This PDF file contains the editorial “The 1.5-Liter Merit Function” for OE Vol. 41 Issue 10

Viewing-angle-enhanced integral 3-D imaging using double display devices with masks is proposed. In this approach, the total system consists of two integral imaging systems with masks, and the integrated images from each system are combined spatially. By using double display devices with masks instead of time-multiplexed images, this scheme does not require the after-image effect or mechanical moving parts. The viewing area where an observer can see the 3-D image without elemental image overlapping is remarkably enhanced by this method.

A new way to integrate two different lasers (LD1: 950 nm and LD2: 990 nm) in a monochip by one-step epitaxial growth is presented. These two lasers are cascaded by a high-doping tunnel junction that separates the two active regions so that a four-terminal device can be implemented, which permits the operation of the lasers electrically independent as LD1, LD2, and LD1+LD2 after fabrication process. High power dual-wavelength operation laser diodes are fabricated, output power as high as 3.1 W at 3 A and 2.4 W at 2 A is obtained, and the slope efficiency can reach 1.38 W/A. As for four-terminal integrated laser diodes, the threshold current is 54, 58, and 65 mA, and the slope efficiency is 0.73, 0.48, and 0.37 for the LD1+LD2, LD1, and LD2, respectively.

We investigate the use of optical systems consisting of either one or two refractive microlens arrays, a collimating and Fourier transforming lens for LED beam homogenization. Due to the small lenslet diameters, diffraction can lead to significant deviations from the desired homogene intensity distribution. Therefore, we analyze the two setups applying scalar diffraction theory and explain the occurring effects using numerical calculations. Our theoretical calculations are supported by experimental results.

We find that the distribution of the spectral correction error on a spectrally corrected CCD array is dependent on the choice of both the correction filter and the lens angle of view. The distribution of the spectral correction error on the spectrally corrected CCD array is analyzed. We present a method of calculating of the spectral coefficient of the monochromatic transmission factor of the filter for CCD array spectral correction. A filter calculated in this manner ensures that the corrected CCD array has a known constant and high spectral accuracy throughout the whole surface of the array.

An alternative technique for rapid prototyping of diffractive optical elements (DOEs) for use with mid-infrared high-power lasers is proposed. The solution is based on the photoablation of an intermediate resist layer on various substrates using an excimer laser in a micromachining station, followed by classical etching of the substrate to form the DOE. Being able to achieve rapid prototyping also depends on having available a rapid, precise, nondestructive profilometry technique in order to optimize each stage of the manufacturing process. After comparing several techniques for structural characterization, we found that coherence probe microscopy was the best one suited for the rapid and precise measurement of the squareness, depth, and surface roughness of the transparent resist mask and the substrate features. Several results of the characterization of transparent resist layers and etched substrates are given. The technique of rapid prototyping by the means proposed is illustrated with the fabrication of a DOE for direct marking of an industrial logo using a high-power CO₂ laser.

This paper introduces the concept of a contrast budget for a head-mounted display, i.e., a list of contrast values of separate contrast-degrading phenomena in the system. The basic phenomena are dynamic range, diffraction, scattered light, and ghost images. By calculating a contrast budget it becomes clear whether or not the display-system contrast will meet specified criteria and which factors contribute the most to the degradation of contrast in the system. An example of a contrast budget is given for the NomadTM head-mounted display system. In this system dynamic range, scanner scattering, and beamsplitter stray light are the dominant contributors, but the overall contrast is comfortably above specification.

A new method for achieving a color electroholographic display has been developed and tested with promising results. In this method, holographic techniques that use a single white light source instead of the traditional three color lasers are employed. A metal halide lamp is used with each of the three color components required for reconstruction, which are extracted using cold mirrors. The separated color images are formed on three liquid-crystal display panels and are combined by a prism to form a single 3-D image. Image position is controlled by a focal adjustment method. Compared with the usual reconstruction methods that use three-color lasers, this new single-white-light approach appears to have improved color clarity.

