This PDF file contains the editorial “Thanksgiving” for OE Vol. 35 Issue 01

It is a great pleasure to have this opportunity to present the readers of Optical Engineering with some of the current research being performed in the field of optical science and engineering in Argentina. The 16 articles in this special section cover a broad range of topics from laser physics to optical metrology, from optoelectronics to the use of optical techniques for studying diverse phenomena.

Recently, a novel wavelet method to reduce speckle noise in synthetic aperture radar images was presented. The method, based on the thresholding of the wavelet coefficients of the transformed image, is computationally efficient and maintains sharp image features. The application of a similar method is explored to reduce speckle noise in TV holography fringes, which is a very important problem in the quantitative analysis of transient phenomena. Several thresholding approaches are used to test the noise reduction algorithm on computer-simulated fringes and results are assessed through the evaluation of two comparative parameters: the image fidelity and the speckle index. It is shown that the method achieves the highest speckle reduction when speckle grains have the same average size of the image pixels.

The properties of a liquid crystal television to be used as a spatial modulator of the polarization of a laser beam are studied. It is found that the model most commonly used for the display description does not adequately reproduce the transmission through crossed polarizers. A new model is proposed that takes into account additional birefringence effects resulting from border effects near the television windows. With this correction, the anomalous behavior of the television is explained and the throughput for different configurations is properly predicted.

Laser backscattering is an excellent tool to investigate size and concentration of suspended particles. It was successfully applied to the analysis of erythrocyte aggregation. A method is proposed that applies laser backscattering to the evaluation of the strength of the immunologic erythrocyte agglutination by approaching the energy required for the mechanical dissociation of agglutinates. Mills and Snabre have proposed a theory of laser backscattering for erythrocyte aggregation analysis. It is applied here to analyze the dissociation process of erythrocyte agglutinates performed by imposing a constant shear rate to the agglutinate suspension in a couette viscometer until a dispersion of isolated red cells is attained, Experimental verifications of the method were performed on the erythrocytes of the ABO group reacting against an anti-A test serum in twofold series dilutions. Spent energy is approached by a numerical process carried out on the backscattered intensity data registered during mechanical dissociation. Velocities of agglutination and dissociation lead to the calculation of dissociation parameters. These values are used to evaluate the strength of the immunological reaction and to discriminate weak subgroups of ABO system.

The modal distribution at the output of a multimode optical fiber behaves as a speckle pattern. Further, a speckle pattern carries information on the pupil diameter of the optical system exit aperture. Pupil measurements by speckle size determination may not be an easy task, because of the statistical nature of the speckle phenomena. The measurement of a multimode fiber core is proposed using a speckle autocorrelation algorithm to obtain the minimum speckle diameter present in the pattern generated by the fiber. Both computer simulations and actual pupils are used to test its performance. Core diameter measurements are carried out showing good agreement with the specifications supplied by the fiber manufacturer.

In dual-plate speckle photography, the images corresponding to both deformation states are recorded on separate photographic plates. In this way, two major limitations of speckle photography, large rigid body displacements of the specimen and the 180-deg displacement direction ambiguity, can be overcome. When both photographic plates are placed with the glass base of one of them between the emulsions and they are illuminated by a narrow laser beam, a pattern of circular fringes is generated. Displacements can be measured by evaluating the distance between the center of the fringe pattern and the undiffracted laser beam, A new algorithm for the fully automatic analysis of such fringe patterns is presented. Its performance is tested using computersimulated fringes and compared with performance obtained using a previously developed algorithm.

A very simple dye laser cavity generates stable single pulses of less than 300-ps duration and several microjoules of energy when pumped by about 50 µJ of an 8-ns pulse from a low-pressure nitrogen laser. The pulses are produced by a mechanism similar to the “quenched resonator” setup. A theoretical description is given, showing good agreement with the observed results. Achievable improvements are described, which would reach the tens of picosecond range.

Spatial filtering, spectral matched inhibition, and recovering of defocused images are implemented by modulation of the induced birefringence of a BSO crystal. The intensity distribution received by the crystal modulates the induced birefringence and the ellipticity of the transmitted light is then modified as it propagates through the crystal. To describe the processor, a 4-f optical system is used. A simple physical model is introduced and numerical and experimental results are presented.

Measurement of optical absorbance is used to study the relaxation of acrylic film doped with Disperse Red 1 and poled with an electric field produced by the corona effect. Time variation of dichroism is studied over 10 days and results are adjusted with different time functions. Second-harmonic generation is presented as a test of the lack of symmetry of the poled film resulting from the orientation process.

