Biometric EncryptionTM is an algorithm which has been developed to securely link and retrieve a digital key using the interaction of a biometric image, such as a fingerprint, with a secure block of data, known as a BioscryptTM. The key can be used, for example, as an encryption/decryption key. The BioscryptTM comprises a filter function, which is calculated using an image processing algorithm, and other information which is required to first retrieve, and then verify the validity of, the key. The key is retrieved using information from an output pattern formed via the interaction of the biometric image with the filter function. Therefore, the filter function must be designed so that it produces a consistent output pattern (and thus, key). The filter function must also be designed to be secure (i.e. information about the fingerprint cannot be retrieved from the filter function). The consistency of the output pattern and the security of the filter function are the two topics discussed in this paper.
We present the derivation of an optimal correlation filter for fingerprint verification. The filter comprises multiple versions of the system user's fingerprint (i.e. it is a composite filter). Also, the characteristics of the filter can be adjusted so that its performance in a correlator is similar to that of a matched-filter or an inverse-filter, or some compromise between the two. It is these attributes that make this filter structure attractive for the task of fingerprint verification. The composite nature of the filter offers distortion tolerance by encompassing several different versions of the fingerprint image, while the tailored characteristics of the filters allows us to produce output correlation planes that can easily be processed. The filter was developed using a 'standard' database, with the objective of separating the two classes of input to the system: 'legitimate users' and 'attackers.' Specifically, the filter is optimized to minimize the probability of error (i.e. misclassification of user). Both the design and the implementation of the optimal fingerprint filter are covered in this paper.
A real-time, VanderLugt-type optical correlator with a single SLM has been developed. A field programmable gate array was used to capture and process images obtained from a CCD camera at a rate of 60 video fields per second. During both enrollment and verification, a finger slides over a glass prism and is input to the system via the frustration of total internal reflection. An auto-enrollment procedure captures the optimal image during each slide. An optimal composite filter is implemented. The correlation detection process comprises real-time tracking of the correlation peak while the finger is sliding, and a decision process based on projective decision boundaries. Real-life tests yielded a false rejection rate of 1% and a false acceptance rate of 0.2%.
Biometric EncryptionTM is an algorithm which has been developed to securely link and retrieve a digital key using the interaction of a biometric image, such as a fingerprint, with a secure block of data, known as a BioscryptTM. The key can be used, for example, as an encryption/decryption key. The Bioscrypt comprises a stored filter function, produced by a correlation-based image processing algorithm, as well as other information which is required to first retrieve, and then verify the validity of, the key. The process of securely linking a key with a biometric is known as enrollment, while the process of retrieving this key is known as verification. This paper presents details of the enrollment and verification procedures.
Liquid crystal televisions (LCTVs) continue to be used as inexpensive spatial light modulators (SLMs) in coherent optical processing systems. The LCTVs are generally manufactured using twisted-nematic liquid crystal materials. The operating curve of a twisted-nematic liquid crystal television can be varied by adjusting the orientations of the polarizer and analyzer relative to the molecular structure of the liquid crystal cell. The usefulness of the LCTV as a SLM in an optical processing system critically depends on the operating curve selected. This is particularly so when the SLM is used in the filter plane of a coherent optical correlator. We present here several criteria that can be used to evaluate the usefulness of an operating curve for a filter-plane SLM.
We present an experimental procedure for the determination of the physical properties of an arbitrary twisted-nematic liquid crystal cell. No assumptions are made about the physical properties of the cell, because the commercially available devices are generally produced under proprietary conditions. The techniques developed were then used to evaluate the Jones matrix of an Epson liquid crystal television, and reasonable agreement is observed between theory and experiment. Knowledge of the Jones matrix for these devices helps to evaluate their performance as spatial light modulators in optical processing systems, and such applications are discussed. It also provides an effective means for determining the spatial quality of the liquid crystal layer. We find that there is significant variation in the birefringence across this cell, and the influence that this has on the use of the cell in optical processing systems is discussed.
Twisted-nematic liquid crystal televisions (LCTVs) are currently widely used as spatial light modulators in coherent optical processing systems. With such devices, `phase-' or `intensity-only' modulation is not easily achieved. In this paper we present procedures for finding some specific operating modes that can be achieved with these commercial LCTVs. The operating modes were found both by studying the Lu and Saleh model of LCTVs, and by referring to previous laboratory measurements. We will introduce a simple experimental procedure that can be used to locate a phase-mostly operating mode. The operating modes are considered in the context of a coherent optical correlator.
We desire to have a joint transform correlator track features in the image of a human retina. Previous binarized digital methods indicated unacceptable limitations in tracking through torsion motions of the eye. To create an extended range of response to eyeball rotation we tried several methods of processing the reference image. We compared laboratory measurements with digital simulations. Based on small statistics and our noiseless models, the results disagree; the digital method has less range, and the optical method has sufficient range (+/- 5 degree(s)) for our purpose.
The complex transmittance of an Epson liquid crystal television (LCTV) is determined as a function of the video drive signal. Several different operating configurations of the LCTV are established, and their usefulness as operating modes for spatial light modulators in the input and filter planes of a hybrid correlator is discussed. A high-stability phase measurement scheme is developed to determine the operating curves, and this system is presented. This measurement scheme is designed so that a fully characterized LCTV could be assuredly moved into the optical correlator. This allows filters to be calculated for the correlator using the exact physical action of the modulators. The transient response of the LCTV to field-rate changes in the video signal is also presented. It is observed that the switching speed of the liquid crystal molecules is a fundamental limitation on the operating speed of these devices.
The classical matched filter used as the basis for optical correlation filters requires a spatial light modulator (SLM) which can effect fully complex modulation. Practical (single) SLMs cannot achieve this modulation, but many can be made to operate in a `phase-mostly' or `amplitude-mostly' mode. Successful correlators have been constructed using phase-mostly SLMs in the filter plane by disregarding the usually small coupling of amplitude. During the last few years, new theories have been developed on the construction of filters for coupled modulators for a variety of metrics. The liquid crystal cell modulator, which can be operated in a variety of coupled modes, presents a unique opportunity to implement these algorithms. This paper reports on such laboratory measurements.
The complex transmittance of an Epson liquid crystal television was determined, as a function of the video drive signal. Several different operating configurations of the LCTVs were established, and their usefulness as operating modes for spatial light modulators in the input- and-filter-planes of a hybrid correlator is discussed. A high-stability phase measurement scheme was developed to determine the operating curves, and this system is presented. The transient response of the liquid crystal television to field-rate changes in the video signal is also presented. It is observed that the switching speed of the liquid crystal molecules presents an obstacle to the operating speed of these devices.