Identification and authentication through individual fingerprints requires three general steps.
- Image Generation/Image Capture
- Feature Extraction
- Matching to an enrollment database
There are many types of fingerprint sensors using different technologies in varying form factors. Techniques for image capture demonstrate an evolution in technology to provide high quality identity imagery. When comparing original ink-on-paper techniques, optical, capacitive, RF Field, and ultrasound sensor technologies, electroluminescent film like Integrated Biometrics’ Light Emitting Sensor (LES) provides advances throughout each step of the capture process.
Ink-on-paper Techniques involve processing subjects for verification using ink applied directly to the fingers and manually rolling across a paper fingerprint card. Persons of interest are identified using an existing fingerprint card or from latent prints recovered through an investigation. These paper cards are then scanned into a computer. Collected fingerprint images are compared manually by trained fingerprint examiners against the stored record of fingerprint cards.
Optical Sensor Technologies make use of Frustrated Total Internal Reflection (FTIR) technology to image the fingerprint placed on the top surface of a glass prism. A finger placed on the top surface of the prism causes ridges of the fingerprint to make contact with the glass. A strong light source shines onto the finger at an angle causing internal reflection at the valley areas and sweat pores of the fingerprint. Optical based sensors using FTIR technology have been the only practical technology to achieve the level of image quality necessary to meet FBI standards for large surface (greater than 0.6 x 0.8 inches) area sensors.
Drawbacks of this technology restrict useful, wide-scale adoption:
- Mobile applications suffer the most due to the size, weight, frailty, and large power consumption of the traditional optical assemblies. Field units require bulky casing to shield and try to protect fragile traditional optical sensor alignments. The light source required to properly illuminate the subject creates additional power requirements that further reduce effectiveness in mobile applications.
- FTIR optical technologies use direct visual imaging inherently prone to poor results in sunny or well-lit conditions and fingerprints must be clean and free of all dirt to provide a clear image. Latent prints left behind on the platen surface can be seen by the camera in subsequent readings and the units require constant cleaning. FTIR-based optical technologies have difficulty with dry fingers with low contrast between the fingerprint ridge and valleys. Dry finger performance is improved with silicone membranes installed over the glass surface. Damage and replacement of these membranes (tears and replacements) require frequent maintenance to remain operational.
- FTIR optical sensors can be readily fooled or spoofed with a latent print or rubber fingerprints. Requirements to shine light on the fingerprint and then capture the refracted light also increase the size of the sensor and can be a disadvantage in non-optimal lighting conditions. For example, daylight use of an optical sensor requires that the sensor be shaded from sunlight or other bright light, so the camera can focus on light from the fingerprint.
- Cold temperature performance is challenging as a warm finger may cause condensation and fogging of the cold glass platen. These conditions greatly impact the clarity of the captured image reducing speed and accuracy of match results.
- “Light emanating” from the sensor lighting source creates a visible marker detrimental for military applications during nighttime operations.
Under ideal conditions, with “human-guided” operation, optical FTIR technology can provide high quality images and direct upload of the image to a digital database.
A capacitive sensor is a two-dimensional array of micro-capacitor plates embedded in a chip. The subject finger acts as the second “plate” to complete a circuit. When a finger is placed on the chip array, electrical charges between the surface of the finger and each of the silicon plates result. The magnitude of these electrical charges depends on the distance between the fingerprint surface and the capacitance plates. The resulting fingerprint “image” from a capacitive sensor is the two-dimensional array of electrical charge values used for matching. The technology is primarily suited for small scale, “one to few” applications.
Poor image quality due to electrical noise and low image resolution rank as the primary disadvantages of capacitive sensor technology. Very few capacitive sensors have passed FBI certification testing. The available certified scanners only meet the requirements for the smallest size of image capture (FAP10).
The semiconductor-based technology is prone to electronic damage due to its frail nature, susceptibility to electrostatic discharge (ESD). Capacitive sensors perform poorly when the subject has dirty or oily fingers; these factors change the capacitance of the finger and impact the captured image. Current commercial capacitive sensors have false reject and false acceptance rates (FRR/FAR) that are much higher than acceptable for legal identification.
