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.
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...)
Two main types exist for governmental applications:
IAFIS FAQ for more details.
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.
It's also possible to simply take a picture of the finger touching a glass, or something else.
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.
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 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:
Red light is able to cross the skin, and so, it is possible to get an image of the lighted skin.
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.
OCT enables to see the structures inside the skin, using interferomety. The interface between the derma and the epiderma shows the same fingerprint structure.
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.