20-06-2013, 03:42 PM
Fingerprint sensing techniques
[attachment=55751]
Fingerprint sensing techniques
Optical readers
Reflection
Transmission
Sweep, sweep with a roller
Touchless
TFT
Electro-optical readers
Capacitance
Silicon chips
TFT
RF field - AC capacitance
Pressure
Resistive membrane on silicon, TFT
Tactile MEMS
Thermal
Ultra-sound
Ink
Ink and paper are the tried-and-true way to take fingerprints, but technology has found ways to eliminate smudges and ink stains.
Optical readers
Reflection
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).
Reflection touchless
TST removed the prism by directly reading the fingerprint, so the finger does not touch anything (but still need a guide to get the right optical distance). Thales (formerly Thomson-CSF) also proposed the same, but with the use of a special powder to put on the finger. The BERC lab from Yonsei University (Korea) also developped a touchless sensor (2004). (2005) TBS launch a touchless sensor with the "Surround Imaging".
Capacitance
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).
Pressure
This is one of the oldest ideas, because when you put your finger on something, you apply a pressure. Piezo-electric material has existed for years, but unfortunately, the sensitivity is very low. Moreover, when you add a protective coating, the resulting image is blurred because the relief of the fingerprint is smoothed. These problems have been solved, and now some devices using pressure sensing are available.
Several solutions, depending on the material, have been proposed:
Conductive membrane on a CMOS silicon chip.
Conductive membrane on TFT.
Micro-electromechanical switches on silicon chip.
Ultra-sound
Ultra-sound fingerprint reading is not common. It may be seen as a kind of echography. Ultra-sound sensing requires quite a big device with mechanical parts, and is quite expensive. Moreover, it takes a few seconds to grab an image. It is not suited for large production volumes at low cost. Its main advantage is the reading of the derma, the sub-surface of the skin, rather than the surface.
[attachment=55751]
Fingerprint sensing techniques
Optical readers
Reflection
Transmission
Sweep, sweep with a roller
Touchless
TFT
Electro-optical readers
Capacitance
Silicon chips
TFT
RF field - AC capacitance
Pressure
Resistive membrane on silicon, TFT
Tactile MEMS
Thermal
Ultra-sound
Ink
Ink and paper are the tried-and-true way to take fingerprints, but technology has found ways to eliminate smudges and ink stains.
Optical readers
Reflection
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).
Reflection touchless
TST removed the prism by directly reading the fingerprint, so the finger does not touch anything (but still need a guide to get the right optical distance). Thales (formerly Thomson-CSF) also proposed the same, but with the use of a special powder to put on the finger. The BERC lab from Yonsei University (Korea) also developped a touchless sensor (2004). (2005) TBS launch a touchless sensor with the "Surround Imaging".
Capacitance
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).
Pressure
This is one of the oldest ideas, because when you put your finger on something, you apply a pressure. Piezo-electric material has existed for years, but unfortunately, the sensitivity is very low. Moreover, when you add a protective coating, the resulting image is blurred because the relief of the fingerprint is smoothed. These problems have been solved, and now some devices using pressure sensing are available.
Several solutions, depending on the material, have been proposed:
Conductive membrane on a CMOS silicon chip.
Conductive membrane on TFT.
Micro-electromechanical switches on silicon chip.
Ultra-sound
Ultra-sound fingerprint reading is not common. It may be seen as a kind of echography. Ultra-sound sensing requires quite a big device with mechanical parts, and is quite expensive. Moreover, it takes a few seconds to grab an image. It is not suited for large production volumes at low cost. Its main advantage is the reading of the derma, the sub-surface of the skin, rather than the surface.