Three optical methods for sizing polymer particles are instrumented to be used in the leather industry. One method is based on the measurement of the light scattered intensity for several angles, and the other two are monochromatic or spectral extinction methods. Uniform latex particles and two commercial products used in leather finishing are measured. Advantages compared with electronic microscopy are discussed.

The full width at half maximum is sometimes used to characterize the autocorrelation function of the time history of a speckle pattern. We propose to include more autocorrelation points to diminish the variability of the measurement. The width of the equivalent rectangle (WER) and the X* LOG X measurements are defined and some simulations and experimental results obtained are shown.

An interesting way to display the temporal evolution of a speckle pattern produced by a slowly changing perturbation is presented. It is based on digital speckle pattern interferometry (DSPI) concepts applied to 1-D slices of each frame of the time-evolving perturbation. The time history of a column of an image is depicted as a function of time. Correlation fringes are obtained by subtracting an unperturbed state from consecutive or alternate columns. Applications are suggested.

A fruitful approach to the studies of a liquid free surface is demonstrated. Using optical interferential techniques, the free surface of nonvolatile and volatile confined liquids is analyzed. Various wall container types are investigated. Besides the classical plane wall, experiments are performed by using cylindrical and toroidal walls. In these last cases, the meniscus effect that affects the free surface of the liquid is compensated. Then, it is possible to obtain completely flat free surfaces of liquids up to 80% of their total. Interferometric experiments are also described to measure the inclination of the site with respect to the local horizon, represented by the liquid flat free surface, and to follow the temporal evolution of such free surfaces affected by different conditions.

The performance of four methods for feature enhancement of mammographic images—optimal adaptive neighborhood image processing, automatic spatial filtering, adaptive histogram equalization, and maximal contrast pixel to pixel—are compared. Each method is tested using a database of 50 mammographic images, which included simulated starshaped nodules. A contrast parameter was used to compare the performance of each processing method. It was found that the maximal contrast pixel-to-pixel method gives the highest contrast value. A mammogram including a real subtle lesion is used to illustrate the usefulness of this last contrast enhancement method in clinical diagnosis.

Extraction efficiency in a folded resonator with overlapping between neighboring beams is analyzed in terms of number of mirrors, small-signal gain, and active length. The medium is assumed to be homogeneously broadened and saturated by the sum of overlapping irradiances, supposed to be uniform across each beam. Extraction efficiency is presented as a function of output coupler reflectivity, total gain, and reflectivity of folding mirrors. Optimum coupling is also computed for a range of gains per amplifying section and folding mirror reflectivities. Finally, efficiencies calculated with this model are compared with those obtained with a nonoverlapping model, showing higher figures, with maximum values shifted to the low-output coupler reflectivity for the presented model.

A homemade TEA CO2 multimode line-tunable laser was used as a radiation source to study the IR absorption of O3 in pure samples and in O3-M mixtures (M= O2, N2, and air) at the initial steps of the excitation by direct calorimetric measurements. The fluence and pressure dependence of the O3 absorption cross section o and the number of photons absorbed (n) were determined for all the available rotational lines P(4) through P(58) of the 9.5-µm emission band. The dependence of (n) on added nonreactive gas pressure over the range O to 750 Torr was studied for three different fluences of the P(30) line.

The analysis of light propagation in optical systems composed of periodic arrays of identical apertures is performed using the diffraction properties of the Walsh functions. By taking into account three main features of these functions, namely, completeness, binary nature, and self-imaging properties, the successive interactions between each aperture and the diffracted field can be adequately explained through the Walsh–Hadamard spectrum associated with the transmitted amplitude distribution.

This PDF file contains the editorial “Visual communications and image processing” for OE Vol. 35 Issue 01

MPEG4 is a natural extension of audiovisual coding, and yet from many perspectives breaks new ground as a standard. New coding techniques are being introduced, of course, but they will work on new data structures. The standard itself has a new architecture, and will use a new operational model when implemented on equipment that is likely to have innovative system architecture. The author introduces the background developments in technology and applications that are driving or enabling the standard, introduces the focus of MPEG4, and enumerates the new functionalities to be supported. Key applications in interactive TV and heterogeneous environments are discussed. The architecture of MPEG4 is described, followed by a discussion of the multiphase MPEG4 communication scenarios, and issues of practical implementation of MPEG4 terminals. The paper concludes with a description of the MPEG4 workplan. In summary, MPEG4 has two fundamental attributes. First, it is the coding of audiovisual objects, which may be natural or synthetic data in two or three dimensions. Second, the heart of MPEG4 is its syntax: the MPEG4 Syntactic Descriptive Language—MSDL.

