Thermal fingerprint sensing
Capture thermique d'empreintes digitales
Thermal fingerprint sensors are taking advantage of the large physical parameter differences
between the skin and the air, namely thermal resistivity and thermal capacitance.
Like other types of sensors, thermal sensing can be
- passive: this is the temperature difference between the sensor and the skin which produces the heat exchange.
static acquisition is a problem as the thermal equilibrium is quickly reached (less than one second).
- active: injecting some heat in the pixel produces a temperature change.
If it is the skin (ridge), the heat is pumped and the final temperature is lower
than in the air (valley) situation.
The first commercial thermal fingerprint sensor was the FingerChip from Atmel
(formerly Thomson-CSF), using PVDF-TrFE
(a pyro-electric material able to convert changes of temperature into charges)
as sensing layer over a silicon chip.
This technology was coming from an uncooled IR camera.
It was also the first swipe sensor in 1997: swiping enabled to use a passive sensor.
Other thermal fingerprint sensors are based on microthermistances,
with or without heating, with or without a cavity (bolometers)
to increase the thermal isolation so the sensitivity.