The necessity to create a circuit eliminates some of the common techniques used for spoofing biometric identification; an advantage over comparable FTIR technologies. The natural advantages of a solid state structure that is thin and lightweight make capacitive sensors a fit for small devices and consumer electronics where one to few matching is prevalent and high quality images is not an application requirement.
Ultrasound sensor technologies uses echography; sending acoustic signals toward the fingertip and capturing the echo signal. The echo signal is used to compute the range image of the fingerprint and, subsequently, the ridge structure itself. This method images the subsurface of the finger skin; therefore, it is resilient to dirt and oil accumulations that may visually mar the fingerprint.
The application uses for this technology are limited due to the cost, power, size, and weight of the scanner devices. The technology is not viable for use with mobile applications and serves as a fixed scanning technology.
Advancements in matching and comparison technologies evolved along with the technologies to capture fingerprint images. What was a complex and manual process for skilled and well-trained human operators is now quickly accomplished with computers and software.
Using digital capture technology prints are converted from an image into a digital template. These templates may be recorded and stored in a database for later comparison to subject fingerprints. Applications for digital matching scale from small databases containing hundreds of records to solutions that require matching against hundreds of thousands of records.
Integrated Biometrics produces a unique sensor type based on electroluminescent Light Emitting Sensor (LES) film to create the fingerprint image. The patented LES film is a multilayer, polymer composite containing particles that luminesce (give off light) in the presence of an electrical field.
When a finger is placed on the film, the ridges of the fingerprint completes the low level electric circuit which causes the particles in the film to luminesce light, producing a highly accurate, high resolution analog image of the fingerprint. The resolution of the fingerprint image is approximately 1500 PPI.
The high-resolution (1500 PPI), image can be captured using a variety technologies. The image is captured creating an Appendix F, FBI-quality image for flat or rolled fingerprints.
Shown above is the underside of the LES film showing the blue glow of the luminescent LES fingerprint image created when a finger is placed in contact with the top surface of the film.
Image capture for Integrated Biometrics’ Thin Film Transistor based sensors is 500 PPI with pixels that are exactly 50.8 microns in spacing. Image generation using LES film is instantaneous.
LES sensor technology handles difficult use cases including dry, wet, and dirty fingers by using adaptive circuitry and dynamic capture algorithms that quickly and automatically optimize image quality.
The LES sensor technology is inherently spoof resistant to many techniques, as the fingerprint ridges and features touching the film must be very close to human skin consistency to activate the luminescence. The ridge of the fingerprint touching the film acts as an electrical ground. Fake fingers using silicone or similar materials and two dimensional images of fingerprints will not create a fingerprint image on the LES sensor.
A silicon “gummy” fake print will not be ‘seen’ by the LES sensor and no image will be collected. Latent prints or dirt left behind will not be seen by the LES sensor. This allows for rapid enrollment and verification of high-quality fingerprints suitable for large applications.
Using proprietary algorithms, the LES sensor technology has extremely low False Acceptance and Reject Rates (FAR/FRR) leading to less frustration for rapid identification as well as authentication of valid users.
Light Emitting Sensor technology by Integrated Biometrics is especially well suited for mobile and outdoor environments. The nature of the technology eliminates the common failures found in alternative solutions. Operation is consistent in all conditions and image generation is not affected by ambient light or direct sunlight. LES performance remains consistent throughout extreme temperature variances. The common problems associated with condensation or fogging on the scanner are eliminated. The LES technology requires no on-board light source to capture fingerprint images so it is ideal for nighttime military operations.
The low power requirements for LES technology empowers integrators to provide smaller, more mobile solutions bringing biometrics into new applications.
LES film is flexible, allowing for sensors that are curved to fit the human finger. The flexible LES film allows for creation of sensors that are customizable to any shape or size.
Light Emitting Sensor film is created by multi-layer screen printing allowing for fingerprint authentication/identification without a distinctly visible sensor or button when printed onto substrates including glass.