This PDF file contains the editorial “Editorial: The International Commission for Optics, Part 2” for OE Vol. 34 Issue 11

This PDF file contains the editorial “Guest Editorial: Optical Remote Sensing and Image Processing” for OE Vol. 34 Issue 11

The paper describes the optimization of the wavelength pair to be used in differential absorption lidar measurements of tropospheric ozone. Along with the usual requirements for optimum lidar performance, aerosol and aerosol gradient effects and sulfur dioxide cross sensitivity have been given special attention. For the measurement and reference, laser-beam wavelengths of 280.91 and 282.72 nm are proposed. Ozone lidar verification measurements with a UV photometric analyzer show excellent agreement and confirm the theoretical finding that ozone concentration uncertainties due to aerosol effects are insignificant under visibility conditions like those prevailing on a warm summer day directly above a deciduous-coniferous mixed forest canopy. The lidar used is described in some detail, and a few examples of measured gas distributions are presented.

A Doppler lidar can measure wind profiles in the atmosphere with excellent accuracy. Invisible atmospheric inhomogeneities such as aircraft wake vortices can be detected and tracked. Lidars are proposed for spaceborne application. The backscatter lidar technology experiment (LITE) was successfully tested in space in 1994. A Doppler lidar in space can give both wind and backscatter information. Examples of proposed spaceborne lidars are discussed.

The ability of a ladar system to resolve two or more separate returns from a combined echo is related to the effective correlation bandwidth of the pulse emitted by the ladar system. Phase modulation of an outgoing pulse introduces additional frequency components, which increases the effective correlation bandwidth of the pulse and thus improves the range resolution of the system. In this paper, we discuss the general theoretical basis for achieving improved range resolution using a modulated waveform and a matched filter receiver. We then demonstrate these concepts by considering the particular case of improved range resolution for a sinusoidally phase modulated carrier with a rectangular amplitude function. We also perform computer simulations with a realistic pulsed ladar envelope possessing the same modulation function. Our calculations indicate that the resolution of a pulsed ladar system may be improved by a factor of 70 with a phase-modulated pulse and a matched-filter receiver.

A theoretical performance analysis of a heterodyne ladar system incorporating a single-rnode fiber receiver has been perforrned. For our purposes, the perforrnance parameters of interest are the coupling and mixing efficiency of the ladar receiver, as they relate to the overall system carrier-to-noise ratio. For a receiver incorporating a single-mode fiber mixer, the received and local-oscillator fields are matched both spatially and temporally at the detector, yielding 100% mixing efficiency. We have therefore focused our efforts on determining an expression for the efficiency with which a diffuse return from a purely speckle target can be coupled into the receiving leg of a monostatic, untruncated cw ladar system. Through numerical analysis, the expected coupling efficiency for a ladar system with negligible truncation of the transmit beam has been determined to be 30.6%.

We introduce and analyze techniques for the reduction of aliased signal energy in a staring infrared imaging system. A standard staring system uses a fixed two-dimensional detector array that corresponds to a fixed spatial sampling frequency determined by the detector pitch or spacing. Aliasing will occur when sampling a scene containing spatial frequencies exceeding half the sampling frequency. This aliasing can significantly degrade the image quality. The aliasing reduction schemes presented here, referred to as microscanning, exploit subpixel shifts between time frames of an image sequence. These multiple images are used to reconstruct a single frame with reduced aliasing. If the shifts are controlled, using a mirror or beam steerer for example, one can obtain a uniformly sampled microscanned image. The reconstruction in this case can be accomplished by a straightforward interlacing of the time frames. If the shifts are uncontrolled, the effective sampling may be nonuniform and reconstruction becomes more complex. A sampling model is developed and the aliased signal energy is analyzed for the microscanning techniques. Finally, a number of experimental results are presented that illustrate the perlormance of the microscanning methods.

Liquid-crystal arrays represents one of the first practical technologies capable of steering light by electronic control only. We use such a device to steer the field of view of a broadband imaging sensor. Unfortunately, dispersion degrades the image quality by smearing out details in the image and by introducing multiple diffraction orders (echoes) at the detector plane. We present a method to compensate for these unwanted effects and thus restore the broadband images obtained with the beam steerer. We use the beam-propagation method to find the wavelength-dependent impulse response, from which we determine the appropriate Wiener filter. When training data are available, we improve the filter with the adaptive least mean square algorithm. We present restored images that demonstrate the capabilities of this technique.

