This PDF file contains the editorial “Editorial” for OE Vol. 40 Issue 02

We propose what we believe is the first all-optical fiber sensing system without any electronic processing. A multiple-mode fiber detects external perturbations and serves as a random phase generator. A crystal serves as the volume hologram recorder and database and provides the interconnections between a specific incoming random phase and the corresponding output pattern indicating the external perturbation on the fiber. All aspects of the sensing system, including detection of the external perturbation, interconnection, and display of the result, are performed by optical means.

The extensive use of charge-coupled devices (CCDs) in astronomical imaging and spectroscopic applications has resulted in techniques being developed to correct for sensitivity variations attributable to the focal plane array detector. One normally overlooked effect is that these sensitivity variations also include intrapixel spatial effects that can influence measurements made with CCDs in certain circumstances. We demonstrate the influence of CCD pixel response nonuniformity on photometric measurements. An investigation is presented of how the relation between pixel size and the image size affects the photometric accuracy. Results are presented for a front-illuminated CCD array using an experimentally measured pixel sensitivity functions.

We study visual quality control in the ceramics industry. In tile manufacturing, it is important that in each set of tiles, every single tile looks similar. Currently, the estimation is usually done by human vision. Our goal is to design a machine vision system that can estimate the sufficient similarity, or same appearance, to the human eye. Our main approach is to use accurate spectral representation of color, and compare spectral features to the rGb color features. A laboratory system for color measurements is built. Experimentations with five classes of brown tiles are presented and discussed. In addition to the k-nearest neighbor (k-NN) classifier, a neural network called the self-organizing map (SOM) is used to provide understanding of the spectral features. Every single spectrum in each tile of a training set is used as input to a 2-D sOm. The SOM is analyzed to understand how spectra are clustered. As a result, tiles are classified using a trained 2-D SOM. It is also of interest to know whether the order of spectral colors can be determined. In our approach, all spectra are clustered in a 1-D SOM, and each pixel (spectrum) is presented by pseudocolors according to the trained nodes. Finally, the results are compared to experiments with human vision.

The design and performances of an EUV spectrometer for diagnostics of high-order harmonics generated by an ultrashort (<30 fs) pulsed laser focused onto a gas jet are presented. The system consists of a toroidal mirror, an entrance slit, a spherical variable-line-spaced grating and a bidimensional detector. The mirror focuses astigmatically the radiation from the gas jet, with the tangential focus on the entrance slit. The grating diffracts the radiation coming out from the slit, spectrally focusing the 5 to 40 nm wavelength range in an almost straight focal curve, where the detector is located. The latter is a 40 mm diameter microchannel-plate intensifier. The spatial focal point of the toroidal mirror lies before the spectral focal plane of the spectrometer, so measurement of the height of the spectral lines gives the beam divergence. The spectral resolution of the spectrometer was measured using the narrow line spectra obtained by a microfocus soft x-ray source. Also the absolute response of the system, that is grating efficiency plus detector efficiency, was measured.

A study of gain improvement in the long-wavelength band (L-band) by a seeding signal from the conventional-wavelength band (C-band) is presented. The seeding signal reduces losses through amplified spontaneous emission and supplies energy for L-band amplification in the underpumped region. Gain improvement is observed for a copumped configuration, where the L-band and seeding signal are colaunched together with minimal noise figure penalty when an optimized seeding signal is used. The seeding signal boosts the L-band gain profile in the underpumped region to the flat-gain profile with gain flatness of less than 1 dB. High flat-gain values of 24 dB with gain variation of less than 0.9 dB when a seeding signal at 1555 nm is applied to the L-band amplifier pumped by only 105 mW from a 980 nm pump laser. A gain improvement of 52 dB is obtained at 1580 nm with this architecture.

