) as well as coronal plane ori-
ented scans (C-scans) [10]. This allows eF-OCT tech-
nology to perform quick alternating imaging sessions in
orthogonal planes by instantly switched scanning re-
gimes [11]. Second, the technology is compatible with
live and simultaneous generation of a conventional fun-
dus image (coronal-plane oriented) [12]. Third, the eF-
OCT is ideal for dynamic focus. While in general, TD-
OCT is compatible with dynamic focus, it is hard to im-
plement dynamic focus in a traditional OCT system,
based on A-scans (axial reflectivity profile). This is be-
cause the focus adjustment needs to be synchronized
with the A-scanning, which is fast and determined by the
line scanning rate. In contrast, with eF-OCT, the demand
for the focus adjustment is relaxed, as the focus adjust-
ment needs to be performed at the frame rate, which is of
much lower frequency rate. The importance of dynamic
focus is expected to become more important in the near
future as progress in the adaptive optics (AO) assisted
OCT accelerates. When the AO loop manages to achieve
ideal correction, A-scan based OCT methods would be
limited to collecting signals from a much reduced depth

M.-R. NASIRI-AVANAKI ET AL.93
range. Therefore, eF-OCT turns out to be a better choice
for combination with AO, than any other OCT methods.
3. Detailed Description of the eF-OCT
Imaging Systems Used
3.1. eF-OCT with Dynamic Focus
The first system presented uses a dynamic focus scheme,
recently assembled in the Applied Optics Group. With
the dynamic focus scheme, the coherence gate is syn-
chronous with the focus point determined by the inter-
face optics, hence the transversal resolution is conserved
throughout the depth and enhanced signal is returned
from all depths. Therefore, images with higher resolution
are collected than using standard OCT. The optical
components of such a system are shown in Figure 2. A
dynamic focus OCT system is especially useful in appli-
cations requiring large depth scanning as well as high
lateral resolution. Such a system produces C-scan and
B-scan images with a lateral image size of up to 5 mm,
with transversal resolution better than 5 microns. The
system operates at 1300 nm and uses an SLD, bandwidth
55 nm, which determines a depth resolution in tissue
better than 14 microns.
3.2. Aberration Corrected eF-OCT System
Using Adaptive Optics
The second system uses adaptive optics to eliminate or
Figure 2. Dynamic focus time domain OCT optical setup.
SLD: superluminescent laser diode, PD: photodiode, C1: 2
× 2 coupler, OF: single mode fibre; PC: personal computer,
BD: balance detection, CL: Collimator lens, MPC: Mirror
positioning controller, MC: Motion controller, PC1, PC2:
polarization controllers.
Figure 3. Dual channel OCT/microscope with closed loop
adaptive optics.
reduce the aberrations in the system. This system can
provide higher resolutions overall. Here, enhanced trans-
versal resolution is targeted. Therefore, the lateral image
size is smaller than in the previous system, up to 1 mm.
As shown in Figure 3, light from the source (824 nm
central wavelength, 20 nm bandwidth) is split by a beam
splitter into the reference and sample arms. Light in the
sample arm is incident on the sample and returned to-
wards two imaging channels. The first channel produces
a confocal image of the sample, used as a guide for the
C-scan OCT images which are produced by a second
channel. A Shack Hartman wavefront sensor (WFS) is
used to sense the aberrations. The signal from the WFS is
processed on a computer which calculates the required
shape of the deformable mirror (DM) to correct for the
aberrations, in closed loop, optimising the resolution of
the system and increasing the intensity of the returned
signal. Measurements have shown that transversal reso-
lution below 3 microns can be obtained while the sample
is positioned at an axial distance larger than 1 cm (at
such large gaps between the sample object and the mi-
croscope objective, much worse resolutions are achiev-
able in micrcopy).
4. Results and Discussion
We imaged the fingertip of a volunteer with and without
a piece of sellotape on it. Intruders may use thin layers to
translate the genuine pattern onto their finger. Such sup-
porting layers can be made from sellotape, wax, gel or
any other material.
Our OCT system can produce two types of images,
and both are useful for the purpose investigated here:
B-scan or cross section images and C-scan or en-face
(microscopy orientation). In Figure 4(a) and (b), C-scan
OCT images of a fingertip are collected with and without

