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Articles de revues sur le sujet « Spatial frequencie »

Auteur : Grafiati
Publié le 9 mars 2023
Mis à jour le 10 mars 2023

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1

Pankil Butala, Pankil Butala, Hany Elgala Hany Elgala et Thomas D. C. Little Thomas D. C. Little. « Sample indexed spatial orthogonal frequency division multiplexing ». Chinese Optics Letters 12, no 9 (2014) : 090602–90606. http://dx.doi.org/10.3788/col201412.090602.

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ICHIHARA, SHIGERU. « Perceived spatial frequency shift after adaptation to compound gratings of two sinusoidals in phase or 180° ; out of phase ». Japanese Psychological Research 29, no 1 (1987) : 10–16. http://dx.doi.org/10.4992/psycholres1954.29.10.

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3

Sagi, Dov, et Shaul Hochstein. « Lateral inhibition between spatially adjacent spatial-frequency channels ? » Perception & ; Psychophysics 37, no 4 (juillet 1985) : 315–22. http://dx.doi.org/10.3758/bf03211354.

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Stancă Ionut, Rizea Ileana Olguta, Popescu Andreea Caterina, Albu Alice, Rus Mihaela et Fica Simona. « The importance of cardiac tomography in the evaluation of cardiac changes and coronary atherosclerosis in patients with betathalassemia major ». Technium Social Sciences Journal 11 (14 septembre 2020) : 602–11. http://dx.doi.org/10.47577/tssj.v11i1.1677.

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Betathalassemic patients demonstrate an increased rate of extracardiac vascular complications, but very low prevalence for coronary artery disease. Computed tomography (CT) achieves excellent tissue characterization, with high spatial resolution and has developed as a gold standard for noninvasive angiography and calcium score assessment. Methods. We examined 7 patients with major beta-thalassemia and 7 patients who had an indication for cardiac CT for resting ECG changes, without symptoms of angina pectoris. We investigated the coronary atherosclerosis by assessing the coronary artery calcium (CAC) and arterial stifness. Usual tests and echocardiography measurement were performed. Cardiac computed tomography determined left ventricular mass, left ventricular ejection fraction (LVEF), coronary calcium score and coronary anatomy. An analysis of myocardial density was also performed. Artery stiffness was assessed by the cardio ankle vascular index (CAVI). Results. Arterial stiffness index in betathallasemic group was higher than control group, R-CAVI index was 6.21± 0.49 vs 5.65±0.37 and L-CAVI index was 6.21± 0.38 vs 5.71±0.31. The assessment of systolic function by echocardiography and cardiac CT examination in the 2 groups, shows that the LVEF in the betathallasemic group was significantly lower than in the control group, which means that some patients already had cardiomyopathy. LV myocardial mass was significantly higher in the group with beta-thalassemia, which is explained by the appearance of myocardial remodeling. The calcium score in patients with major beta-thalassemia was 0 and 8,5± 5,9 in the control group. Only 3 patients (42,8%) in the control group had a calcium score > 10U. No atherosclerotic lesions were observed in patients with major beta-thalassemia, whereas the control group showed mild coronary atherosclerotic lesions. If myocardial density can be determined, calcium or iron deposits can be detected in the myocardium. In patients with beta-thalassemia, the density of the myocardium was higher, both in the left ventricle (49.29 8.87±HU) and in the septum (56.71± 8.1 HU). Calculation of Pearson’s correlation coefficient revealed a good association between CT and echocardiography, reproducibility of CT was significantly higher on an intra-observer level for LVEF and LV Mass. Conclusions: Patients with β–thalassemia major have a similar calcium score compared to control subjects, but they have an increase in arterial stiffness. However, zero frequencie of coronary heart disease, denotes coronary protection mechanisms in thalassemia, so future research should focus on the anti-atherogenic potential of blood lipids at these patients. The ability of cardiac tomography to detect calcifications and changes in myocardial density should be valued, as it can be a good tool for establishing the diagnosis of cardiomyopathy by iron loading.
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Zhou, Y. X., et C. L. Baker. « Spatial properties of envelope-responsive cells in area 17 and 18 neurons of the cat ». Journal of Neurophysiology 75, no 3 (1 mars 1996) : 1038–50. http://dx.doi.org/10.1152/jn.1996.75.3.1038.

