Multispectral imaging is a sophisticated technology that was developed to overcome the fingerprint capture problems conventional imaging systems have in less-than-ideal conditions. The core problem was that conventional technologies rely on unobstructed and complete contact between the fingerprint and the sensor, a condition that is elusive in the real world. The more effective solution was based on using multiple spectrums of light and advanced polarization techniques to extract unique fingerprint characteristics from both the surface and subsurface of the skin. The nature of human skin physiology is such that this subsurface information is both relevant to fingerprint capture and unaffected by surface wear and other environmental factors. The net result is that Lumidigm sidesteps the problems that conventional technologies face in real world conditions. Additionally, our subsurface capability allows us to discriminate a real finger from an imposter or “spoof” fingerprint, making Lumidigm a leader in liveness detection.
Multispectral imaging looks at and beyond the skin surface to the subsurface foundation of the fingerprint ridges. Different wavelengths of visible light interact with the skin in different ways, enabling significantly enhanced data capture. The fingerprint pattern on the surface echoes the subsurface structures from which they arose during development. Multispectral imaging exploits the dependent relationship between surface and subsurface fingerprint patterns; subsurface data collected by multispectral imaging technology supports and augments surface data to create the highest-quality fingerprint image available.
The basic operation of the multispectral sensor is straightforward. The sensor consists of two main components: a light source, which provides the light to illuminate the finger resting on a platen; and an imaging system, which images this region of the platen onto a digital imaging array. While these components are similar to those of a conventional optical fingerprint sensor, the configuration of the multispectral sensor is expressly designed to avoid the optical phenomenon of total internal reflectance (TIR) because it depends on unobstructed and complete contact between the fingerprint sensors and the platen to work.
The multispectral illumination system consists of a source of multiple illumination wavelengths rather than the quasi-monochromatic illumination commonly used for TIR imaging. Linear polarizers are used in the illumination and detection portions of the sensor. The polarizers are arranged in an orthogonal configuration (a.k.a. polarizer-analyzer) to emphasize the light that penetrates the surface of the skin and undergoes multiple scattering events before emerging from the skin toward the image array.
To more fully understand the significance of this technology’s subsurface imaging capability, it may help to examine the nature of how a human fingerprint is created. The fingerprint ridges that we see on the surface of the finger have their foundation beneath the surface of the skin, in the capillary beds and other sub-dermal structures.The fingerprint ridges we see on our fingertips are merely an echo of the foundational “inner fingerprint”. Unlike the surface fingerprint characteristics that can be obscured by moisture, dirt or wear, the “inner fingerprint” lies undisturbed and unaltered beneath the surface. When surface fingerprint information is combined with subsurface fingerprint information and reassembled in an intelligent and integrated manner, the results are more consistent, more inclusive and more tamper resistant.
Multispectral Fingerprint Image Acquisition (1.3 MiB)
This whitepaper discusses biometric multispectral imaging technology in depth.