The use of structured light illumination techniques for three-dimensional (3-D) data acquisition is, in many cases, limited to stationary objects due to the multiple pattern projections needed for depth analysis. High speed N-pattern projections require synchronization between the camera and the projector and have the added expense of these high speed devices. The composite pattern (CP) method allows multiple structured light patterns to be combined via spatial frequency modulation, thereby enabling measurement and rendering of a 3-D surface model of an object using only a single pattern. The capture speed of a single pattern does not require synchronization and is only limited by the camera speed which is N times less than the N-pattern techniques. When used on partially translucent materials such as human skin, the CP weighting is corrupted thereby degrading the 3-D reconstruction. The method described herein, termed modified CP, extends the CP design with the addition of a stripe encoding pattern to be insensitive to the internal scattering of human skin. This stripe pattern, used in conjunction with a new spatial processing method, allows for less contrast sensitivity, less sensitivity to human skin spatial frequency response and thus higher resolution performance. The resolution performance is experimentally measured based on a measure our group has developed, referred to as the depth matched transfer function. Measurements and practical applications are demonstrated.
Structured Light Illumination is a widely used 3D shape measurement technique in non-contact surface scanning. Multi-pattern based Structured Light Illumination methods are the most accurate measuring techniques, but are sensitive to object motion during the pattern projection. To reduce this sensitivity, Composite Pattern was introduced as a single pattern Structured Light Illumination technique. Composite Pattern technique spatially modulates several Phase Measuring Profilometry patterns into a single pattern but demonstrated sensitivity to surface contrast and an object’s Spatial Modulation Transfer Function in the form of banding error. The Modified Composite Pattern was developed based on Composite Pattern, but used an imbedded binary gray code to minimize sensitivity to an object’s Spatial Modulation Transfer Function and banding at the expense of lateral resolution. We present a novel method utilizing an MCP pattern for non-ambiguous phase followed by a single sinusoidal pattern. The surface phase modulates the single sinusoidal pattern which is demodulated using a quadrature demodulation technique and then unwrapped by the MCP phase result. A single sinusoidal pattern reconstruction inherently has banding error. So the final step is to use an existing de-banding algorithm. The mathematical implementation is given in detail for the two pattern algorithm and experimental results are presented.
Our group believes that the evolution of fingerprint capture technology is in transition to include 3-D non-contact fingerprint capture. More specifically we believe that systems based on structured light illumination provide the highest level of depth measurement accuracy. However, for these new technologies to be fully accepted by the biometric community, they must be compliant with federal standards of performance. At present these standards do not exist for this new biometric technology. We propose and define a set of test procedures to be used to verify compliance with the Federal Bureau of Investigation’s image quality specification for Personal Identity Verification single fingerprint capture devices. The proposed test procedures include: geometric accuracy, lateral resolution based on intensity or depth, gray level uniformity and flattened fingerprint image quality. Several 2-D contact analogies, performance tradeoffs and optimization dilemmas are evaluated and proposed solutions are presented.
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