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1. Many neurons in areas 17 and 18 respond to spatial contrast envelope stimuli whose Fourier components fall outside the cell's spatial-frequency-selective range. The spatial properties of such envelope responses are investigated here and compared with responses to conventional luminance-defined gratings to explore the underlying receptive-field mechanism. 2. Three spatial properties of envelope responses are reported more extensively in this paper. First, the envelope responses were selective to the carrier spatial frequency in a narrow range of frequencies higher than a given cell's luminance spatial frequency selective range (luminance passband). Second, a given cell's dependence on envelope spatial frequency often differed from its luminance passband. Last, the optimal carrier spatial frequency did not shift systematically with the envelope spatial frequency, supporting the hypothesis that the carrier and envelope spatial-frequency dependencies were mediated by distinct mechanisms. 3. In contrast to the direction selectivity to the envelope motion in many envelope-responsive cells, no direction preference to carrier motion was found for envelope responses. The direction of carrier motion did not alter the direction selectivity for envelope motion, further supporting the hypothesis that the carrier and envelope temporal properties were mediated by separate mechanisms. 4. The distributions of the optimal carrier and luminance spatial frequencies among envelope-responsive cells were analyzed. The optimal carrier spatial frequencies were randomly distributed from five times the cell's optimal luminance spatial frequency to the upper resolution limit of the X-retinal ganglion cells at the same retinal eccentricity, suggesting that the selective ranges of envelope responses and luminance responses are not strongly correlated over the population of envelope-responsive cells. 5. Our data support a "two-stream" receptive-field model for envelope-responsive cells. One stream is a conventional, spatially linear receptive-field mechanism, mediating luminance responses for the cell; the other mediates envelope responses and consists of a two-stage processing: a set of spatially small and distributed nonlinear neural subunits whose outputs are spatially pooled at the second stage. 6. In conclusion, this study indicates that envelope responses in area 17 and 18 neurons cannot be due to a nonlinearity that is common to all visual stimuli before narrowband spatial-frequency-selective filtering; instead, a specialized processing stream, parallel to the conventional luminance response stream, is needed to supplement the traditional luminance processing stream in these cells. This specialized stream responds to the envelope stimuli and is selective to their carrier and envelope spatial frequencies. The distributions of the optimal luminance and carrier spatial frequencies indicate a rich variety of possible integration between luminance and envelope information.
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Dan Mai, 但迈, 刘美慧 Liu Meihui et 高峰 Gao Feng. « 单像素空间频域成像的实时化 ». Chinese Journal of Lasers 49, no 5 (2022) : 0507207. http://dx.doi.org/10.3788/cjl202249.0507207.

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COHEN, LEON. « Time-Frequency Spatial-Spatial Frequency Representations ». Annals of the New York Academy of Sciences 808, no 1 Nonlinear Sig (janvier 1997) : 97–115. http://dx.doi.org/10.1111/j.1749-6632.1997.tb51655.x.

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Dev, Ashwani, et George A. McMechan. « Spatial antialias filtering in the slowness-frequency domain ». GEOPHYSICS 74, no 2 (mars 2009) : V35—V42. http://dx.doi.org/10.1190/1.3052115.