The International Telecommunications Union (ITU) has prepared a series of draft recommendations defining a multimedia videoconferencing/ videophone terminal transmitting over the Public Switched Telephone Network (PSTN). Recommendation H.324 defines the overall system structure and therefore the device commonly known as an H.324 terminal. The author outlines the key technical characteristics of the entire set of five draft recommendations—H.324, G.723, H,263, H.246, and H.223. Of particular interest is the new H.263 video coding algorithm, which outperforms the H.261 video coder by approximately 2 or 3 to 1. The H.263 coder employs a wide range of advanced coding techniques, including half-pel motion compensation, 8 x 8-pel motion vectors, overlapped block motion compensation, and PB frames. It is likely that the H.263 recommendations will be used for other applications at higher bit rates as well as for the H.324 PSTN. The ITU has also begun work to adapt the H.324 PSTN terminal for mobile radio applications. The primary effort is the addition of error control measures to the multiplexer (and possibly the source coders) to address the more challenging error characteristics in the mobile environment.

Image coding using quadtrees is useful in many applications and is well known in computer vision and graphics. We draw a parallel between quadtrees and rational languages that are recognizable by finite state automata (FSA), also known as automata. The FSA are shown to be an interesting alternative to the corresponding quadtrees. Binary image (O= black, 1 = white) coding using a FSA follows these two steps: (1) automaton construction based on the image quadtree, (2) automaton minimization. The display of the coded image is simply obtained by stepping through the automaton. We demonstrate how simple image operations can be efficiently implemented directly with FSA. Examples are also presented to illustrate the relationship between the FSA complexity and the coded images. Finally, the generalization of this approach to gray-level images, color images, and volume data is discussed. We believe that this automaton coding approach will be useful in developing efficient image and data compression algorithms.

The discrete sine and cosine transforms (DSTS and DCTS) are powerful tools for image compression. They would be even more useful if they could be used to perform convolution. As we have recently shown, these transforms do possess a convolution-multiplication property and therefore can be used to perform digital filtering. The convolution is a special type, called symmetric corrvo/ution. This paper reviews the concepts of symmetric convolution and then elaborates on how to use the operation, and thereby DSTS and DCTS, to implement digital filters for images. An indication of how to compute the transforms and what is the complexity of their use is also given.

The wavelet transform is a valuable tool in video processing because of its flexibility in representing nonstationary signals. Waveletbased compression has the advantages of efficient decorrelation of image frames and reduced-complexity multiresolution motion estimation (MRME). We propose three techniques to improve motion estimation in a wavelet-based coder. First, we propose to use an adaptive threshold for coding the motion vectors of the high-pass subimages. Secondly, we propose a bidirectional motion estimation (BMRME) technique in the wavelet transform domain. In BMRME, we estimate the temporal (i.e., direction information) flags only for the blocks in the lowest-resolution subimages and use the same information for the corresponding blocks in the higher-resolution subimages. Finally, we propose a fast multiresolution motion estimation technique where the directional subimages at each level of the wavelet pyramid are combined into a single subimage. Multiresolution motion estimation is then performed on the newly formed subimages. The proposed techniques improve the coding performance significantly over the baseline MRME technique. In addition, they further reduce the computational complexity of the MRME technique.

An object-based coding scheme is proposed for coding the right channel of a stereoscopic image sequence, u.wng motion and disparity reformation. A multiresolution block-based motion estimation approach is used for initialization, while disparity estimation is performed using a pixel-based hierarchical dynamic programming algorithm. A split and merge segmentation procedure based on both 3-D motion and disparity is then used to determine regions with similar motion and depth parameters. The segmentation part of the algorithm is interleaved with the estimation part in order to optimize the coding performance of the segmentation procedure. This is combined with an efficient depth modeling method that offers full depth information at the decoder site. In order to reduce the computational load of the merge phase of the algorithm, a fast algorithm is implemented which speeds up the merge procedure considerably. Motion and depth model parameters are then quantized and transmitted to the decoder along with the segmentation information. An object-based motion-compensating scheme is then used to reconstruct the original image, based on the objects created by the segmentation approach.