We present an algorithm that improves the overall performance of compression algorithms by applying low-pass filtering before the compression process and inverse filtering after decompression. Prefiltering images prior to encoding removes high spatial frequencies from the original image, thus increasing the compression ratio of the encoder. The image degradation caused by the filter together with the nonlinear transformation of a typical compression algorithm reduces the entropy of the original image, thus increasing the compression ratio. In addition to improving the compression ratio, the suggested scheme reduces the blocking effect that is common to block-based compression algorithms, such as the Joint Picture Experts Group (JPEG).

We describe an efficient method for the coding of the affine motion parameters. Conventional direct quantization method consists of a linear quantizer and an entropy coder. Although this is the easiest way to code affine parameters, it has some problems. First, its implementation requires too many quantizers and entropy code tables. Second, the direct quantization method does not take into account the interrelationship between the parameters. Therefore, it shows much degradation after motion compensation if the bit rates allocated to the parameters for the transmission or the storage are reduced. To overcome these problems, we propose a new method that deduces the real translational motion from the six affine motion parameters, which have more physical meaning and a strong temporal correlation. In addition, the scaling and rotational motion are transformed into 2-D vector elements, and then the vector quantization method is used to code them. Simulation results show that our proposed scheme provides better performance than the conventional direct quantization method.

Four schemes to detect masses in digitized mammograms based on textural information are demonstrated and compared. Enhancement and segmentation of the image is performed using hexagonal wavelets, steerable filters, and Laws texture energy maps. Hexagonal wavelets are used because spectral orientation is partitioned into three bands covering the frequency domain equally. The lifting scheme is used to construct these wavelets, because it can be extended to uneven sampling, is not susceptible to boundary conditions, and is computationally efficient. It is found that with Laws, the specific convolutional mask is not as important as the pre-enhancement used. Steered filter enhancement shows the best results, closely followed by adaptive histogram equalization, with wavelet enhancement performing the poorest. Oriented filters are used for enhancement and orientation detection. A set of filters are designed that can be efficiently steered along the dominant orientation of the image.

An approach is presented for separating and compensating wideband signatures of radar targets. The signatures thus modified are processed into focused radar imagery. In this approach, the time- frequency representation of superimposed signatures is utilized to define selective binary filters that are enhanced and labeled using computer vision techniques. On filtering, the signature of each target is synthesized from its short-time Fourier transform representation. Following synthesis, the filtered signature is motion compensated in the frequency domain. Radar images can then be generated for scenarios in which multiple moving targets cannot be imaged otherwise. Data collected with an actual stepped frequency radar system is processed to demonstrate that combined signatures of aircraft are effectively separated, compensated and transformed into individual inverse synthetic aperture radar (ISAR) images.

This paper presents an efficient per-pixel technique for synthesizing the IR images of desert backgrounds. The technique is based on semiempirical thermal models using a small database of the statistical features of the model coefficients. It is shown that jointly normal Markov random fields can be used to model these coefficients, if the thermophysical parameters of desert backgrounds are independent normal random Markov fields. A new algorithm is introduced for generating the coefficients with desired covariance matrix and correlation lengths. Simulation and experimental results with the new technique illustrate accuracy comparable to that of the conventional per-pixel approach, while the database reduction is several orders of magnitude.

Traditionally, the stability of the output of a laser is controlled through scientific means or by a simple feedback loop. For multiinput multioutput control and for medium- to high-power lasers, however, these control schemes may break down. We report on the use of a fuzzy logic control scheme to improve the stability of a pulsed nitrogen laser. Specifically, the nitrogen laser is modeled as a two-input two-output system. The two input parameters are the discharge voltage (V and nitrogen pressure (P), and the two output parameters are the pulse energy (E) and pulse width (PW). The performance of the fuzzy logic controller is compared with a decoupled two-channel PID (proportional+integral + derivative) controller. In our experiment, the long-term stabilities of the open-loop system are 1.82% root mean square (rms) for pulse energy and 4.58% rms for pulse width. The PID controller achieves better performance with long-term stabilities of 1.46% rms for pulse energy and 4.46% rms for pulse width. The fuzzy logic controller performs the best with long-term stabilities of 1.02% rms for pulse energy and 4.24% rms for pulse width, respectively.