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A rigorous, explicit spatial antialias filter is designed and applied to spatially coarsely sampled seismic data by removing all energy above the first Nyquist wavenumber, and aliased energy that is folded back across the Nyquist, in the horizontal slowness-frequency domain. The spatial filtering in the slowness-frequency domain is explicit, free from any event linearity assumption, and does not require any interpolation. The spatially aliased energy is dispersive, and present at small and large slownesses. Comparison of the output data after antialias spatial filtering, with output data after conventional antialias frequency filtering, shows that the filter removes the spatially aliased frequencies selectively at each slowness; antialias low-pass frequency filtering under- or overcorrects for spatial aliasing at all slownesses. A seismic gather can be spatially dealiased only at the expense of wavelet spectral changes; dealiasing and preservation of amplitude variations with offset are not simultaneously possible.
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Pigliucci, Massimo, et Guido Barbujani. « Geographical patterns of gene frequencies in Italian populations of Ornithogalum montanum (Liliaceae) ». Genetical Research 58, no 2 (octobre 1991) : 95–104. http://dx.doi.org/10.1017/s0016672300029736.

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SummaryGeographic variation was studied at 15 electrophoretic loci (40 alleles) in Italian populations of Ornithogalum montanum Cyr. ex Ten. (Liliaceae). Homogeneity of allele frequencies was assessed by G tests; gene-frequency patterns were described by spatial autocorrelation statistics; matrices of genetic and environmental distance were compared through a series of Mantel's tests, and the zones of highest overall gene-frequency change per unit distance (steep multi-locus clines, or genetic boundaries) were identified. Nineteen allele frequencies appear heterogeneously distributed, but only 3 of them show significant spatial structure. Only 2 allele frequencies are correlated with 1 environmental parameter. Large genetic differences are observed between spatially close populations. These findings support a model of differentiation in which the genetic relationships between isolates do not depend on their spatial distances, but reflect mainly population subdivision and restricted gene flow.
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Mayer, Melanie J., et Charlene B. Y. Kim. « Smooth frequency discrimination functions for foveal, high-contrast, mid spatial frequencies ». Journal of the Optical Society of America A 3, no 11 (1 novembre 1986) : 1957. http://dx.doi.org/10.1364/josaa.3.001957.

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Shulman, Gordon L., et James Wilson. « Spatial Frequency and Selective Attention to Spatial Location ». Perception 16, no 1 (février 1987) : 103–11. http://dx.doi.org/10.1068/p160103.

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The effect of spatial attention on the detectability of gratings of different spatial frequency was measured using a probe technique. Three experiments are reported in which the detectability of full-field probe gratings was measured while subjects analyzed stimuli presented in either the central or the peripheral visual field. Selective attention to peripheral stimuli produced a facilitation at low frequencies and a decrement at high frequencies. These effects disappeared under forced-choice presentation.
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Liu Meihui, 刘美慧, 但迈 Dan Mai et 高峰 Gao Feng. « 基于目标形貌测量的空间频域成像校正方法 ». Laser & ; Optoelectronics Progress 58, no 10 (2021) : 1011027. http://dx.doi.org/10.3788/lop202158.1011027.

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Wang Yan, 王岩, et 牛宏伟 Niu Hongwei. « 基于光学空频域变换的自适应图像分块隐藏技术 ». Laser & ; Optoelectronics Progress 58, no 16 (2021) : 1609001. http://dx.doi.org/10.3788/lop202158.1609001.

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Zhong Xiaoxue, 钟晓雪, 黄国武 Huang Guowu, 缪弘波 Miu Hongbo, 胡城豪 Hu Chenghao, 刘威 Liu Wei, 孙春容 Sun Chunrong, 陈志华 Chen Zhihua et al. « 基于空间频域成像的烧伤程度无创定量评估 ». Chinese Journal of Lasers 49, no 24 (2022) : 2407205. http://dx.doi.org/10.3788/cjl202249.2407205.

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Bensmaïa, S. J., J. C. Craig et K. O. Johnson. « Temporal Factors in Tactile Spatial Acuity : Evidence for RA Interference in Fine Spatial Processing ». Journal of Neurophysiology 95, no 3 (mars 2006) : 1783–91. http://dx.doi.org/10.1152/jn.00878.2005.