M.-R. NASIRI-AVANAKI ET AL. 94
(a) (b)
Figure 4. C-scynamic focus
llotape on it, respectively. These images are sampled
eF-OCT sys-
te
d Z-depth. The sellotape layer could not be
se
in more detailed information, the second sys-
te
depth profiles obtained
fr
an images collected with the d
TD-OCT. (a) without sellotape (b) with sellotape.
se
from different depths, according to the axial position of
the coherence gate and this is why they may look differ-
ently, however they do not disclose the existence of the
sellotape. Images with similar aspect are however col-
lected using conventional systems at the border control
points. What we show here is that OCT can explore the
fingerprint in depth and this may be an extra feature not
utilised so far in security. The pattern is clear and can
serve the purpose of identification.
As mentioned above in Section 3, our
ms are characterised by the fact that they could easily
be switched between C-scan and B-scan regime, by re-
configuring the signals sent to the three scanners:
X-lateral,
Y-lateral an
en in the C-scan images above, however could be traced
in cross section images (B-scans). Such B-scan images
are shown in Figure 5. Now, the sellotape layer is
clearly distinguishable, covering the finger and following
its shape.
To obta
m was used. C-scan and B-scan OCT images are shown
in Figure 6 and in Figure 7 respectively. The sweat
ducts are much larger and tiny feature can be better iden-
tified. The sellotape layer is clearly seen in the cross sec-
tion image in Figure 7 (bottom).
Figure 8 shows a graph of two
om the images in Figure 7. An extra peak in the A-scan
profile is identified, marking the axial position of the
sellotape. The figure also shows how the addition of a
mask (the sellotape) changes the optical properties of the
compound sample, made from the sellotape and the fin-
ger. The reflection at the surface is reduced, allowing
more light to penetrate to the lower layers. Further re-
search may allow interpreting such changes and quanti-
fying the values of the indices of refraction involved.
Figure 5. B-scan OCT images collected with the dynamic
focus eF-OCT system. (top): without sellotape; (bottom)
with sellotape.
(a) (b)
Figure 6. C-sAO-eF-OCT
. Conclusions
he two systems above represent state of the art eF-OCT.
can images collected with the
system. (a) without sellotape; (b) with sellotape. The white
blobs shown by the arrows ar e swe a t duc ts.
5
T
We explored the capability of dynamic focus and adap-
tive optics to improve the potential of deciphering fake
layers among genuine skin layers. The depth of the cov-
ering material is disclosed and the thickness of the layer,
measured. We have proven above that the two regimes of
operation, characteristic to the eF-OCT only, can advan-
tageously be used to detect any extra layers (using the
B-scan regime) and perform the standard operation of

M.-R. NASIRI-AVANAKI ET AL.95
Figure 7. B-scan OCT images collected with the corrected
aberration OCT. (top) without sellotape: (bottom) with
sellotape.
Figure 8. Depth profiles of a thumb, with and without a
ngerprint recognition (C-scan regime). Their enhanced
ology of Fingerprint
onventional means do not provide any information
c methods,
ba
tion.
Th
d possible direction is that in detecting stress.
A
requires a special
so
sellotape mask, obtained from images in Figure 7.
fi
capabilities in terms of resolution can be extended to
other aspects of biometry in order to improve the reli-
ability of security decisions.
6. Impact on the Techn
Readers
C
from inside the finger. This is the aspect which OCT
addresses uniquely. The extra depth information pro-
vided by OCT has not been translated yet to commer-
cially available fingerprint imaging systems.
As opposed to most of the existing biometri
sed on imaging superficial features, OCT can enhance
the reliability of fingerprint imaging. Imaging systems
based on OCT eliminate the security flaws and could not
be easily spoofed by fingerprint dummies. OCT can ex-
tract features in multilayer objects and therefore could be
extended to other security applications. OCT can even
“see” the genuine fingerprint behind the faked one.
Another possible direction is the liveness detec
e system can be easily modified to perform Doppler
OCT, and become sensitive to blood flow. Moving flow
can be used as a contrast mechanism. With such a system,
the security centre can check the liveness of the finger,
and in this way, can prevent cheating by using a dead
finger [13]. A dead finger would have no motion within
it.
A secon
s shown in images above, the sweat ducts are well visi-
ble, as white blobs in the C-scans and as helicoidal struc-
tures in the B-scans. Usually, an intruder should manifest
excess sweetening which could be picked up by OCT
when restricting the imaging process to the sweat ducts.
There are two types of human sweating: thermal and
mental sweating. This investigation process can be ap-
plied to both types of sweating. Thermal sweating is
stimulated by external heat, while mental sweating oc-
curs in response to mental or physical stress [14]. In re-
sponse to the stress stimulus, only a small amount of
sweat is secreted due to mental sweating. However, it has
been proven that OCT can successfully detect such small
values of sweat [15] and therefore OCT can complement
a “Lie detector testing device”...
To implement OCT technology
urce, two or three scanners and an interferometer. In
the last 5 years, the cost of OCT systems decreased con-
siderably due to the increase in the numbers of ophthal-
mology practices employing OCT, as well as due to the
expansion of OCT outside ophthalmology. Preventing
criminals from accessing restricted places is priceless.
This can justify for the moment, investment into adding
OCT readers to the conventional recognition machines.
The second system, using adaptive optics is still rela-
tively expensive. This was used here to investigate the
value of a higher resolution image only and not as a
suggestion to expand it towards commercialization. The

M.-R. NASIRI-AVANAKI ET AL. 96
ve also shown here, that eF-OCT can also per-
fo
. Acknowledgements
. Meadway, M. R. Nasiri-Avanaki and A. Bradu ac-
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