A scientific effort is currently underway to assess tropical forest degradation and its potential impact on Earth's climate. Because of the large continental regions involved, Advanced Very High Resolution Radiometer (AVHRR) imagery and its derivative vegetation index products with resolutions between 1 and 12 km are typically used to inventory the Earth's equatorial vegetation. Archival AVHRR imagery is also used to obtain a temporal baseline of historical forest extent. Recently however, 50-km Seasat-A Scatterometer (SASS) Ku-band imagery (acquired in 1978) has been reconstructed to ≈ 4-km resolution, making it a supplement to AVHRR imagery for historical vegetation assessment. In order to test the utility of reconstructed Ku-band scatterometer imagery for this purpose, seasonal AVHRR vegetation-index and SASS images of identical resolutions were constructed. Using the imagery, discrimination experiments involving 18 vegetation categories were conducted for a central South America study area. The results of these experiments indicate that AVHRR vegetation-index images are slightly superior to reconstructed SASS images for differentiating between equatorial vegetation classes when used alone. However, combining the scatterometer imagery with the vegetation-index images provides discrimination superior to any other combination of the data sets. Using the two data sets together, 90.3% of the test data could be correctly classified into broad classes of equatorial forest, degraded woodland/forest, woodland/savanna, and caatinga.

A perceptual-based multiresolution image fusion technique is demonstrated using the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) hyperspectral sensor data. The AVIRIS sensor, which simultaneously collects information in 224 spectral bands that range from 0.4 to 2.5 μm in approximately 10-nm increments, produces 224 images, each representing a single spectral band. The fusion algorithm consists of three stages. First, a Daubechies orthogonal wavelet basis set is used to perform a multiresolution decomposition of each spectral image. Next, the coefficients from each image are combined using a perceptual-based weighting. The weighting of each coefficient, from a given spectral band image, is determined by the spatial-frequency response (contrast sensitivity) of the human visual system. The spectral image with the higher saliency value, where saliency is based on a perceptual energy, will receive the larger weight. Finally, the fused coefficients are used for reconstruction to obtain the fused image. The image fusion algorithm is analyzed using test images with known image characteristics and image data from the AVIRIS hyperspectral sensor. By analyzing the signal-to-noise ratios and visual aesthetics of the fused images, contrast-sensitivity-based fusion is shown to provide excellent fusion results and to outperform previous fusion methods.

The submicroscan interpolation image-processing technique is analyzed to determine the effects of additive noise on the quality of the output imagery. Both temporal and fixed-pattern spatial noise are evaluated assuming a white Gaussian noise model. Closed-form solutions for the power spectral density of the output noise are derived for both one- and two-dimensional submicroscanning. It is found that temporal noise degrades the output imagery and determines the minimum usable submicroscan image shift. For the case of fixed-pattern noise, submicroscan interpolation causes a spectral redistribution of the noise power spectrum that tends to improve image quality.

Optical scanning holography (OSH) is a holographic recording technique that uses active optical heterodyne scanning to generate holographic information pertaining to an object. The holographic information manifests itself as an electrical signal suitable for real-time image reconstruction using a spatial light modulator. The electrical signal that carries the holographic information can also be digitized for computer storage and processing, allowing the image reconstruction to be performed numerically. In previous experiments with this technique, holographic information has been recorded using the interference pattern of a plane wave and a spherical wave of different temporal frequencies to scan an object. However, the proper manipulation of the pupil functions in the recording stage can result in real-time processing of the holographic information of an object during recording. We propose a holographic edge extraction technique as an important example of real-time preprocessing of holographic information that utilizes alternate pupils in the OSH recording stage. We investigate the theory of holographic preprocessing using a spatial frequency-domain analysis based on the recording system's optical transfer function. The theory is reinforced through computer simulation.

When invariant moments are used as numerical features for image description and recognition, the noncentral moments generated by correlation can be used for scale, rotation, and shift-invariant pattern recognition. A search in the feature-vector space for matching the input and reference feature vectors can be fast. The new method can apply to multiple-image input without segmentation of the images, and the noncentral moments are more robust than the central moments to noise.

A novel wavelet-based joint transform correlator (WJTC) for rotation-invariant pattern recognition and applications in optical image processing and remote sensing is investigated. First an optimal set of filter parameters and a mother wavelet filter are selected. These are used to extract features at different resolution from a set of rotationally distorted training images. Then a composite reference feature is formulated from these features for use in the WJTC. Simulation results for both noisy and noiseless environments are presented to verify the effectiveness of this technique.