A simple method to suppress the zero-order diffraction in the reconstructed image of digital holography is presented. In this method, the Laplacian of a detected hologram is used instead of the hologram itself for numerical reconstruction by computing the discrete Fresnel integral. This method can significantly improve the image quality and give better resolution and higher accuracy of the reconstructed image. The main advantages of this method are its simplicity in experimental requirements and convenience in data processing.

Advanced enterprise servers require dense packaging of their fiber optic input/output (I/O) interconnections. This can be accomplished using small-form-factor (SFF) optical connectors and transceivers such as the MT-RJ and LC. Previously, we presented a performance comparison between some of the major types of multimode SFF connectors; at the time, none of these designs was mature enough to offer equivalent performance to established connectors or to support a single-mode interface. We extend the previous work to include recent design enhancements in multimode connectors, as well as the first performance comparison between single-mode SFF connectors. Fiber optic jumper cables are tested using industry standard test procedures and bench marked against the industry standard SC Duplex connector. Initial loss data as well as stress testing are performed. Variations are found in the performance of the connectors types, with the mechanical stress tests of axial and off-axial pull being the most demanding application tests. Because SFF connectors are more susceptible to pull forces, we describe a new type of strain relief developed for high-density I/O interconnects. These connectors demonstrated for the first time design points that are suitable to replace the SC Duplex interface in both multimode and single-mode enterprise system applications.

A method of stretching optical fiber holds interest for measuring strain in smart structures where the physical displacement may be used to tune optical fiber lasers. A small, lightweight, tunable fiber laser is ideal for demodulating strain in optical fiber Bragg gratings (FBGs) attached to smart structures such as the reusable launch vehicle that is being developed by NASA. A method is presented for stretching optical fibers using the thin-layer composite unimorph ferroelectric driver and sensor (THUNDER) piezoelectric actuators invented at NASA Langley Research Center. THUNDER actuators use a piezoelectric layer bonded to a metal backing to enable the actuators to produce displacements larger than the unbonded piezoelectric material. The shift in reflected optical wavelength resulting from stretching the FBG is presented. Means of adapting THUNDER actuators for stretching optical fibers are discussed, including ferrules, ferrule clamp blocks, and plastic hinges made with stereo lithography.

We investigate the wavelength selection of dual fiber grating sensors for strain and temperature measurements. Temperature and strain coefficients of fiber gratings are measured in the wavelength range of 600 to 1600 nm in Corning Flexcor-980^TM fiber. Theoretical fits are estimated using Ghosh's temperature-dependent refractive index model for optical fiber glasses. Moreover, the effect of apparent thermal strain due to the structural thermal expansion on the temperature coefficient in a surface-bonded fiber grating sensor is investigated, and numerical results for the accuracy of temperature and strain are presented. It is shown that the structural thermal properties have a large influence on the resolution and measurement accuracy of dual fiber grating sensors.

The interaction between random birefringence and the Kerr effect is investigated numerically with dispersion-managed (DM) solitons. In zero-averaged-path DM optical fiber links, because linear dispersion is fully compensated, the Kerr effect becomes a detrimental residual factor to the transmission of DM soliton and results in a narrower optical pulse. However, under the proper conditions, an optical pulse can propagate better through the balance between random birefringence and the Kerr effect. After optimally mapping the optical pulse power and birefringence parameter of an optical fiber, the polarization-mode dispersion resulting from random birefringence can essentially be balanced by the Kerr effect. In addition, considering polarization-dependent losses, we find that the pulse width's fluctuation and broadening are very large even with little loss difference in the two polarization components.

A measurement procedure developed for quarter-pitch gradient index lenses is presented. It is based on the use of a polarization interferometer and a waveguide refractometer. The use of a commercial waveguide refractometer allows the identification of the refractive index profile associated with each polarization mode and the selection of the reference refractive index profile. Using a conoscopic setup and the reference refractive index profile (radial polarization mode), the refractive index profile of the tangential polarization mode is determined. The measurements performed with the polarization interferometer are especially valuable on axis, while the waveguide refractometer provides a higher precision at the edge of the cylindrical sample.