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We investigated the extent to which subjects’ ability to perceive the fine spatial structure of a stimulus depends on its temporal properties (namely the frequency at which it vibrates). Subjects were presented with static or vibrating gratings that varied in spatial period (1–8 mm) and vibratory frequency (5–80 Hz) and judged the orientation of the gratings, presented either parallel or perpendicular to the long axis of the finger. We found that the grating orientation threshold (GOT)—the spatial period at which subjects can reliably discriminate the orientation of the grating—increased as the vibratory frequency of the gratings increased. As the spatial modulation of SA1 and RA afferent fibers has been found to be independent of vibratory frequency, the frequency dependence of spatial acuity cannot be attributed to changes in the quality of the peripheral signal. Furthermore, we found GOTs to be relatively independent of stimulus amplitude, so the low spatial acuity at high flutter frequencies does not appear to be due to an inadequacy in the strength of the afferent response at those frequencies. We hypothesized that the RA signal, the strength of which increases with vibratory frequency, interfered with the spatially modulated signal conveyed by SA1 fibers. Consistent with this hypothesis, we found that adapting RA afferent fibers improved spatial acuity, as gauged by GOTs, at the high flutter frequencies.
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Jacobson, Lowell D., et Harry Wechsler. « Joint spatial/spatial-frequency representation ». Signal Processing 14, no 1 (janvier 1988) : 37–68. http://dx.doi.org/10.1016/0165-1684(88)90043-6.

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Yin, Ling Xiao, et Jing Ling Chen. « Spin Spatial Frequency Response of Atomic Magnetometer ». Key Engineering Materials 787 (novembre 2018) : 81–86. http://dx.doi.org/10.4028/www.scientific.net/kem.787.81.

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We describe a method for measuring the spin spatial frequency response in a Cs vapor cell by using a digital micro-mirror device (DMD) to modulate the pumping light both spatially and temporally. An equivalent space-alternative magnetic field is created by this way. The pumping light through the Cs vapor cell is measured and analyzed in spatial frequency domain. We obtain the spatial frequency response of the Cs vapor cell from 1.4 cm-1to 364.9 cm-1. The theoretical results of the spatial frequency response according to Fick's second diffusion law agree with the experimental results. This method provides an alternate approach for spatial characterization and three-dimensional imaging of spins.
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Baro, John A., et Stephen Lehmkuhle. « The effects of a luminanace-modulated background on the grating-evoked cortical potential in the cat ». Visual Neuroscience 3, no 6 (décembre 1989) : 563–72. http://dx.doi.org/10.1017/s0952523800009895.

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AbstractAveraged grating-evoked cortical potentials were recorded from area 17 of awake cats. Peak latency of early components of the visual-evoked potential (VEP) response to stimulus onset increased as a function of spatial frequency, while amplitude tended to be largest at intermediate spatial frequencies. Latency increased and amplitude generally decreased to lower spatial-frequency stimuli (<0.25 cycle/deg) in the presence of a uniform flickering field (UFF). The UFF had a relatively small or opposite effect on peak latency and amplitude for higher spatial-frequency stimuli (>0.50 cycle/deg). The VEP response to stimulus offset was present only at low spatial frequencies and was virtually eliminated by the presence of the UFF. The effects were similar whether the target and UFF background were simultaneously presented or briefly separated; however, the UFF had no effect when the two were spatially separated. The effects of the UFF background on VEP onset response increased with increasing temporal frequency from 2–8 Hz; offset responses were affected similarly at all temporal frequencies. These effects are similar to those observed in humans and suggest that two spatio-temporally tuned mechanisms contribute to the early VEP response. In the cat, the mechanisms seem to correspond to X and Y cells in the dorsal lateral geniculate nucleus.
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Frishman, L. J., A. W. Freeman, J. B. Troy, D. E. Schweitzer-Tong et C. Enroth-Cugell. « Spatiotemporal frequency responses of cat retinal ganglion cells. » Journal of General Physiology 89, no 4 (1 avril 1987) : 599–628. http://dx.doi.org/10.1085/jgp.89.4.599.