A hybrid character recognition system that uses a feature-extraction method is proposed. The features are extracted using a wavelet transform, preclassified using a k-nearest-neighbor-based neural net and subsequently postprocessed using an optical correlator. This feature-based neural wavelet optical architecture is then tested on blurred character images.

Synthesis of a spatial-domain bipolar composite filter (SBCF) using simulated annealing algorithm is presented. Since a SBCF is a spatial-domain filter, it is very suitable for implementation in a joint transform correlator. To alleviate the stringent control requirement of a phase-modulated spatial light modulator (SLM), a position-encoding technique is used to generate the SBCF as well as the input objects. Experimental demonstrations are also provided, in which it is shown that the SBCF has a higher discrimination capability of extracting the target objects against the antitarget objects.

generalized fringe-adjusted joint transform correlator (JTC) is proposed, which employs a new family of real-valued filters called the fractional power fringe-adjusted filters (FPFAFs). By suitably adjusting the parameters used in the FPFAF formulation, we can obtain JTC-based classical matched filtering, inverse filtering, and amplitude-modulated phase-only filtering. An efficient JTC architecture for optical implementation of the proposed technique is also developed. The proposed technique is especially suitable for real-time image-processing and remote-sensing applications, since it avoids the limitations of the VanderLugt-type correlators and it has the inherent capability to process both noise-free and noisy input scenes.

A novel low-f-number, wide-field-of-view imaging spectrometer has been designed for measuring the day-glow spectrum over the wavelength range of 260 to 870 nm with spectral resolutions of 0.5 and 0.03 nm. The zero-obscuration all-reflective design is an f/2.0 imaging spectrograph using commercial gratings. The field of view along the spatial direction is 6 deg, with a spatial resolution of 0.1 mrad. The spectrometer is designed to work with a commercially available 1037 x 1340 CCD detector with 6.8 x 6.8-μm pixel size. The imaging spectrometer optics consists of an aspheric toroidal telescope, a slit, an aspheric toroidal collimator, a planar reflective grating, and three off-axis higher-order aspheric imaging mirrors. Significant improvements in the performance have been achieved by introducing aspheric toroidal elements in the design. The weight and size have been reduced by a factor of 20 as compared to previous similar instruments. A virtual prototype of the instrument has also been modeled by using integrated optical and mechanical design software.

The surveillance of large areas with high resolution is limited by the CCD technology, the readout electronics, and the data storage rate. An imaging concept that includes an optical camera and a survey strategy is proposed for remote landing-site certification. A 1-m-diam telescope performs diffraction-limited imaging over its field of view, imaging an area of 0.25 x 0.25 m of the (Martian) surface on a 7 x 7-μm pixel. With the sensor at 350 km above the 10 x 10-km site, this pushbroom imaging configuration incorporates only five sensor passes over the site. The mission time is decreased by nearly 50% from the previously proposed concept for the site certification imaging.

We discuss a technique based on nonlinear and adaptive optics for simulation of phase-distortion effects in imaging systems. This technique uses a nonlinear two-dimensional optical feedback system to produce a controllable spatially and temporally varying chaotic intensity distribution. The intensity pattern is converted into a spatially varying thin phase screen using an optically addressed spatial phase modulator. A chaotic phase distortion is then introduced into an imaging system's output image by propagation through the phase screen. A deformable mirror with computer control is used for simulation of large-scale phase distortions.

The overall atmospheric modulation transfer function (MTF) is essentially the product of the turbulence and aerosol MTFs. Models describing meteorological dependences of both C²n (where Cn is the refractive index structure coefficient) and the coarse aerosol size distribution have been developed previously. Here, they are used to predict the turbulence MTF, aerosol MTF, and overall atmospheric MTF according to the weather. Comparison of predictions with measurements yields very low mean squared normalized error and suggests that such prediction models can also be very useful in image restoration based on weather data at the time and general location in which the image was recorded. An interesting aspect of this work is that measurements of the aerosol MTF with different imaging instrumentation are very different, as expected from theory developed previously concerning the practical aerosol MTF actually recorded in the image. This is dependent on instrumentation parameters. This experimental verification with two different imaging systems supports the model that the "practical" aerosol MTF is very dependent on instrumentation.

