Multispectral Imaging Technology

Lumidigm’s sensors use multispectral imaging to collect information from below the surface of the skin. To understand how multispectral imaging technology from Lumidigm has revolutionized fingerprint identification and verification, it is helpful to review how conventional optical technology works.

Conventional optical technology

Fingerprint images are commonly collected by optical sensors that utilize the phenomenon of total internal reflectance (TIR). The TIR schematic at the right illustrates one TIR sensor configuration. This figure shows an illumination source (perhaps a strip of light-emitting diodes) on the left sending light across the prism to the right where it reflects off a diffuse reflective coating and illuminates the top surface of the prism, called the platen. If there is nothing on the platen, the light does not cross it. Rather, TIR causes the light to reflect off the platen and brightly illuminate an imaging system located to the lower left side of the prism (not shown). When a finger touches the platen, the points of contacts (i.e., the ridges of the fingerprint) allow light to cross the platen. This light is not reflected back to the imager. Because TIR still occurs in the valleys of the fingerprint (shown in the figure inset), the resulting image shows relatively dark features at the points of contact (ridges) and bright features elsewhere (valleys).

Revolutionary Multispectral Imaging Technology

Lumidigm has developed a multispectral imaging technology that is able to collect additional information from below the surface of the skin (see skin physiology). As shown in the schematic at the right, the multispectral imaging sensor has two main components: a light source and an imaging system. Unlike the TIR sensor, these components are designed and configured expressly to avoid the total internal reflectance phenomenon. The multispectral imaging system utilizes multiple illumination wavelengths rather than the quasi-monochromatic illumination commonly used for TIR imaging. The orthogonal configuration of linear polarizers emphasizes this multispectral light, which penetrates the surface of the skin. The light then undergoes multiple scattering events before emerging from the skin toward the image array. In avoiding the optical phenomenon of TIR, the multispectral imaging sensor is able to collect more identifying data from the finger than the TIR sensor. The multispectral imaging sensor can enhance TIR images through seamless integration into a TIR system, or it can stand alone.

Is subsurface information necessary?

TIR sensors are accurate when good-quality and authentic fingerprint ridges are presented. However, this situation is not as common as you might think. When presented with missing or poorly-defined fingerprint features due to dry skin, lack of optical contact, etc, an image from a TIR sensor will be incomplete. Conversely, multispectral imaging sensors from Lumidigm can use information from below the skin to enhance fingerprint images. TIR sensors image whatever ridges are presented, even if they are fake; because theLumidigm sensor can look below the surface, it can detect spoofs easily. Multispectral imaging technology is even dependable in extreme environments that thwart conventional imagers, such as high humidity and bright light.

For more details about Lumidigm’s multispectral technology, please download the white paper Multispectral Sensing for High-Performance Fingerprint Biometric Imaging (pdf, 537k) or contact us.

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