How to acquire accurate and reliable motion parameters from an image sequence is a knotty problem for many applications in image processing, image recognition, and video coding, especially when scenes involve moving objects with various shapes and sizes as well as very fast and complicated motion. In this paper, an improved pel-based motion estimation (ME) algorithm with smoothness constraints is presented, which is based on the investigation and the comparison of different existing pel-based ME (or optical flow) algorithms. Then, in order to cope with various moving objects and their complex motion, a hierarchical ME algorithm with smoothness constraints and postprocessing is proposed. The experimental results show that the motion parameters obtained by the hierarchical ME algorithm are quite creditable and seem to be close to the real physical motion fields if the luminance intensity changes are due to the motion of objects. The hierarchical ME algorithm still provides approximate and smooth vector fields even for scenes that involve some motion-irrelevant intensity changes or blurring caused by violent motion.

Object-oriented analysis-synthesis coding subdivides each image of a sequence into a number of moving objects and compensates the motion of each object. Thus it can reconstruct real motion better than conventional motion-compensated coding techniques at very low bit rates. It uses a mapping parameter technique for estimating motion information of each object. Since a mapping parameter technique employs gradient operators, the estimated parameters are sensitive to redundant details and noise. To determine mapping parameters accurately, we propose a new analysis method using integral projections for estimation of gradient values. Also, to reconstruct the local motion correctly, the proposed algorithm divides an image into segmented objects, each having uniform motion information, whereas the conventional one assumes a large object having the same motion information. Computer simulation results with several test sequences show that the proposed image analysis method in object-oriented analysis-synthesis coding gives better performance than the conventional one.

There are two kinds of scan rate up-conversion for a cinematic sequence: frame-based motion-compensated interpolation and fieldbased scan rate up-conversion. The latter deinterlaces every field in an interlaced sequence which can be displayed using a progressive format or interlaced format to double the field rate. This paper proposes a new frame-based interpolation method that consists of segmentation and interpolation modules. The first module divides the difference between two successive frames into changed and unchanged regions. Each changed region is further differentiated into moving objects, covered, and uncovered regions. The second module generates the intermediate frames by using the direct motion-based interpolation method and indirect motionbased interpolation method to fill the changed regions, and the forward/ backward insertion method to fill the covered and uncovered regions. The proposed interpolation method can deal with the interframe zooming effect as well as the object-background ambiguity problem. Finally, it is shown that this new method is computationally simpler than the other frame interpolation methods and the interpolated frames are qualitatively and quantitatively better than results obtained with the other methods.

Segmented image coding (SIC) produces images of better subjective quality than techniques such as the Joint Photographic Experts Group standard at very low bit rates. It segments images into regions of similar texture and codes their contour and texture separately, Most SIC techniques represent the texture as a linear combination of orthonormal base functions, which differ from region to region. In earlier SIC methods, the orthonormal bases were generated by the Gram-Schmidt (GS) procedure, whose excessive computational requirements are unacceptable in many applications, This paper introduces a new class of weakly separable (WS) orthonormal bases, The WS base functions are the product of two components, which can be generated very quickly and using a modest amount of memory. The class of WS base functions includes polynomials, cosines, and warped polynomials. The paper describes the spatial properties of these base functions. Experimental data show that for typical coding parameters, WS SIC is 8 to 40 times faster than traditional SIC, at the expense of a 3% to 6’% larger root-mean-square error. However, the subject quality of the compressed images is only slightly smaller for the WS method. Considering its computational advantages, WS SIC is therefore a promising technique for fast, low bit rate coding.

A major problem with a vector-quantization-based image compression scheme is its codebook search complexity. Recently, a new vector quantization (VQ) scheme called the predictive residual vector quantizer (PRVQ) was proposed, which gives performance very close to that of the predictive vector quantizer (PVQ) with very low search complexity. This paper presents a new variable-rate VQ scheme called the entropy-constrained PRVQ (EC-PRVQ), which is designed by imposing a constraint on the output entropy of the PRVQ. We emphasized the design of the EC-PRVQ for bit rates ranging from 0.2 to 1.00 bits per pixel. This corresponds to compression ratios of 8 through 40, which is the range likely to be used by most of the real-life applications permitting Iossy compression. The proposed EC-PRVQ is found to give a good rate-distortion performance and clearly outperforms the state-of-the-art image compression algorithm developed by the Joint Photographic Experts Group (JPEG). The robustness of the EC-PRVQ is demonstrated by encoding several test images taken from outside the training data. The EC-PRVQ not only gives better performance than JPEG, at a manageable encoder complexity, but also retains the inherent simplicity of a VQ decoder.