An adaptive optical system (AOS) with a feedback loop closed via a feedforward neural network (NN) is considered. The vector of the wavefront corrector control signals is computed by the network from two vectors of the intensity moments measured in two near-field planes by two matrix photodetectors. The NN is trained with a back-propagation algorithm to predict the vector of adaptative mirror signals from the measured intensity vectors. During training, the network forms an optimal control algorithm for a given configuration of an optical system, taking into account misalignments and nonlinearities of the hardware used. A numerical model of a multichannel AOS controlled by a multilayer NN was built, trained, and run for difterent input aberrations. The neural control permits a direct conversion of the intensity distribution measured in the near field into control signals of a wavefront corrector. High efficiency of control has been demonstrated for a model of a 16-channel adaptive optical system for arbitrary input aberrations having a limited spatial spectrum.

An accurate method for measuring charge transfer efficiency (η) and photoresponse nonuniformity (PRNU) of linear charge-coupled devices is developed. The apparatus is described. A finely focused light spot is adopted to irradiate each photosensitive element to get its response output signal. Data are collected automatically by scanning peak-searching and processed by linear best fitting. As examples, measurements for some types of devices are reported and factors influencing the results are discussed.

A CCD imaging system has been developed for precision laser beam pointing. The system contains a high frame rate CCD camera operated in windowing mode under command of an embedded processor. A laser pointing and tracking update rate of 2 kHz has been achieved.

The first experimental demonstration of a single-mode optical fiber double-loop resonator with approximately equal recirculating loop lengths and two identical or nonidentical 2 x 2 fiber couplers is presented. The theory of the double-loop resonator with nonidentical 2 x 2 fiber couplers is developed as well. The dependence of the output intensity on the intensity coupling coefficients of the 2 x 2 fiber couplers is investigated. It is found that the experimental and theoretical results are in fairly good agreement.

An adaptive approach to edge detection using the transform domain and bandpass filtering is discussed. The method is also extended to 3-D and shown to yield better edge maps. The discrete symmetric cosine transform (DSCT) is shown to be the best transform for accurate edge detection. The theory is first discussed in 1-D. Then, the adaptive algorithm utilizing both the gradient and the Laplacian information is developed in 2-D and 3-D. Computer simulations with regular scenes and magnetic resonance imaging images are provided. The extension of the method to 3-D leads to improved noise immunity and better edge contours.

An extension of the reduced-difference pyramid (RDP) algorithm is derived and proposed for lossy image compression with upperbounded maximum error (semilossy). Quantization has been introduced into the basic RDP in two different ways: approximations are made either before or after taking the differences among pairs of adjacent nodes. The resulting peak-error (PE) propagation along the pyramid is theoretically investigated, and quantization strategies are suggested. Tests on the standard gray-scale Lena image show significant and steady rate-distortion improvements over the original RDP. As few fixed-point operations are required per pixel, on-line compression/decompression is feasible on general-purpose computers.

The LiNbO₃ optical intensity modulator is a promising key device in global optical communication systems which demand high reliability. Although performance of the modulator has been considerably improved to meet practical use, there remain problems related to reliability issues such as unpredictable fluctuation of drift phenomena. In this regard, our recent experimental data on the fabrication of Ti:LiNbO₃ modulators are presented here to clarify problems in the device characteristics caused by the device materials. At present, there is little understanding of the origins and mechanisms of the fluctuations, and the estimation and assurance of device quality throughout the long term are difficult. In order to obtain as great a reliability as possible, screening tests for all produced devices are needed.

The optical transfer function (OTF) of calibration lenses must be measured with a high accuracy, requiring complex setups. Thermal expansion effects of the measuring system are a limiting influence, resulting in changes of the imaging state (e.g., defocusing) during the period of measurement. A new concept is presented to overcome these disturbing effects. By taking a rapid tomography sequence of image planes, the three-dimensional irradiance distribution in the image space is captured and stored in a computer, analogous to a flash photo of a moving object in conventional photography. After this, there is time to find the desired imaging state in the data. This "flash" concept has been successfully implemented in a typical OTF measuring setup, improving the reproducibility by one order of magnitude.

Two-dimensional Fourier transform profilometry (2-D FTP) for the automatic measurement of three-dimensional (3-D) object shapes is presented and verified by the experiment in this paper. In some applications, in which there are specklelike structures and discontinuity in the observed fringe pattern, 2-D FTP provides a better separation of the height information from noise.