We present a novel robust layered image transmission (LIT) design algorithm for noisy channels. In the algorithm, the layered embedded zerotree wavelet (LEZW) coding technique is used to encode the images for the transmission of each layer. A new error protection allocation scheme based on genetic programming is then employed to determine the degree of protection for each layer so that the average distortion of the reconstructed images after transmission can be minimized. Simulation results show that, subject to the same amount of redundancy bits for error protection, the new algorithm outperforms other existing algorithms where equal-protection schemes are adopted.

Spin-casting liquid mixtures followed by in situ polymerization offers an elegant and inexpensive method of generating front surface paraboloidal mirrors of almost any focal length, aperture, and on- or off-axis character, requiring no post polishing. Mirrors have been made from a variety of substrates including epoxy-composite mixtures, with focal lengths ranging from 2 to 80 m, and apertures from 5 to 76 cm. The goal of this project was to make spin-cast mirrors for use as reflectors, telescopes, and cat’s-eye retroreflectors in open path Fourier transform infrared (OP/FT-IR) spectrometry. These mirrors were designed for portable outdoor use, and thus have been optimized for maximum ruggedness and mechanical strength, and minimal mass, coefficient of thermal expansion, and cost. After spin-casting, the only preparation prior to silvering is a thermal post-cure. Results are presented giving details of the materials, hardware, and procedures used to spin-cast paraboloidal mirrors.

A method for measuring the cell gap of liquid-crystal displays (LCDs) is presented. It is based on analyzing the interference spectrum of white light reflected by the LCD. It can be used before or after filling the LCD. The method is applicable for LCDs whose operation is based on the polarization or the scattering of light. Four methods for the calculation of the cell gap are presented. The first method is based on the wavelengths corresponding with the peaks in the interference spectrum. Two other methods use analog and digital filtering techniques to filter out all the irrelevant information in the interference spectrum; the cell gap is then calculated with fast Fourier transform (FFT) or the inverse FFT. In the fourth method an empirical model is fitted to the measured interference spectrum.

We discuss methods of improving the quality and resolution of passive millimeter-wave images, particularly those obtained using the Defense Evaluation and Research Agency (DERA) MITRE imager and the more recent MERIT imager. This later real time imager consists of some novel optics followed by a conical scanner in the form of a disk rotating about an axis through its center and tilted with respect to its normal. A horizontal array of receivers is scanned such that each receiver performs a conical scan pattern in the scene. The resulting image, which has a 40x20 deg field of view, consists of a series of circles whose centers are uniformly displaced horizontally. Each receiver is calibrated initially using a two point correction but then drifts in time and a scene based correction is applied. Following preprocessing the images are superresolved using nonlinear restoration techniques. Two such techniques are described. The first is an error energy reduction algorithm that uses background subtraction and the second assumes a Lorentzian distribution of gradients within an image. Both are able to superresolve without significantly enhancing the noise. These various processes are described and images presented.

A novel optoelectronic quotient-selected modified signed-digit division technique is proposed. This division method generates one quotient digit per iteration involving only one shift operation, one quotient selection operation and one addition/subtraction operation. The quotient digit can be selected by observing three most significant digits of the partial remainder independent of the divisor. Two algorithms based on truth-table look-up and binary logic operations are derived. For optoelectronic implementation, an efficient shared content-addressable memory based architecture as well as compact logic array processor based architecture with an electron-trapping device is proposed. Performance evaluation of the proposed optoelectronic quotient-selected division shows that it is faster than the previously reported convergence division approach. Finally, proof-of-principle experimental results are presented to verify the effectiveness of the proposed technique.