Block truncation coding (BTC) is a successful image compression technique due to its simple and fast computational burden. The bit rate is fixed to 2.0 bits/pixel, whose performance is moderate in terms of compression ratio compared to other compression schemes such as discrete cosine transform (DCT), vector quantization (VQ), wavelet transform coding (WTC), etc. Two kinds of overheads are required for BTC coding: bit plane and quantization values, respectively. A new technique is presented to reduce the bit plane overhead. Conventional bit plane overhead is 1.0 bits/pixel; we decrease it to 0.734 bits/pixel while maintaining the same decoded quality as absolute moment BTC (AMBTC) does for the "Lena" image. Compared to other published bit plane coding strategies, the proposed method outperforms all of the existing methods.

We propose three correlation-based methods to simultaneously detect the centroids of multiple objects in an input scene. The first method is based on the modulus of the moment function, the second method is based on squaring the moment function, and the third method works with a single intensity filter. These methods are invariant to changes in the position, orientation, and scale of the object and result in good noise-smoothing performance. We use spatial light modulators (SLMs) to directly implement the input of the image and filter information for the purpose of these approaches. We present results showing simulations from different approaches and provide comparisons between optical-correlation- and digital-moment-based methods. Experimental results corresponding to an optical correlator using SLMs for the centroid detection are also presented.

An efficient method for two-level thresholding is proposed based on the Bayes formula and the maximum entropy principle, in which no assumptions of the image histogram are made. An alternative criterion is derived based on maximizing entropy and used for speeding up the searching algorithm. Five forms of conditional probability distributions—simple, linear, parabola concave, parabola convex, and S-function—are employed and compared to each other for optimal threshold determination. The effect of precision on optimal threshold determination is discussed and a trade-off precision = 0.001 is selected experimentally. Our experiments demonstrate that the proposed method achieves a significant improvement in speed from 26 to 57 times faster than the exhaustive search method.

Loss of resolution due to image blurring is a major concern in electron microscopy. The point spread function describing that blur is generally unknown. We discuss the use of a recently developed fast Fourier transform (FFT)-based direct (noniterative) blind deconvolution procedure, the APEX method, that can process 512×512 images in seconds of CPU time on current desktop platforms. The method is predicated on a restricted but significant class of shift-invariant blurs, consisting of finite convolution products of heavy-tailed Lévy probability density functions. Such blurs considerably generalize Gaussian and Lorentzian point spread functions. The method is applied to a variety of original scanning electron microscopy (SEM) micrographs and is shown to be useful in enhancing and detecting fine detail not otherwise discernible. Quantitative sharpness analysis of "ideal sample" micrographs shows that APEX processing can actually produce sharper imagery than is achievable with optimal microscope settings.

We develop a regularized mixed-norm image restoration algorithm to deal with various types of noise. A mixed-norm functional is introduced, which combines the least mean square (LMS) and the least mean fourth (LMF) functionals, as well as a smoothing functional. Two regularization parameters are introduced: one to determine the relative importance of the LMS and LMF functionals, which is a function of the kurtosis, and another to determine the relative importance of the smoothing functional. The two parameters are chosen in such a way that the proposed functional is convex, so that a unique minimizer exists. An iterative algorithm is utilized for obtaining the solution, and its convergence is analyzed. The novelty of the proposed algorithm is that no knowledge of the noise distribution is required, and the relative contributions of the LMS, the LMF, and the smoothing functionals are adjusted based on the partially restored image. Experimental results demonstrate the effectiveness of the proposed algorithm.

A Fourier-based solution to the problem of figure-ground segmentation in short baseline binocular image pairs is presented. Each image is modeled as an additive composite of two component images that exhibit a spatial shift due to the binocular parallax. The segmentation is accomplished by decoupling each Fourier component in one of the resultant additive images into its two constituent phasors, allocating each to its appropriate object-specific spectrum, and then reconstructing the foreground and background using the inverse Fourier transform. It is shown that the foreground and background shifts can be computed from the differences of the magnitudes and phases of the Fourier transform of the binocular image pair. While the model is based on translucent objects, it also works with occluding objects.