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Spatiotemporal frequency responses were measured at different levels of light adaptation for cat X and Y retinal ganglion cells. Stationary sinusoidal luminance gratings whose contrast was modulated sinusoidally in time or drifting gratings were used as stimuli. Under photopic illumination, when the spatial frequency was held constant at or above its optimum value, an X cell's responsivity was essentially constant as the temporal frequency was changed from 1.5 to 30 Hz. At lower temporal frequencies, responsivity rolled off gradually, and at higher ones it rolled off rapidly. In contrast, when the spatial frequency was held constant at a low value, an X cell's responsivity increased continuously with temporal frequency from a very low value at 0.1 Hz to substantial values at temporal frequencies higher than 30 Hz, from which responsivity rolled off again. Thus, 0 cycles X deg-1 became the optimal spatial frequency above 30 Hz. For Y cells under photopic illumination, the spatiotemporal interaction was even more complex. When the spatial frequency was held constant at or above its optimal value, the temporal frequency range over which responsivity was constant was shorter than that of X cells. At lower spatial frequencies, this range was not appreciably different. As for X cells, 0 cycles X deg-1 was the optimal spatial frequency above 30 Hz. Temporal resolution (defined as the high temporal frequency at which responsivity had fallen to 10 impulses X s-1) for a uniform field was approximately 95 Hz for X cells and approximately 120 Hz for Y cells under photopic illumination. Temporal resolution was lower at lower adaptation levels. The results were interpreted in terms of a Gaussian center-surround model. For X cells, the surround and center strengths were nearly equal at low and moderate temporal frequencies, but the surround strength exceeded the center strength above 30 Hz. Thus, the response to a spatially uniform stimulus at high temporal frequencies was dominated by the surround. In addition, at temporal frequencies above 30 Hz, the center radius increased.
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Regan, D. « Storage of spatial-frequency information and spatial-frequency discrimination ». Journal of the Optical Society of America A 2, no 4 (1 avril 1985) : 619. http://dx.doi.org/10.1364/josaa.2.000619.

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Hague, Michael T. J., Heidi Mavengere, Daniel R. Matute et Brandon S. Cooper. « Environmental and Genetic Contributions to Imperfect wMel-Like Wolbachia Transmission and Frequency Variation ». Genetics 215, no 4 (16 juin 2020) : 1117–32. http://dx.doi.org/10.1534/genetics.120.303330.

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Maternally transmitted Wolbachia bacteria infect about half of all insect species. They usually show imperfect maternal transmission and often produce cytoplasmic incompatibility (CI). Irrespective of CI, Wolbachia frequencies tend to increase when rare only if they benefit host fitness. Several Wolbachia, including wMel that infects Drosophila melanogaster, cause weak or no CI and persist at intermediate frequencies. On the island of São Tomé off West Africa, the frequencies of wMel-like Wolbachia infecting Drosophila yakuba (wYak) and Drosophila santomea (wSan) fluctuate, and the contributions of imperfect maternal transmission, fitness effects, and CI to these fluctuations are unknown. We demonstrate spatial variation in wYak frequency and transmission on São Tomé. Concurrent field estimates of imperfect maternal transmission do not predict spatial variation in wYak frequencies, which are highest at high altitudes where maternal transmission is the most imperfect. Genomic and genetic analyses provide little support for D. yakuba effects on wYak transmission. Instead, rearing at cool temperatures reduces wYak titer and increases imperfect transmission to levels observed on São Tomé. Using mathematical models of Wolbachia frequency dynamics and equilibria, we infer that temporally variable imperfect transmission or spatially variable effects on host fitness and reproduction are required to explain wYak frequencies. In contrast, spatially stable wSan frequencies are plausibly explained by imperfect transmission, modest fitness effects, and weak CI. Our results provide insight into causes of wMel-like frequency variation in divergent hosts. Understanding this variation is crucial to explain Wolbachia spread and to improve wMel biocontrol of human disease in transinfected mosquito systems.
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Kim, Jin-Young, Hyun-Han Kwon et Jeong-Yeul Lim. « Development of Hierarchical Bayesian Spatial Regional Frequency Analysis Model Considering Geographical Characteristics ». Journal of Korea Water Resources Association 47, no 5 (31 mai 2014) : 469–82. http://dx.doi.org/10.3741/jkwra.2014.47.5.469.