This paper describes a feature-correction two-stage vector quantization (FC2VQ) algorithm for the compression of photo ID pictures. The FC2VQ method treats different regions in a facial image differently. A region of facial features (ROFF), containing the eyes and the mouth, is detected and rendered more accurately than the rest of the image. The technique can compress a 128x 128x 8-bit (16,384 bytes total) ID image to an average size of 350 bytes. The quality of the compressed images is far superior to that obtained by other methods, including the JPEG standard, at similar compression ratios.

This paper proposes an image coding algorithm using a tree structure which takes advantage of the geometric similarity among the hierarchical bands of the wavelet transform (WT). A concept of conditional height difference is introduced in order to further reduce the actual number of bits to transmit the tree structure with a height attribute. This paper also proposes a method to appropriately incorporate human visual characteristics into the proposed coding algorithm. This is achieved by determining the human visual weight of each frequency band into which WT decomposes an image. Simulations are used to show the muchimproved performance of the above scheme over the well-known compression algorithms both in terms of peak signal-to-noise ratio (PSNR) and in other senses. For example, the 512x 512 “Lena” image is shown to be compressed at 1 bit/pixel with a PSNR higher than 40 dB, with less hardware complexity.

In this paper, a new subband coding system called subband absolute moment block truncation coding (SAMBTC) is introduced to compress monochrome images and color images recorded in Y/a and L*u*v* uniform color spaces. The SAMBTC incorporates full-band absolute moment block truncation coding (AMBTC) into subbands along with a new subband dynamic bit allocation algorithm which is derived from the Shannon rate-distortion bound. Simulation results show that the proposed Shannon-bound-based bit allocation algorithm outperforms the commonly used standard-deviation-based bit allocation scheme. Compared with AMBTC, SAMBTC achieves superior imaging without blocking artifacts at low bit rates.

Admission control and bandwidth allocation mechanisms have been widely investigated for asynchronous transfer mode (ATM) networks. The criteria for a new call to be accepted should guarantee the required performance for the new call without degrading the quality of service of the existing calls. In order to efficiently manage the network resources, an architecture for a structured logical partition of network resources among traffic classes with different grades of service has been proposed by Bolla et al. in 1993. In this paper, a new dynamic bandwidth allocation scheme called the minimum overflow traffic block algorithm (MOTA) is proposed to assign the bandwidth for each traffic class in the hierarchical admission control structure in an ATM network. The traffic types can be video, image, voice, and so on, An overflow traffic function Q is used to adjust the bandwidth assignment each time a new connection is required, This algorithm sets the mean blocked traffic over the ATM link as the optimization object function Q and adjusts the bandwidth assignment to each traffic class by optimizing Q. This paper uses mean cell loss probability as the quality of service (QoS) for the performance measurement of the system. This novel approach can minimize the mean cell loss probability of the system by optimizing Q when the cell delay constraints are satisfied.

Previous work has demonstrated the effectiveness of the expectation-maximization algorithm to restore noisy and blurred singlechannel images and simultaneously identify its blur. In addition, a general framework for processing multichannel images using single-channel techniques has been developed. The authors combine and extend the two approaches to the simultaneous blur identification and restoration of multichannel images. Explicit equations for that purpose are developed for the general case when cross-channel degradations are present. An important difference from the single-channel problem is that the cross power spectra are complex quantities, which further complicates the analysis of the algorithm. The proposed algorithm is very effective at restoring multichannel images, as is demonstrated experimentally.

The mismatch between dynamic ranges of intensities in non-uniformly illuminated portrait scenes and of intensities reproducible on common display devices is addressed. In particular, such mismatches appear quite often in videophone and videoconferencing situations, Displayed portrait images with no details visible in some facial regions result from them, and lead in turn to a reduced impression of telepresence. A novel image processing approach for dynamic range compression of portrait images is presented in this paper. Unlike conventional methods, it provides compressed images with a most natural appearance. The basic idea is to simulate a more uniform scene illumination in the video signals of portrait images. This requires knowledge about the scene’s reflectance function. Based on scale-space filtering, an edge discrimination technique is developed that distinguishes intensity edges caused by reflectance from edges caused by illumination at smoothly curved surfaces. This facilitates an estimation of the reflectance component from the intensity distribution of portrait images using an extension of the classical lightness computation approach.