We describe the experimental setup of a multispectral color image acquisition system consisting of a professional monochrome CCD camera and a tunable filter in which the spectral transmittance can be controlled electronically. We perform a spectral characterization of the acquisition system taking into account the acquisition noise. To convert the camera output signals to device-independent color data, two main approaches are proposed and evaluated. One consists in applying regression methods to convert from the K camera outputs to a device-independent color space such as CIEXYZ or CIELAB. Another method is based on a spectral model of the acquisition system. By inverting the model using a principal eigenvector approach, we estimate the spectral reflectance of each pixel of the imaged surface.

Focusing schlieren with white light extended sources, an optical processing technique used to visualize phase images, is mathematically formalized in terms of Fourier optics and therefore classified as a typical procedure of optical image processing. The visualization is obtained by completely or partially stopping the image of the light source. In these two situations, the relation between the resulting image intensity and the phase that must be visualized is described by different equations. In the first case, the result is similar to that obtainable by the strioscopy (dark-field technique) operated in coherent optics; in the second case, the result resembles that of the coherent schlieren. The mathematical treatment is carried out using an extended white light source codified by a grid: the filter is the negative of the source grid image, centered or shifted with respect to the image itself. Experimental phase images are visualized by means of the codified source and by an incandescent bulb.

Industrial plants and accreditation laboratories implementing quality systems, according to international standards ISO/IEC 17025, ISO/QS 9001-9004, and EN 45001-45003, are required to evaluate the uncertainty of measurements. Manufacturers of thermal cameras do not offer any data enabling estimation of the measurement uncertainty of these imagers. The difficulty in determining measurement uncertainty is a significant limitation on thermal cameras for applications in industrial plants and cooperating accreditation laboratories implementing these quality systems. In this paper a set of parameters for characterization of commercial thermal cameras, a description of measuring apparatus, some test results on such cameras, a mathematical model of the temperature measurement uncertainty, and software for its quick calculation are presented.

We present a two-step integral imaging system to obtain 3-D orthoscopic real images. By adopting a nonstationary micro-optics technique, we demonstrate experimentally the potential usefulness of two-step integral imaging.

Integral photography (IP) is one of the most attractive methods for autostereoscopic display. We experimentally study an IP system, adopting a commercial liquid-crystal display panel by testing several lens arrays that have different f numbers. The viewing angle in each case is calculated and measured. We show the limitation of the conventional lens array with respect to viewing angle, and suggest the method using an array of small-f-number aspheric Fresnel lenses to avoid aberration for wide viewing angles. The blur noise caused by the array is also discussed.

A better understanding of the fundamental principles of photon migration in highly scattering media is of great importance for developing new methods for disease diagnosis, imaging, and tomography. In this paper we present a most-favorable-path approach to the problem of light propagation in turbid media, based on the diffusion equation. The most favorable path, on which photons will be found, can be obtained from the path of the net flux propagation. The diffusion intensity and the direction of the net flux can be calculated with the finite-element method, which can deal with heterogeneous problems and frequency-domain problems within complex geometry, and can introduce boundary effects as well. Simulations were conducted for phantoms with different optical parameters and source-detector separations. Experiments have been performed to verify this new method. The most favorable path is found to be more sensitive to the source-detector separation and the absorption coefficient of the medium than to the scattering coefficient. The feasibility of using the theory in optical tomography for the recognition of objects hidden in turbid media is discussed.

