Light-based fingerprint sensing
Capture d'empreintes digitales basé sur la lumière
Many live-scan fingerprint sensors were using optical means in the past,
taking advantage of the frustrated reflection on a glass interface.
This is still the main technique for governmental applications where
it is important to acquire a large image, and many clever variations exist.
But regular optical sensors are too large and expensive when addressing the
smartphone market: electrical/capacitive sensors are more adapted.
But we may see a comeback of optical sensor in the close future,
when they will be integrated with the (optical) display.
FTIR: frustrated total internal reflection / Reflection with contact / Réflexion avec contact |
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The oldest 'live-scan' readers use frustrated refraction over a glass prism
(when the skin touches the glass, the light is not reflected but absorbed).
The finger is illuminated from one side with a LED while the other side transmits
the image through a lens to a camera. (FTIR: frustrated total internal reflection).
La plus ancienne technique de capture d'empreinte "live" est l'usage de
la réflexion totale sur un prisme en verre (lorsque la peau touche le verre,
la lumière n'est plus réfléchie, elle est absorbée). Le doigt est éclairé à l'aide
d'une DEL (diode électroluminescente) par un coté du prisme, tandis que
l'image est récupérée de l'autre coté avec une lentille et un capteur d'image.
(Ce schéma date de 1996, les capteurs CMOS étaient fort rares...)
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(2000) Here is the example of the Compaq fingerprint reader (CMOS camera).
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Two main types exist for governmental applications:
- FAP20: one fingerprint
- FAP60: 4 fingers (slap fingers)
IAFIS FAQ for more details.
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Many companies are offering such FTIR devices, this is a common technology. Some are offering some variants less common.
Cette technologie de capture est très courante, et de nombreuses compagnies l'utilisent. Quelques-unes offrent des variantes moins courantes.
Nagoya University uses a micro-collimator instead of the fiber optic, enabling direct fingerprint comparison.
L'université de Nagoya utilise un micro-collimateur à la place de la fibre optique, afin de réaliser une comparaison directe de l'empreinte.
(1999) Fingerprint Sensor with Micro Collimator (Nagoya Univ.)
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Direct image / Image directe |
It's also possible to simply take a picture of the finger touching a glass, or something else.
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(2015) Vkansee introduces a 2000dpi reader, UTFIS,
using advanced pinhole imaging techniques to take a high-resolution image.
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(2018 Jun) The Vivo NEX flagship smartphone has an in-display fingerprint sensor.
It has been a big surprise for me to see that it's a regular camera with a very short focus lens
or a pinhole.
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Reflection with sweep / Réflexion avec balayage |
Sweep is a variant of the regular FTIR acquisition: the finger must swipe over the
sensor to get a full image. This enables to make smaller, thinner acquisition devices.
Kinetic Sciences
(1999) Kinetic Sciences
and Cecrop/Sannaedle have proposed sweep optical sensors.
Kinetic Sciences et Cecrop/Sannaedle ont proposé des capteurs optiques à balayage.
Paper describing the sensor (dead link):
http://www.smta.org/files/telecom_solutions02_immega.pdf
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Casio
Casio
+ Alps Electric uses a roller with the sensor inside. The roller acts like the prism.
Casio + Alps Electric utilise une roulette contenant le capteur. La roulette agit comme prisme.
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Reflection with direct sensor contact (lensless) |
To avoid any lens to focus the image, and make a flat sensor, a direct contact of the
skin with the sensor is proposed: it is important the skin to be close to the sensor, in
general a photodiode, less than the pixel pitch (about 50 microns). The light is coming
from below the skin (see further transmission through the skin).
TFT
TFT on glass substrate is a less expensive technology compared to silicon, and is flat:
this was the first proposal.
(1997) Philips replaces the CMOS sensor by a TFT display, used as sensor.
Philips has also worked with Ethentica
to combine their technologies, but without known product. LG has also studied a TFT sensor.
Some other companies proposing TFT fingerprint sensors:
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Polymer organic photodetector
(2005 Jul) NanoIdent unveils a flexible fingerprint
sensor using a polymer organic photodetector.
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Transmission through the skin / Transmission à travers la peau |
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Red light is able to cross the skin, and so, it is possible to get an image of the lighted skin.
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NEC, Delsy, Holtek
NEC and
Delsy (and Secom?)
use a CMOS camera which is the size of the finger,
with a fiber optic in-between, the light is coming from the edges in this case.
NEC et Delsy (et Secom?) utilisent un capteur optique CMOS
qui est nécessairement de la taille du doigt, avec une fibre optique déposée sur le capteur,
la lumière provenant des bords dans ce cas.
(2015 Dec) Holtek is using this principle with the GH8111
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Reflection touchless / Réflexion sans contact |
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Instead of touching a platen, the idea is to acquire directly the fingerprint image with
an adapted camera. This is avoiding the distorsion linked to the finger pushed against
the glass platen. But this is not that easy because of the low contrast between
ridges and valleys -much less than FTIR. To compensate, structured light has been proposed.
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BERC
(2004) The BERC lab from Yonsei University (Korea) also developped a touchless sensor.
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TBS
(2005) TBS launches a touchless sensor with the "Surround Imaging".
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AOS
(2011 Jan) AOS Advanced Optical Systems develops AIRprint,
a device that detects fingerprints by shining polarized light onto a person's hand
and analyzing the reflection using two cameras configured to detect different polarizations,
able to scan fingerprints from up to two meters away.
(2013) OnePrint & IDAir
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RICE university
(2017 Apr)
Rice graduate student Yicheng Wu demonstrates the SAVI prototype,
which is able to capture fine details of an object from a distance,
effectively replacing a large telephoto lens.
The prototype camera is on a motorized track in the foreground at left,
while a laser at right creates a speckle pattern on the target, a fingerprint.
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Structured light / fringes / Lumière structurée |
Idemia
(2014 Aug) Finger On the Fly®
from Safran/Morpho, now Idemia, is using structured light.
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OCT Optical Coherence Tomography |
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OCT enables to see the structures inside the skin, using interferomety. The interface between
the derma and the epiderma shows the same fingerprint structure.
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Papers:
Illinois University
(2013) Real-time in vivo computed optical interferometric tomography / Ahmad & al.
a, Three-dimensional rendering comparing OCT (left) and ISAM (right).
The spiral structures of the sweat ducts appear with higher resolution and higher SNR
in the ISAM data set
b, Representative en face planes (OCT left and ISAM right) with enlarged representative
regions indicated by colour-coded arrows (blue, OCT; yellow, ISAM) showing the cross-section
of the sweat ducts. The diameter of the sweat ducts obtained with ISAM more closely matches
the known anatomical range of diameters.
c, En face planes (OCT left and ISAM right) at a (optical) depth of 780 µm below
the surface, showing enhanced resolution deeper inside the superficial dermis.
Scale bars, 500 µm.
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Langevin Institute
(2015 Oct) The Langevin Institute has made an OCT (optical coherence tomography) device
specifically to acquire fingerprints.
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