The resolution and precision of an optical encoder are determined by three factors: (1) the distance between the photodetector and the slit disk, (2) the ratio of the width of the active area of the photodetector to the width of the slit of the slit disk, and (3) the concentricity of the photodetector and the slit disk. A quantitative analysis of these factors generally requires a grasp of sophisticated analytical techniques, including an incoherent spectrum, the optical system of the light emitting diode (LED), and diffraction. We propose a breadboard system and techniques for obtaining the same output signal as that of the actual encoder using the image data of the light intensity as it permeates the slit disk of the encoder, without the aid of the advanced analytical techniques generally required. We then compare the analytical result obtained with this technique with the output signal from the actual encoder to confirm the validity of this breadboard system and technique.

A perfect zero reference mark is essential for an incremental measurement system to achieve absolute coordinates. An optical zero reference mark based on a phase jump is presented. The zero reference position signal is achieved using two same-frequency focused beams to cross an edge on a simple scale. The phase difference between these two beams is prefixed at 180 deg. The use of a heterodyne interference technique enables the phase jump to be detected. The zero signal is a phase jump of 180 deg, and the theoretical slope of the phase jump is infinite. Because the phase differences of the signal on two sides of the phase jump are 180 deg, the direction can be also obtained. The zero signal is high in resolution and has excellent disturbance resistance. At the same time, the zero reference mark can be naturally integrated with a commercially available positioning system. Both the theoretical design and experimental verification are conducted and the results are presented in detail.

We present a new two-image photometric stereo method that is capable of determining surface deformations when extended light sources are used and the imager and light sources are close to the surface. Surface reflectance is modeled using a linear combination of Lambertian, backscatter, and new extended-source Phong specular reflection component models. The ambiguity resulting from using only two light sources is resolved in the surface normal integration phase, in which the resulting surface is constrained to being a valid image-based triangular facet surface. The method has been tested on an application involving waste-storage drum inspection and yields good accuracy.

A photophysical model for describing the absorption of light by melanin based on the mechanisms of sequential two-photon absorption is investigated. The process involves a conversion of the photon energy in a photochemical effect such as photodissociation of the biological molecules. The collinear photothermal lensing technique is employed to obtain stable, reliable and accurate detection of nonlinear photon absorption in melanin. In these experiments, a He-Ne laser and an argon-krypton ion laser are used to provide two different excitation wavelengths simultaneously. The experimental results were consistent with the theoretical model described.

The continual deformation of an object will lead to a temporal speckle pattern. By analyzing this time-dependent speckle pattern the temporal deformations of the object can be obtained. We propose a method for measuring the displacement components caused by deformations using the temporal speckle pattern interferometry (TSPI). By capturing a series of speckle interference patterns related to the object deformations, we can get the fluctuations in the intensity of the interference patterns. Further, the phase maps for whole-field object displacements are calculated with the inversion of the temporal interference intensities by estimating both the average intensity and the modulation. In this way, one can quantitatively measure temporal displacements simply using a conventional electronic speckle pattern interferometry (ESPI) system without phase shifting or a carrier. An elaboration of the TSPI is presented and out-of-plane displacements caused by pressure and thermal deformations are measured.

Single pulsed subtraction electronic speckle-shearing pattern interferometry is used to measure the time evolution of deformations induced by periodic impulse loading on a metal plate. This is accomplished with a stroboscopic system based on a pulsed laser and an interline-transfer CCD camera running at 60 frames/s. In a few seconds, a sequence of hundreds of interferograms is recorded with an effective sampling time interval of some tens of microseconds. We describe the experimental setup used to generate the transient deformation on the metal plate and also the synchronization of laser pulses and image acquisition. Several correlation fringe patterns are presented, showing the time evolution of the deformation. Finally, these data are used to calculate optical phase maps and displacement derivative fields for different times after the application of the dynamic load on the object.

Recently the author of this paper presented an application of image-processing techniques to identify and to measure the parameters of the bubble in bubbly flow.1 In this paper, a new technique—a noninvasive measurement, using digital image analysis techniques, based on multiframe record to measure the instantaneous bubble rise velocity—is presented.

This PDF contains the communication section including "Encoding complex diffractive optical elements onto a phase-only liquid-crystal spatial light modulator".