The interaction of a focused laser beam with a step on a conducting surface is investigated by a vector method. The 2-D problem is solved by using a model of fictitious current filaments, while 3-D problems are treated by using a model of fictitious dipoles as the sources of the scattered field. Computer simulations indicate a difference between the two polarization components (TE and TM), but with the samples investigated in this work, these differences are too small for practical measurements. The scattered field distribution is found to be strongly dependent on the step height relative to the observation plane and on its position relative to the beam waist. Thus, measurements of the intensity distribution in the observation plane can provide information about these parameters with high sensitivity. Experimental investigation confirms that the height of a step in the range λ/10 to λ/4 can be measured with an accuracy of 5%, including its sign (up or down). The position of the step with respect to the beam can be estimated with an accuracy of about 1/100 of spot size.

The possibility of using selectively low emission to reduce the thermal signature of radomes is identified. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High-density, polycrystalline beryllium oxide is identified as a material with low emittance in the upper atmospheric window at 8 to 13 μm, owing to a strong reststrahlen band. Radiometer measurements and bulk reflectance spectra are reported for ceramic BeO and are used to calculate the thermal infrared emission factors as well as the radiance for a 50°C BeO surface. The results depend heavily upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3- to 5-μm and 8- to 13-μm ranges. The possibility of reducing the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured results for a 0.8-μm silicon overlayer on BeO are reported.

The spatial coherence function of a beam exiting from a wave-guiding device can be used to determine the transverse modal composition of the fields which are supported within the structure. The strength of the spatial coherence technique is that it directly yields the relative modal weights which exist within the cavity. While this technique has received little attention in the past due to the experimental difficulties in measuring spatial coherence, a recently developed technique overcomes several of these problems. We demonstrate how the relative modal amplitudes may be extracted from the spatial coherence function for the case where the field distribution is described by Hermite-Gaussian polynomials. We point out some limitations of this method and discuss how these may be circumvented.

Tungsten oxide films were prepared by sol-gel and sputtering processes. The refractive indices and extinction coefficients of the films were calculated from the transmittance and reflectance spectra in the visible and infrared regions. The films were transparent in the visible and infrared regions when they were prepared by the 501-gel process and fired at 300°C and 500°C, and by the sputtering process under the conditions of O₂/Ar=0.05 and 0.1 at room temperature. The refractive indices of the films depended on preparation conditions. The extinction coefficients of the films were almost zero in the visible and infrared regions. On the other hand, film prepared by the sputtering process under the condition of O₂/Ar=0.01 was blue colored. The refractive index of the film increased with increasing wavelength in the visible and infrared region. The extinction coefficient ofthe film showed a large value at 1450 nm, which corresponds to the minimum of the transmittance spectrum of the film.

The optical properties (absorption coefficient, scattering coefficient and the anisotropic factor of scattering) and fluorescence characteristics of normal and abnormal bronchial tissue were measured in vitro. After adding additional blood optical properties to in vitro optical properties of tissue, the in vivo bronchial fluorescence was simulated and analyzed by Monte Carlo modeling. The Monte Carlo simulation results showed that with an appropriate illumination and fluorescence collection geometry, the distortion of in vivo fluorescence spectra of tissue caused by variations of optical properties at different wavelengths could be much reduced. Based on these results, a spectrofluorometry system was developed for the collection of in vivo laser-induced fluorescence spectra of tissue during endoscopy. In comparing the in vivo fluorescence spectral shape of bronchial tissue collected by this system with the intrinsic one obtained in vitro, we found no obvious distortion in the in vivo spectra. This was completely consistent with the analysis of Monte Carlo modeling. The in vivo measurement results demonstrated that significant differences in fluorescence intensity between normal and diseased bronchial tissue (dysplasia, carcinoma in situ) can be used to differentiate them from each other. Also, changes in fluorescence intensity are more robust for detecting abnormal tissues than the differences in spectral characteristics.

This PDF file contains the letter “Letter: Importance of boundary reflections in the theory of diffusive light scattering [see 33(12)3849-3852(Dec1994)]” for OE Vol. 34 Issue 11

This PDF file contains the letter “Letter: Response to Importance of boundary reflections in the theory of diffusive light scattering” for OE Vol. 34 Issue 11

This PDF file contains the letter “Letter: Comment on the paper: Elimination of systematic error in subpixel accuracy centroid estimation [see 30(9)1320-1331(Sep1991)]” for OE Vol. 34 Issue 11

This PDF file contains the editorial “Book Rvw: Acousto-Optic Signal Processing: Fundamentals and Applications," by Pankaj K. Das and Casimer M. DeCusatis for OE Vol. 34 Issue 11

This PDF file contains the editorial “Book Rvw: High-Power Optically Activated Solid State Switches," by Arye Rosen and Fred Zutavern for OE Vol. 34 Issue 11