The vanadium dioxide (VO₂) thin film, usually employed as an infrared detector, exhibits hysteresis in its resistance-temperature characteristic. Considering a polycrystalline VO₂ thin film as a composite medium, containing semiconducting and metallic microcrystals, the well-known effective-medium approximation theory is employed to relate the volume fraction of the semiconducting microcrystals to the effective film resistance. A phenomenological model is first proposed for describing the hysteretic dependence of volume fraction on temperature. From this, a model for hysteresis in the resistance-temperature characteristic is then derived, and a procedure for estimating the model parameters is outlined. The model reproduces the more important hysteretic characteristics such as the major, minor, and nested loops, in good agreement with the experimental characteristics.

We demonstrate the generation of cw millimeter-wave signals in LN optical channel waveguides using optical technology. The spectral linewidth of the generated millimeter-wave signals is 2 kHz. The power of the measured cw millimeter-wave signals is almost in proportion to the power multiplication of two input optical signals. In this experiment, the frequency difference of two optical signals is 10 to 40 GHz, and the power of millimeter-wave signals showed variation within 4 dBm.

A new method for computing precise estimates of the motion vector field of moving objects in a sequence of images is proposed. Correspondence vector-field computation is formulated as a matching optimization problem for multiple dynamic images. The proposed method is a heuristic modification of dynamic programming applied to the 2-D optimization problem. Motion-vector-field estimates using real movie images demonstrate good performance of the algorithm in terms of dynamic motion analysis.

Optical sensors for noncontact velocity measurement require imaging systems with constant optical magnification in spite of the variable distance between the sensor and the object of measurement. This requirement can be met using telecentric systems. When such systems are set up, several design considerations must be taken into account. These concern the decision as to whether a single or a double telecentric system shall be used, as well as the choice of the optical magnification and the dimensioning of the aperture stop. The results of the theoretical analysis are confirmed by experiments with a prototype velocity sensor for belt drives.

An efficient one-step digit-set-restricted modified signed-digit (MSD) adder based on symbolic substitution is presented. In this technique, carry propagation is avoided by introducing reference digits to restrict the intermediate carry and sum digits to {1,0} and {0,1}, respectively. The proposed technique requires significantly fewer minterms and simplifies system complexity compared to the reported one-step MSD addition techniques. An incoherent correlator based on an optoelectronic shared content-addressable memory processor is suggested to perform the addition operation. In this technique, only one set of minterms needs to be stored, independent of the operand length.

In many imaging applications, it is highly desirable to replace mechanical beam-steering components (i.e., mirrors and gimbals) with a nonmechanical device. One such device is a nematic liquid crystal optical phased array (LCOPA). An LCOPA can implement a blazed phase grating to steer the incident light. However, when a phase grating is used in a broadband imaging system, two adverse effects can occur. First, dispersion will cause different incident wavelengths arriving at the same angle to be steered to different output angles, causing chromatic aberrations in the image plane. Second, the device will steer energy not only to the first diffraction order, but to others as well. This multiple-order effect results in multiple copies of the scene appearing in the image plane. We describe a digital image restoration technique designed to overcome these degradations. The proposed postprocessing technique is based on a Wiener deconvolution filter. The technique, however, is applicable only to scenes containing objects with approximately constant reflectivities over the spectral region of interest. Experimental results are presented to demonstrate the effectiveness of this technique.

A mask with periodic apertures imaging system is adopted very widely and plays a leading role in modern technology for uses such as pinhole cameras, coded imaging systems, optical information processing, etc. because of its high resolution, its infinite depth of focus, and its usefulness over a broad frequency spectra ranging from visible light to x rays and rays. While the masks with periodic apertures investigated in the literature are limited only to far-field diffraction, they do not take the shift of apertures within the mask into consideration. Therefore the derivation of the far-field diffraction for a single aperture cannot be applied to a mask with periodic apertures. The far-field diffraction formula modified for a multiaperture mask has been proposed in the past, the analysis remains too complicated to offer some practical guidance for mask design. We study a circular mask with periodic rectangular apertures and develop an easier way to interpret it. First, the near-field diffraction intensity of a circular aperture is calculated by means of Lommel's function. Then the convolution of the circular mask diffraction with periodic rectangular apertures is put together, and we can present a simple mathematical tool to analyze the mask properties including the intensity distribution, blurring aberration, and the criterion of defining the far- or near-field diffraction. This concept can also be expanded to analyze different types of masks with the arbitrarly shaped apertures.

A new design for a prism-based optoelectronic refractometer proposed earlier has been investigated. Experimental results are found to be in close agreement with theoretical calculations. For a given refractive index of the prism material, the divergence of the incident conical beam and angles of the prism determine the range of refractive index of the ambient over which the refractometer remains highly sensitive. Incident conical beams of higher divergences are found to enlarge this range. This refractometer is more robust and user-friendly than other such refractometers reported in the literature. Such a device can be useful for on-line measurements of refractive indices in process control systems.

A novel integrated intelligent pixel processor that combines the reception and transmission of video signals is a key component for future mobile communication systems. Such a processor requires an optical interface capable of simultaneous formation of real and virtual images. A composite lens system capable of such dual imaging is described.

We have designed, built, and flight-tested a new star camera for daytime guiding of pointed balloon-borne experiments at altitudes around 40 km. The camera and lens are commercially available, off-the-shelf components, but require a custom-built baffle to reduce stray light, especially near the sunlit limb of the balloon. This new camera, which operates in the 600- to 1000-nm region of the spectrum, successfully provides daytime aspect information of ≈10 arcsec resolution for two distinct star fields near the galactic plane. The detected scattered-light backgrounds show good agreement with the Air Force MODTRAN models used to design the camera, but the daytime stellar magnitude limit was lower than expected due to longitudinal chromatic aberration in the lens. Replacing the commercial lens with a custom-built lens should allow the system to track stars in any arbitrary area of the sky during the daytime.

The effects of two inclined grating planes illuminated by a plane wave in Talbot interferometry are studied. In the more realistic situation, the two grating planes are first rotated around their own axis parallel to the line direction, and then the detector grating is rotated by an angle around the normal of the grating plane. The theoretical analysis presented indicates that the tilt-angle of the resultant moiré fringes is sensitive to the inclination of the gratings. The results are compared with those in our earlier work and suggest that the approximation made then is reasonable for the conventional Talbot interferometry, from an optical engineering viewpoint.

A new "indirect" laser-driven ultrashort hard-x-ray source based on the combination of a high-repetition-rate femtosecond laser system with a conventional x-ray tube is demonstrated. By irradiation of a metal cathode with femtosecond laser pulses electron bunches are produced and accelerated toward a high-Z target material in a high voltage electric field. Hard x rays are generated via electron bremsstrahlung and characteristic line emission inside the target. The influence of laser parameters on thermionic electron emission and on the hard-x-ray generation efficiency is studied. A theoretical analysis and a comparison with available ultrashort hard-x-ray sources are given.

During the operation of a laser in an unstable resonator with a large Fresnel number, as the output power of a chemical oxygen iodine laser (COIL) is increased, instability in the output laser beam will occur, behaving mainly as far-field beam spot drift and deformation. To interpret this phenomenon, a simulation of the output beam mode of this kind of laser was developed by using the method of fast Fourier transform. From the result of calculation, we propose that the presence of the nonuniform gain in the COIL produces a skewed output intensity distribution, which causes the mirror to tilt and bulge due to thermal expansion. As the output power is increased, the mirror surface, especially the back of the scraper mirror, will absorb more and more heat, causing thermal expansion of the mirror front surface to occur more seriously, thus resulting in the tilting and deformation of the far-field beam spot. The initial misalignment of the direction is also an important factor leading to far-field beam spot drift and deformation.

Digital watermarking has been proposed for copyright protection in our digital society. We propose an adaptive digital watermarking scheme based on the human visual system model and a fuzzy logic technique. The fuzzy logic approach is employed to obtain the different strengths and lengths of a watermark by the local characteristics of the image in our proposed scheme. In our experiments, this scheme provides a more robust and imperceptible watermark.

This PDF contains the communication "Long-term stability of optical surfaces."