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Bilotta, J., et I. Abramov. « Spatial properties of goldfish ganglion cells. » Journal of General Physiology 93, no 6 (1 juin 1989) : 1147–69. http://dx.doi.org/10.1085/jgp.93.6.1147.

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We systematically classified goldfish ganglion cells according to their spatial summation properties using the same techniques and criteria used in cat and monkey research. Results show that goldfish ganglion cells can be classified as X-, Y-, or W-like based on their responses to contrast-reversal gratings. Like cat X cells, goldfish X-like cells display linear spatial summation. Goldfish Y-like cells, like cat Y cells, respond with frequency doubling at all spatial positions when the contrast-reversal grating consists of high spatial frequencies. There is also a third class of neurons, which is neither X- nor Y-like; many of these cells' properties are similar to those of the "not-X" cells found in the eel retina. Spatial filtering characteristics were obtained for each cell by drifting sinusoidal gratings of various spatial frequencies and contrasts across the receptive field of the cell at a constant temporal rate. The spatial tuning curves of the cell depend on the temporal parameters of the stimulus; at high drift rates, the tuning curves lose their low spatial frequency attenuation. To explore this phenomenon, temporal contrast response functions were derived from the cells' responses to a spatially uniform field whose luminance varied sinusoidally in time. These functions were obtained for the center, the surround, and the entire receptive field. The results suggest that differences in the cells' spatial filtering across stimulus drift rate are due to changes in the interaction of the center and surround mechanisms; at low temporal frequencies, the center and surround responses are out-of-phase and mutually antagonistic, but at higher temporal rates their responses are in-phase and their interaction actually enhances the cell's responsiveness.
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Moraglia, Giampaolo. « Visual Search : Spatial Frequency and Orientation ». Perceptual and Motor Skills 69, no 2 (octobre 1989) : 675–89. http://dx.doi.org/10.2466/pms.1989.69.2.675.

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Observers searched for a Gaussian-windowed patch of sinewave grating (Gabor pattern) through displays containing varying numbers of other such patterns (distractors). When the spatial frequencies of target and distractors differed by ± 2 octaves and their orientations by ± 60°, the search proceeded spatially in parallel irrespective of whether the target could be discriminated in terms of spatial frequency differences alone, orientation differences alone, or their combination. However, when target and distractors differed by only ±.5 octave in spatial frequency and by ± 15° in orientation, the search was serial and self-terminating, again irrespective of the nature of the target-distractor differences. These findings show that, contrary to some suggestions, the preattentive detection of targets defined by conjunctions of spatial frequency and orientation may occur, but only when the spectral distance between target and distractors allows their encoding by independent mechanisms.
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Djerir, Wahiba, Tarek Boutkedjirt, Ali Badidi Bouda et A. Satour. « Application of Inverse Methods for Spatial Deconvolution of Pulsed Ultrasound Fields Radiated in Solids ». Materials Science Forum 636-637 (janvier 2010) : 1541–47. http://dx.doi.org/10.4028/www.scientific.net/msf.636-637.1541.

