After optical sensing, the measurement of the capacitance between the skin and the pixel is the most physical effect used to acquire fingerprints. Where there is a ridge or a valley, the distance varies, as does the capacitance. Because an electrical field is measured and the distance between the skin and the pixel must be very low to provide enough sensitivity, the coating must be as thin as possible (a few microns). A significant drawback is vulnerability to strong external electrical fields, the most dangerous being ESD (Electro-Static Discharge).
Après la lecture optique, la mesure de la capacité électrique entre la peau et le pixel est l'effet physique le plus souvent mis en oeuvre: la capacité électrique varie comme l'inverse de la distance, ce qui permet de distinguer crêtes et vallées. Comme le champ électrique mesuré entre la peau et le pixel est très faible, la protection surfacique doit être très mince (quelques microns) afin d'obtenir une sensiblité correcte. Un défaut connu est la sensibilité aux forts champs électriques, comme ceux provoqués par les décharges électrostatiques ESD.

TFT / Flat panel

Flat panel technology rather than bulk silicon has been studied by Philips Korea and some others, using capacitive sensing.

La technologie des écrans plats a été étudiée par Philips comme substitut du silicium, en utilisant un effet capacitif.

Sometimes confused with capacitance sensing, the common point between RF field sensing and capacitance sensing is the "capacitance" connection of the signal.
Parfois confondu avec les capteurs capacitifs, le seul point commun entre un capteur RF et un capteur capacitif est la liaison électrique "capacitive" du signal.

A low radio frequency (RF) signal is injected into the finger, then read by the pixels on the silicon acting like antennas. The signal strenght depends on the capacitance/resistive connection, so from the distance between the skin and the pixel.
Un faible signal radiofréquence est injecté dans le doigt, puis lu par les pixels sur le silicium qui agissent comme des antennes. La puissance du signal lu dépend de la liaison capacitive/résistive, donc de la distance entre la peau et le pixel.

Many clever variations exist, particularly about the readout circuitry to increase to signal to noise ratio, especially when the coating is becoming thick (greater than the pixel pitch, so 50 microns).

Because of the required sensitivity, very often pixels are done directly on a silicon chip: it helps to have many good transistors directly where we need to read the signal. But the silicon is expensive, and it is better to have large area sensor to enhance the recognition levels. So some companies proposed to use simple copper lines (or equivalent) over a low cost substrate such as Kapton, and connect the lines to a silicon chip: the silicon area is not depending on the captured fingerprint area. But at the cost of a higher noise, so a lower image quality.

Active RF sensing requires to send a signal in the skin: very often, this is done using a metal ring (e.g. Apple).

When the frequency is zero = a constant voltage, this is the simple impedance/resistance that is used.

Prussian blue is known to visualize latent prints by selective electrochemical deposition:

It is also possible to capture a fingerprint in one roll: