This PDF file contains the editorial “Tracing Rays” for OE Vol. 48 Issue 06

Face clustering is gaining ever-increasing attention due to its importance in optical image processing. Because traditional clustering methods do not specify the particular characters of the face image, they are not suitable for face image clustering. We propose a novel approach that employs the trace ratio criterion and specifies that the face images should be spatially smooth. The graph regularization technique is also applied to constrain that nearby images have similar cluster indicators. We alternately learn the optimal subspace and the clusters. Experimental results demonstrate that the proposed approach performs better than other learning methods for face image clustering.

We address the problem of unequal error protection (UEP) for SNR enhancement layer network abstraction layer (NAL) units of scalable video coding extension of H.264/AVC standard over wireless packet-erasure channel. We develop a UEP scheme by jointly selecting SNR NAL units and allocating unequal amounts of protection to selected NAL units for every group of pictures in the sequence. A simple heuristic algorithm is proposed to quickly derive the protection pattern. Experimental results demonstrate the proposed UEP scheme provides significant error resilience.

Finding good new local minima in the merit function landscape of optical system optimization is a difficult task, especially for complex design problems where many minima are present. Saddle-point construction (SPC) is a method that can facilitate this task. We prove that, if the dimensionality of the optimization problem is increased in a way that satisfies certain mathematical conditions (the existence of two independent transformations that leave the merit function unchanged), then a local minimum is transformed into a saddle point. With SPC, lenses are inserted in an existing design in such a way that subsequent optimizations on both sides of the saddle point result in two different system shapes, giving the designer two choices for further design. We present a simple and efficient version of the SPC method. In spite of theoretical novelty, the practical implementation of the method is very simple. We discuss three simple examples that illustrate the essence of the method, which can be used in essentially the same way for arbitrary systems.

A long-wave infrared division of amplitude imaging Stokes polarimeter is presented. For the first time, to our knowledge, application of microbolometer focal plane array (FPA) technology to polarimetry is demonstrated. The sensor utilizes a wire-grid beamsplitter with imaging systems positioned at each output to analyze two orthogonal linear polarization states simultaneously. Combined with a form birefringent wave plate, the system is capable of snapshot imaging polarimetry in any one Stokes parameter (S₁, S₂, or S₃). Radiometric and polarimetric calibration procedures for the instrument are provided, and the reduction matrices from the calibration are compared to rigorous coupled wave analysis and ray-tracing simulations. Image registration techniques for the sensor are discussed, and data from the instrument are presented, demonstrating the ability to measure intensity variations corresponding to polarized emission in natural environments. As such, emission polarimetry can be exploited at significantly reduced cost, sensor size, and power consumption over instruments based on more expensive mercury-cadmium telluride FPAs.

A simple method is reported to manufacture a planar compound eye using a microlens array. The compound eye, inspired by insects, consists of a microlens array and a waveguide coupled with it. A microlens array with lenses of 50 µm in diameter is fabricated by melting AZ1500 photoresist and then transferring it onto SU-8. With the self-focus method applied, a waveguide array is formed, and each is exactly coupled to a lens. The formation of the waveguide is simulated using finite difference time domain (FDTD) arithmetic, resembling the ommatidia produced in our experiment. The ommatidia is also testified to astrict beam, just as the natural compound eyes do.

A glancing-incidence interferometric test for large x-ray mirror mandrels, using two computer-generated holograms (CGHs), is described. The two CGHs are used to form a double-pass glancing-incidence system. One layout of the CGH-cavity glancing-incidence interferometer is studied and factors relating to its design are analyzed. A semianalytical expression for the phase function is derived that avoids high-order ripple errors and improves the efficiency of the CGH pattern generation. A system of coarse and fine fiducials for establishing the CGH-cavity and aligning the tested mandrel is discussed. Compared with traditional measurement devices, such as coordinate measuring machines (CMMs) or other scanning metrology systems, the CGH-cavity glancing-incidence interferometer can measure a mandrel surface without mechanical contact, at a high spatial sampling frequency, with lower uncertainty, and at high speed.

It has been experimentally demonstrated that a three-dimensional axisymmetric-folded combination carbon dioxide laser with five discharge tubes can offer a desired output power and good-quality light beam. The output laser has five sub-beams that start from a mutual point on the inside surface of the output plane lens. The collimation and convergence of the five sub-beams are obtained. In our results, the measured output laser power from a folded resonator was 37 W, and the total output of the laser was 76 W with 10% efficiency.

A three-wavelength method is developed to interrogate an extrinsic Fabry-Pérot interferometer (EFPI) for the measurement of a dynamic measurand. Three signals 120 deg out of phase are created by using three fiber Bragg gratings (FBGs). A phase demodulator, which is the same as that used for demodulating a 3×3 coupler-based interferometer, is used to measure the phase change. A minimum detectable phase of 0.43 μrad/Hz^1/2 is achieved.

We present an all optical approach allowing the realization of microscale logic exclusive or (XOR) gate based on special interferometer configuration. The operation device includes two close optical waveguides realized on a silicon chip. A thin metallic layer of a few nanometers that separates the two waveguides causes a relative phase shift of between them. The structure performs interference of the two input beams while demonstrating XOR logic gate operation at the amplitude of the resulted output. The logic operation is realized after small propagation distance of <1.8 μm.

An all-optical upstream carrier generation scheme based on a Brillouin/erbium fiber laser (BEFL) is proposed for wavelength-independent optical network units in a wavelength-division multiplexed passive optical network. Experimental measurements show that the BEFL mainly operates in a single longitudinal mode and occasionally undergoes mode hopping given environmental fluctuations. Employing the proposed scheme, bidirectional transmission at 1.25-Gb/s over 20-km single-mode fiber is performed. Bit error rate measurement shows no crosstalk originating from Rayleigh backscattering and back-reflections in the network, due to a 0.08-nm-wavelength shift between the downstream and upstream carriers.

A novel all-optical orthogonal frequency division multiplexing (AO-OFDM) scheme is proposed and demonstrated. Ultrashort optical pulses are used as samples for optical discrete Fourier transform (DFT) and the inverse DFT process. Different subcarrier channels can be parallelly processed by fiber Bragg gratings. A 20-Gb/s two subcarrier AO-OFDM experiment is carried out with narrowband filtering and optical cyclic postfixes (CP) inserted. Experimental results show that this scheme has good spectral efficiency. Furthermore, the received signals have better eye diagrams and bit error rate performance with the help of CP. This scheme can be used in high-speed optical transmission systems.

The availability of free space optics (FSO) systems in dependence on weather conditions and on FSO link parameters, such as transmitted optical power, beam divergence, receiver sensitivity or link path distance, is discussed. A number of phenomena in the atmosphere, such as absorption, scattering, and turbulence, can affect beam attenuation, but in the case of wavelengths typical of FSO systems operation, only scattering and turbulence are appropriate to be taken into consideration. We model the power loss caused by turbulence by using the Rytov scintillation theory. Attenuation due to scattering, which can be expressed as a function of the link distance, wavelength, and meteorological visibility, is calculated from visibility data collected at several airports in Europe. Statistical evaluation of the attenuation caused by scattering and the power link margin calculated from FSO link parameters are used for calculating the link availability.

The Key Laboratory of Ocean Remote Sensing of the Ministry of Education of China, Ocean University of China, has developed the first mobile Doppler wind lidar in China. As an important component of meteorological services for the Good Luck Beijing 2007 Qingdao International Regatta, the mobile Doppler wind lidar was used to measure the sea surface wind (SSW) with 100 m^*100 m spatial and 10-min temporal resolution in Qingdao from 15 to 23 August 2007. We present the results from two aspects of this campaign. First, the lidar was operated in the fixed-direction mode and compared to SSW simultaneously measured by a collocated buoy. Second, we present lidar wind measurements throughout the regatta and show good agreement with the match situation of the International Regatta. In addition, we present a case study, accounting for the observation of sailboats stopped by the headwind. With considerable data accumulated, we have shown that the mobile Doppler wind lidar can indeed provide near real-time SSW in support of the sailing games. The lidar has also provided meteorological services for the 2008 Olympic sailing games from 8 to 22 August and Paralympics Sailing Games from 8 to 13 September 2008 in Qingdao.

A measure for objectively assessing the performance of color image fusion methods is proposed. Two different aspects are considered in establishing the proposed measure-namely, the amount of common information between the source images and the final fused image as well as the distribution of color information in the final image in order to achieve optimal color representation. Mutual information and conditional mutual information are employed in order to assess information transfer between the source images and the final fused image. Simultaneously, the distribution of colors in the final image is explored by means of the hue coordinate in the perceptually uniform CIELAB space. The proposed measure does not depend on the use of a target-fused image for the objective performance evaluation. It is employed experimentally for objective evaluation of fusion methods in the cases of medical imaging and night vision data.

In this paper, a new intrafield deinterlacing algorithm with an edge-direction vector (EDV) in the image block is introduced. The EDV, which is computed by a Sobel mask utilized edge map, was first introduced so that finer resolution of the edge direction could be acquired. The Sobel operator is performed on the "top and bottom" or "left and right" adjacent pixels to detect the most likely edge direction of the missing pixel. The proposed EDV-oriented deinterlacing system operates by identifying small pixel variations at five orientations, 26.5, 45, 90, 135, and 153.5 deg. The EDV values work as inputs of the S-type small membership function, and the weight values were obtained for each edge direction. Based on the fuzzy rule and edge-direction confidence parameter, the missing pixels were computed. These weight values were multiplied by the candidate deinterlaced pixels to successively build approximations of the deinterlaced sequence. The results of computer simulations demonstrated that the proposed method outperforms a number of previously documented intrafield deinterlacing methods.

A novel optical security technique for safeguarding user passwords based on an optical fractal synthesizer is proposed. A validating experiment has been carried out. In the proposed technique, a user password is protected by being converted to a fractal image. When a user sets up a new password, the password is transformed into a fractal pattern, and the fractal pattern is stored in authority. If the user is online-validated, his or her password is converted to a fractal pattern again to compare with the previous stored fractal pattern. The converting process is called the fractal encoding procedure, which consists of two steps. First, the password is nonlinearly transformed to get the parameters for the optical fractal synthesizer. Then the optical fractal synthesizer is operated to generate the output fractal image. The experimental result proves the validity of our method. The proposed technique bridges the gap between digital security systems and optical security systems and has many advantages, such as high security level, convenience, flexibility, hyper extensibility, etc. This provides an interesting optical security technique for the protection of digital passwords.

We use nonlinear composite filters in object recognition, even when they have rotation, scale, noise, and illumination distortions. We generated 936 images of the letters E, F, H, P, and B. The images consisted of these letters scaled from 70% to 130% and rotated 360 deg. The maximum number of images supported by these filters was determined by a numerical experiment. Considering a system confidence level of at least 80%, the maximum number of images is around 216. We found a "rotation problem" when the filter contained the letter rotated 360 deg, since circles were artificially introduced, and this creates complications when working with images that also have circles in their spectrum. Due to this, we propose a segmented filter that breaks the circular symmetry. Experiments where carried out in order to find the noise tolerance of each filter, and the use of Spearman's rank correlation (in conjunction with the nonlinear method, SNM) is proposed in order to increase that tolerance. We also made an assessment of the impact that illumination changes had in the correlation output, in the problem image, and we propose the use of SNM to obtain illumination invariance. We tested these filters with two real-life problems; nonlinear composite filters can recognize the target in the presence of distortions.

We propose a method to detect a face with different poses under various environments. On the basis of skin color information, skin regions are first extracted from an input image. Next, the shoulder part is cut out by using shape information and the head part is then identified as a face candidate. For a face candidate, a set of geometric features is applied to determine if it is a profile face. If not, then a set of eyelike rectangles extracted from the face candidate and the lighting distribution are used to determine if the face candidate is a nonprofile face. Experimental results show that the proposed method is robust under a wide range of lighting conditions, different poses, and races. The detection rate for the HHI face database is 93.68%. For the Champion face database, the detection rate is 95.15%.

We present an approach to integrate a preprocessing step of the region of interest (ROI) localization into 3-D scanners (laser or stereoscopic). The definite objective is to make the 3-D scanner intelligent enough to localize rapidly in the scene, during the preprocessing phase, the regions with high surface curvature, so that precise scanning will be done only in these regions instead of in the whole scene. In this way, the scanning time can be largely reduced, and the results contain only pertinent data. To test its feasibility and efficiency, we simulated the preprocessing process under an active stereoscopic system composed of two cameras and a video projector. The ROI localization is done in an iterative way. First, the video projector projects a regular point pattern in the scene, and then the pattern is modified iteratively according to the local surface curvature of each reconstructed 3-D point. Finally, the last pattern is used to determine the ROI. Our experiments showed that with this approach, the system is capable to localize all types of objects, including small objects with small depth.

A palmprint identification system recognizes a query palmprint image by searching for its nearest neighbor from among all the templates in a database. When applied on a large-scale identification system, it is often necessary to speed up the nearest-neighbor searching process. We use competitive code, which has very fast feature extraction and matching speed, for palmprint identification. To speed up the identification process, we extend the cover tree method and propose to use a set of cover trees to facilitate the fast and accurate nearest-neighbor searching. We can use the cover tree method because, as we show, the angular distance used in competitive code can be decomposed into a set of metrics. Using the Hong Kong PolyU palmprint database (version 2) and a large-scale palmprint database, our experimental results show that the proposed method searches for nearest neighbors faster than brute force searching.

A chessboard image can be used as a pattern for camera calibration and pose estimation. We propose an algorithm for the automatic detection of a chessboard using one circle as a probe. We first generate candidates using the Lucas-Kanade feature detector algorithm. Finally, we find control points on the chessboard using properties inherent on a chessboard pattern. We suggest that a distinct edge along the border line and black and white sequence on the chessboard can be used as constraints. A circle is used as a probe for checking those properties. We show that the proposed algorithm can be used successfully in camera calibration and pose estimation. Experimental results using images acquired under the variation of scale, pose, and illumination shows the robustness of proposed algorithm.