Raymond C. Daley

Channel capacity model of binary encoded structured light-stripe illumination

A common approach to structured light-illumination measurement is to encode a surface topology successively with binary light-stripe patterns of variable spatial frequency. Each surface location is thereby encoded with a binary sequence associated with its height. By analyzing the lateral displacements of the reflected encoded pattern, one can reconstruct the surface topology without ambiguity. We present a model for multistripe analysis in terms of an information channel for which the maximum spatial stripe frequency is related to channel capacity and maximized accordingly by use of Shannons theorems. The objective is to improve lateral resolution through optimized spatial frequency while maintaining a fixed range resolution. Given an optimized spatial frequency, a technique is presented to enhance lateral resolution further by multiplexing the light structure. Theoretical and numerical results are compared with experimental data.

RANDOM PHASE ENCODING OF COMPOSITE FULLY COMPLEX FILTERS

The mapping of complex-valued functions onto phase-only spatial light modulators is examined. Random phase encoding effectively adds amplitude control to the phase-only filter and can be used to trade off systematic errors of the phase-only filter for random errors. This is illustrated for the problem of recognizing a threedimensional object from arbitrary views. The complex-valued composite filters that constitute a filter bank design are encoded by phase-only and pseudorandom methods. The best recognition probabilities are achieved by blending the two methods so that only the smallest amplitudes are randomly encoded.

Application of communication theory to high-speed structured light illumination

Structured light illumination has been used for several decades to extract three-dimensional information from surface topology. Most of the research and development has been in the light structuring methodology and the electronic processing while a unified theoretical description has been lacking. With the advent of programmable spatial light modulators having high frame rates, structured light illumination methods using spatial and temporal patterns are practical. We present structured light systems using spatial light modulation as communication systems and use communications theory in their description. This theory is applied to the specific method of successive binary light striping and the tradeoffs between surface encoding quality and processing speeds are discussed. Shannons theorem of channel capacity provides an objective measure to evaluate some of these tradeoffs and compare a variety of different approaches to structured light illumination. Another result of our analysis is the unification of structured light projection with pattern recognition. Methods of image recognition using Fourier expansion via orthogonal pattern projection are presented. The results of this analysis establish some physical limitations which guide us to effectively utilize both the methodology and the technology applicable to both 3-D data acquisition as well as pattern recognition. Both numerical and experimental results are presented to demonstrate concepts.

Improved light sectioning resolution by optimized thresholding

A common approach to structured light illumination is light stripe projection onto a surface topology and then analyzing the lateral displacements of the reflected pattern to reconstruct the surface topology. A single spatial frequency of a light stripe pattern may be used to illuminate a relatively flat surface. In the case of rough surfaces, the surface topology is encoded with a sequence of light stripe patterns with successively higher spatial frequencies. In both approaches, the maximum resolution is limited by the maximum spatial frequency used. However, the tradeoff between SNR blurring and spatial frequency limits the final reconstruction accuracy. That is, as spatial frequency increases, the projection systemss blurring function causes the light stripes to be coupled thereby decreasing the SNR of the reflected image. We present both mathematical and numerical models for this phenomenon which indicates that by laterally moving the light stripe pattern across the surface and optimally thresholding the image, we can achieve measurement density and accuracy beyond that achieved by increasing the frequency of a stationary light stripe pattern. the numerical model to be calibrated will accept experimental data. Theoretical and numerical results will be compared with experimental results.

Topographical analysis with time-modulated structured light

We present a range-finding method for determining surface topography by using time modulated structured light illumination. By illuminating a surface with a time-modulated light structure of several transmitters and triangulating relflected light onto a single point receiver, we are able to determine the surface height at a point by determining which light source intersects the surface at that point. In essence, the surface height acts to multiplex the projected signals onto the receiver. We increase the system resolution beyond the number of light sources by overlapping the image intensity of the light sources. The resulting signature allows for sub-pixel resolution. Time modulating the structurted light allows for demodulation with a high signal-to-noise ratio without the use of high intensity light sources thereby reducing system cost and complexity. This method of topographical analysis can be scaled to high end systems capable of real-time, high-resolution imaging. In addition to system geometry and resolution capacity, we discuss the advantages and disadvantages of various modulation and coding schemes that can be applied to this approach.

Performance characterization of structured light-based fingerprint scanner

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.

Using pseudorandom phase-only encoding to approximate fully complex distortion-invariant filters

In this paper, we introduce a unified model-based pattern recognition approach that can be formulated into a variety of techniques to be used for a variety of applications. Complex phasor addition and cancellation are incorporated into the design of filter(s) to perform implicit logical operations using linear correlation operators. These implicit logical operations are suitable to implement high-level gray-scale morphological transformations of input images. In this way we effectively project nonlinear decision boundaries into the input signal space yet maintain the mathematical simplicity of linear filter designs. We apply this approach to the automatic distortion- and intensity-invariance object recognition problem. We introduce a set of shape operators or complex filters that are logically structured into a filter bank architecture to accomplish the distortion and intensity-invariant system. This synthesized complex filter bank is optimally sensitive to fractal noise representing natural scenery. The sensitivity is optimized for a specific fractal parameter range using the Fisher discriminant. The output responses of the proposed system are shown for target, clutter, and pseudo-target inputs to represent its discrimination and generalization capability in the presence of distortion and intensity variations.

Non-contact 3D fingerprint scanner using structured light illumination

As crime prevention and national security remain a top priority, requirements for the use of fingerprints for identification continue to grow. While the size of fingerprint databases continues to expand, new technologies that can improve accuracy and ultimately matching performance will become more critical to maintain the effectiveness of the systems. FlashScan3D has developed non-contact, fingerprint scanners based on the principles of Structured Light Illumination (SLI) that capture 3Dimensional data of fingerprints quickly, accurately and independently of an operator. FlashScan3D will present findings from various research projects performed for the US Army and the Department of Homeland Security.

Complex linear morphology for intensity- and distortion-invariant pattern recognition

Any desired diffraction pattern can be produced in the Fourier plane by specification of a corresponding input plane transparency. Complex-valued transmittance is generally required, but in practice, phase-only transmittance is used. Many design procedures use numerically intensive, constrained optimization. We, instead, use a noniterative procedure that directly translates the desired, but unavailable, complex transparency into an appropriate phase transparency. At each pixel the value of phase is pseudorandomly selected from a random distribution whose standard deviation is specified by the desired amplitude. We apply the pseudorandom phase-only encoding to hybrid composite filter design. These filters are used in a filter bank architecture to perform intensity- and distortion-invariant pattern recognition.