John S. Loomis

Computer-Generated Holography And Optical Testing

A computer-generated hologram is a geometric pattern that can be used as a precise reference in an optical test. Computer-generated holograms can be used to make reference wavefronts that would be very difficult and expensive to make by other methods. This paper reviews the development of computer-generated holograms for optical testing. Various encoding methods are discussed. Examples are given to demonstrate some of the properties of such holograms. A review of interferometer design leads to a discussion of how the hologram functions as a part of the interferometer and of the limitations to the computer-generated hologram as used in optical testing.

Transform -ratio ternary phase -amplitude filter formulations for character recognition

Five types of discrete-valued correlation filters, including binary phase-only and ternary phase-amplitude types, were tested in simulations addressing specific character recognition problems. The filters were evaluated for classification accuracy and correlation efficiency. Transform-ratio filter ternary phase-amplitude filters provided the best classification performance but also the lowest correlation efficiency.

Wide-angle decentered lens beam steering for infrared countermeasures applications

A beam-steering system consisting of three cemented achro- matic doublets is presented. Intended for use in IR countermeasure ap- plications, our system is designed to operate over the 2- to 5-mm spec- trum with minimum angular dispersion. We show that dispersion can be minimized by using doublet lenses fashioned from AMTIR-1 and germa- nium. Our system is designed to be compact and lightweight, with no internal foci, while allowing steering to 622.5 deg. We also maintain a minimum 2-in. clear aperture for all steering angles, and a nominal di- vergence of 1 mrad. Plane wave and Gaussian beam analyses of our system are presented.

Cluster merging based on weighted mahalanobis distance with application in digital mammograph

A new clustering algorithm that uses a weighted Mahdlanobis distance as a distance metric to perform partitional clustering is proposed. The covariance matrices of the generated clusters are used to determine cluster similarity and closeness so that clusters which are similar in shape and close in Mahalanobis distance can be merged together serving the ultimate goal of automatically determining the optimal number of classes present in the data. Properties of the new algorithm are presented by examining the clustering quality for codebooks designed with the proposed method and another common method that uses Euclidean distance. The new algorithm provides better results than the competing method on a variety of data sets. Application of this algorithm to the problem of detecting suspicious regions in a mammogram is discussed.

Optimal reconstruction of missing-pixel images

A basis-function technique for reconstructing images with missing pixels is described. This technique yields optimal reconstructed image smoothness in that each basis-function width is maximized consistent with an acceptable level of computational effort.

Design of a real-time unskeletonized autostereoscopic display system enabled by new technology

The design of a 3D display system that is simultaneously autostereoscopic, look-around, raster-filled, and dynamic and that is enabled by new digital micromirror device technology is discussed.

Aberration production using a high-resolution liquid-crystal spatial light modulator

Phase-only liquid-crystal spatial light modulators can provide a powerful means of wavefront control. With high resolution and diffractive (modulo 2 π) operation, they can accurately represent phase maps with a large dynamic range. Because of this, they are an excellent means of producing electrically controllable, dynamic, and repeatable aberrations. However, proper calibration is critical in achieving accurate phase maps. Consequently, several calibration methods from previous literature were considered. With simplicity and accuracy in mind, we selected one method for each type of necessary calibration and augmented one of the methods with a new step. Then using our preferred calibration methods, we evaluated the performance of the spatial light modulator in the laboratory. We studied various phase maps using interferometry and atmospheric aberrations using a Shack- Hartmann wavefront sensor. All of these measurements show good agreement with theoretical expectations.

Camera principal point estimation from vanishing points

Calibration is a fundamental task in computer vision and photogrammetry. One of the most important intrinsic camera parameters is the principal point, which is the intersection between the optical axis and the image plane. The proposed method, which uses simple properties of vanishing points, provides a new technique for accurate identification of the principal point. It is done independently of all the other camera parameters. Checkerboard image corner points are located as saddle points, and the Hough transform is applied to remove spurious points and group them into rows and columns. The vanishing point for the columns lie on a horizon line. A group of images, created by rotating the checkerboard while holding the camera stationary, is processed in this manner to create multiple vanishing points on the same horizon line. A perpendicular line to the horizon is then projected back through the principal point. Repeating this for several image groups, corresponding to different camera orientations, allows for accurately locating the principal point as the intersection of the perpendiculars. Our approach does not need any prior information about the cameras being used, and does not require any manual user interaction. Experiments to evaluate the performance of this approach on real test images indicates that the uncertainty for the principal point location overlaps and is smaller than the region found by Bougets toolbox.

Accurate Estimation of Principal Point Using Three Mutually Orthogonal Horizon Lines

Accurate estimation of the principal point (PP) is critical for many camera calibration applications. Taking the PP as the intersection of the optical axis with the image plane, it is possible to identify this location as the orthocenter of three orthogonal vanishing points. This paper presents a method for accurately identifying the three vanishing points as the intersections of three horizon lines. The technique utilizes groups of images of a checkerboard test pattern collected from three orthogonal planes. Each group consists of images of the checkerboard rotated to different positions in the same plane, which can be used to identify a single horizon line. This is achieved by locating checkerboard corner points as saddle points, and using a Hough transform to group them into rows and columns. The vanishing points generated from the rows and columns lie along the same horizon line. Applying this technique to the rotated images within the group allows accurate estimation of the horizon line. Repeating this for image groups on three orthogonal planes creates horizon lines that effectively intersect at three orthogonal vanishing points, allowing for identification of the PP. The advantage of this technique is that it indirectly finds the three orthogonal vanishing points using horizon lines that are accurately found using fits to multiple vanishing points. Experiments with this technique indicate that it significantly reduces the error in finding the PP.

System performance of PSI technique as a function of key parameters

Phase stepped interferometry (PSI) technique uses a charge coupled device (CCD) camera for intensity image acquisition and allow for near real-time full field object displacement measurements. Since this technique deals with nanometer level displacements, accuracy is a big factor for this method. System parameters of this technique control the accuracy of the measurement and hence determine the systems performance. In this paper an effort is made to explain the characteristics of the key parameters and their role in determining systems performance. The optimum value of the parameters have been chosen after a careful observation. A simulated result has also been presented using the valve of the parameters in order to show the importance of key parameters on systems performance.