Object size inspection is an important task and has various applications in computer vision. For example, the automatic control of stone-breaking machines, which perform better if the sizes of the stones to be broken can be predicted. An algorithm is proposed for image segmentation in size inspection for almost round stones with high or low texture. Although our experiments are focused on stones, the algorithm can be applied to other 3-D objects, We use one fixed camera and four light sources at four different positions one at a time, to take four images. Then we compute the image differences and binarize them to extract edges. We explain, step by step, the photographing, the edge extraction, the noise removal, and the edge gap filling. Experimental results are presented.

The results of a study of fiber Raman lasers that make use of very simple and elegant feedback mechanisms based on fiber optic coupiers are presented, Traveling-wave and standing-wave resonator structures based on couplers have been numerically simulated to determine optimum design parameters for silica- and germania-based fibers.

The positive resist image by dry etching (PRIME) process is a high-resolution positive resist system incorporating deep-UV (DUV)/ebeam exposure, silylation (gas/liquid phase), and dry development. A new method to calculate the silylation profile in the PRIME process is presented. New software modules have been added to the 2-D simulator SLITS (simulation of lithography on topographic substrates) to simulate the silylation and dry-developed profiles in the PRIME process. The siIylation and dry-developed profiles for the PRIME process are simulated and compared to experimental results. Simulations were carried out for both e-beam and DUV exposures. Under e-beam exposure, the maximum percentage error between the simulated and experimental results was 13%. Under DUV exposure, the silylation depth at the mask edge can be reduced by increasing the dose thus effectively controlling the resist linewidth.

Two methods of static image compression—the Joint Photographic Experts Group (JPEG) and a fractal-based method—are compared in terms of the detectability of simple targets following compression and decompression of the images containing such targets. Targets consisted of rectangles of various sizes and contrasts that were embedded in images of natural terrain. Using compression ratios from Oto 35, it was found that the loss in detectability of targets in images compressed using the fractal technique was significantly greater than the loss for the JPEG-compressed images.

Based on birefringence, a building-block stacking technique is suggested in this paper. A solid-state optical morphological processor module is thus developed, which is an integration of a beam array generator submodule, an optical connector submodule, and a Pockets readout optical modulator. It is shown that the technique is compact in construcfion, simple for fabrication, and insensitive to the environment.

The dependence of the reflectivity on the distance of the spots corresponding to the two incident beams on crystals is measured and analyzed in self-reading geometry conjugators (SRGCS) with Cu-doped single-domain photorefractive (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 CrYStals. Double peaks in some dependence are observed and used to investigate the mechanism of SRGCS. The dynamic features of the two phase-conjugate beams and the decrease of the specular reflectivity of the probing beam in SRGCS are observed and analyzed. A bump in the probing beam’s phase-conjugate replica as a result of a short pause of the incident pumping beam is analyzed in terms of the pyroelectric effect and photovoltaic effect. A comparison of results on conjugation fidelity, response rate, and threshold between CAT self-pumped phase conjugators and SRGCS is given. Real-time vertical and horizontal high-pass filtering, real-time double-exposure interferometry, and detection of the blemishes caused by amplitude defects in optical systems are demonstrated experimentally. The corresponding theoretical analyses are also given.

One of the most difficult problems encountered currently in the design of photosensitive systems in the field of UV curing and in the area of imaging science (such as 3-D stereolithography, photopolymers in laser-to-plate imaging systems, holographic recording films, or holographic optical elements) is to extend the spectral sensitivity of the photoinitiating system to the visible light emission. The design of efficient photosensitizer/photoinitiator combinations is partly based on a better understanding of the basic mechanism that is involved. The timeresolved laser absorption spectroscopy combined with steady state experiments appears as a very convenient tool to improve the efficiency of photoinitiating systems. The discussion centers on the excited state processes in selected examples of efficient photosensitive systems for laser-induced polymerization reactions that are of great practical use, e.g., in holographic recording or laser imaging.

All blocks of the given image were divided into six bands, with the first band containing only the dc coefficient, the second band containing the first five ac coefficients, the third band the next nine ac coefficients, and so on. Figure 1 shows this division of DCT coefficients in bands. The number and code size of all DCT coefficients and various EOB markers were calculated. The dc and ac coefficients and all EOB markers in tested images accounted for are 7.3%, 75.5%,and 17.2% of coded symbols, respectively, whereas the EOB(0) alone contributed 15.6% of the coded symbols.