TruePrint™
TruePrint™ Technology
The Fundamentals
DRS \\ 7jun02
1
techieDetail16.ppt
TruePrint RF Imaging Technology
Starting from first principles, connect an AC signal generator to 2 parallel
conductive plates, generating an electric field between the plates. The
wavelength will be much larger than all dimensions of the plates, so the field will
be purely electric with no magnetic component, and no electromagnetic effects.
The system can be treated as a quasi-static electric field with the equipotential
contours (shown in dashed red) representing lines of constant signal amplitude.
Conductive
plates
Equipotential
contours
DRS \\ 7jun02
2
techieDetail16.ppt
TruePrint RF Imaging Technology
If we now corrugate one of the conductive surfaces, the RF electric field will
follow the shape of the conductive boundary. As illustrated here, the
equipotential contours within the field will take on shapes that are an attenuated
form of the shape of the conductive plate. A planar array of RF electric field
sensors, essentially very small antennas, placed in the electric field region will
acquire voltages that represent the shape of the corrugated conductive surface.
Corrugated
Conductive plate
Shaped
Equipotential
contours
An E-field sensor array can measure that shape
DRS \\ 7jun02
3
Antenna array
techieDetail16.ppt
The Structure of the Skin
z
Now let’s examine the structure
of the skin to see how to apply
this RF imaging mechanism
z
Dry dead skin cells have low
electrical conductivity. This
region behaves as a dielectric
z
The boundary region where the
live cells begin turning into
keratinized skin is moist and
electrically conductive.
DRS \\ 7jun02
Air
Dead
dry
skin
Live
skin
4
techieDetail16.ppt
TruePrint RF Imaging Technology
Applying the principle to fingerprint imaging.
An signal generator on chip applies a small RF signal between the finger and
the adjacent semiconductor.
The signal is coupled into the live conductive layer of the skin by a
conductive surface (called the finger drive ring) positioned around the outside
of the active imaging region of the sensor.
Finger drive
Part of a finger
Conductive layer just beneath surface of skin
Ridges and valleys on finger surface
Semiconductor
DRS \\ 7jun02
5
techieDetail16.ppt
TruePrint RF Imaging Technology
RF Field (between finger and chip) mimics shape of
conductive (live) skin layer
Conductive layer just beneath surface of skin
DRS \\ 7jun02
6
techieDetail16.ppt
TruePrint RF Imaging Technology
Sensors near ridges
measure higher signals
DRS \\ 7jun02
Sensors near valleys
measure lower signals
7
techieDetail16.ppt
TruePrint RF Imaging Technology
The pixel antennas have characteristic impedances in the
teraohm range. Therefore ultra high input-impedance
sense amps are used under pixel to read the antennas’
voltages and drive the switched data busses.
Cross section of
finger skin
Live skin cell layer
Outer dead skin
layer
(dielectric)
surface of the skin
Pixel antennae array
Semiconductor
substrate
Excitation signal
reference plane
Excitation
Generator
Hi input impedance
sense amps
Cross section of antenna array reading finger skin.
DRS \\ 7jun02
8
techieDetail16.ppt
Backup
DC Capacitive Overview
For comparison purposes, this diagram shows the pixel structure and
electric field geometry for a typical DC capacitive fingerprint sensor. In
one mode of operation, a fixed charge is placed on the active pixel plate
and the voltage generated is measured. Note that this is a capacitive
fringing field, so the field geometry is hemispherical and it is confined to a
region very close to the sensor surface.
Cross section of
finger skin
Capacitive sensors measure the difference in permittivity
between the ridge surface skin and the air in the valleys.
Live skin cell layer
Outer dead skin layer
(dielectric)
surface of the skin
Electric field geometry
Air gaps at valleys
Pixel sensor plate array
Currently active
pixel sensor plate
c
Semiconductor
substrate
Classic capacitive fingerprint sensor - cross section of sensor
array reading finger skin.
DRS \\ 7jun02
9
techieDetail16.ppt
Advantages of TruePrint Technology
over standard DC capacitive sensing
TruePrint imaging does not depend upon air in the valleys
z
If the valleys are worn away, filled with oil or dirt, or if
they are smashed flat, the sensor still images
z
Even very dry skin can be imaged successfully
TruePrint technology uses coherent planar field structures
between the finger and the sensor
z
Minimizes crosstalk between sensors, 250 sensors per
inch in TruePrint technology generates a real 250 ppi
information content. Most other sensors lose
information due to crosstalk.
z
The sensing fields penetrate thick and callused skin
z
The sensors can work through thicker protective
coatings
TruePrint technology is very flexible and can be
automatically adjusted to adapt to a wide range of different
skin types and environments.
DRS \\ 7jun02
10
techieDetail16.ppt
Dynamic optimization™ -- How it works
Uses the flexibility of the TruePrint technology to
adapt to the person’s current skin condition
The system takes several image frames in
sequence
z each one better optimized than the previous
Process continues until the image is good
enough to accept or reject confidently
DRS \\ 7jun02
11
techieDetail16.ppt
Dynamic optimization
Example of a typical dry finger
4
This example took
4 frames
3
Executed in about
½ second on a PC
2
4
In Slow
Motion . . .
Adjust A/D references
3
1
Increase amplifier gain
2
Increase drive signal
1
DRS \\ 7jun02
12
techieDetail16.ppt
For more information …
Click here to learn
about fingerprint
Matching methods
Click here to learn
how very small
fingerprint
sensors work
Back to Beginning
DRS \\ 7jun02
13
techieDetail16.ppt