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When measuring the ultrasound field, the signal provided by the receiving transducer is affected by its spatial properties. Particularly, the displacement normal to its surface is spatially averaged because of the receiver finite size. In this study, we show using a numerical simulation, the effectiveness of the spatial deconvolution of these effects for a rectangular transducer. For that, three methods allowing the inversion of the aperture effect are tested 1) Wiener’s method; 2) the power spectral equalization (PSE) method, and 3) the maximum a posteriori (MAP) method. The obtained results show that the three methods are able to reconstruct the ultrasound field from the spatially averaged values and the quality of the reconstruction depends strongly upon the signal to noise ratio (SNR) and the spatial frequencies of the ultrasound field investigated
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Rosli, Yanti, Suzanne M. Bedford et Ted Maddess. « Low-Spatial-Frequency Channels and the Spatial Frequency-Doubling Illusion ». Investigative Opthalmology & ; Visual Science 50, no 4 (1 avril 2009) : 1956. http://dx.doi.org/10.1167/iovs.08-1810.

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Yager, Dean, et Patricia Kramer. « A model for perceived spatial frequency and spatial frequency discrimination ». Vision Research 31, no 6 (janvier 1991) : 1067–72. http://dx.doi.org/10.1016/0042-6989(91)90210-v.

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Schor, Clifton M., et Peter A. Howarth. « Suprathreshold Stereo-Depth Matches as a Function of Contrast and Spatial Frequency ». Perception 15, no 3 (juin 1986) : 249–58. http://dx.doi.org/10.1068/p150249.

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Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg−1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies. Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15–9.6 cycles deg−1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10–100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of crossed disparity were required to match broad than narrow DOGs to the apparent depth of the standard black line. The matched disparity was greater at low than at high contrast levels. When perceived contrast of all the DOGs was matched to standard contrasts ranging from 5–72%, disparity for depth matches became similar for narrow and broad DOGs. 200 ms pulsed presentations of DOGs with equal perceived contrast further reduced the disparity of low-contrast broad DOGs needed to match the standard depth. A perceived-depth bias in the uncrossed direction at low spatial frequencies was noted in these experiments. This was most pronounced for low-contrast low-spatial-frequency targets, which actually needed crossed disparities to make a depth match to an uncrossed standard. This bias was investigated further by making depth matches to a zero-disparity standard (ie the apparent fronto-parallel plane). Broad DOGs, which are composed of low spatial frequencies, were perceived behind the fixation plane when they actually subtended zero disparity. The magnitude of this low-frequency depth bias increased as contrast was reduced. The distal depth bias was also perceived monocularly, however, it was always greater when viewed binocularly. This investigation indicates that contrast fading of low-spatial-frequency stimuli changes their perceived depth and enhances a depth bias in the uncrossed direction. The depth bias has both a monocular and a binocular component.
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Hirsch, Joy, et Ron Hylton. « Spatial-frequency discrimination at low frequencies : evidence for position quantization by receptive fields ». Journal of the Optical Society of America A 2, no 2 (1 février 1985) : 128. http://dx.doi.org/10.1364/josaa.2.000128.

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Aiken, Bonnie E., et Michael S. Loop. « Visual reaction time of cats to different spatial frequencies ». Visual Neuroscience 5, no 6 (décembre 1990) : 557–64. http://dx.doi.org/10.1017/s0952523800000717.

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AbstractIf physiological mechanisms similar to cat Y and X cells explain faster detection of low spatial frequencies by humans, then cats should show the same effect. We have tested this prediction by determining the visual reaction time of cats over a range of spatial frequencies and contrasts by training them to respond quickly when a vertical sine-wave grating was presented. At 50% contrast, the cat's visual reaction time increased monotonically from 0.25–2.0 cpd (cycle/deg). At every spatial frequency tested, the cat's reaction time increased monotonically as contrast decreased. By determining contrast threshold (70% detection) at each spatial frequency, it was possible to determine reaction times for different spatial frequencies at equal physical contrasts and equal “threshold equivalent” contrasts. Some of the cat's faster detection of low spatial frequencies was due to sensitivity differences and some was not. To determine if faster detection of low spatial frequencies was based upon Y cells, we took advantage of the fact that Y cells show a strong peripheral effect while X cells do not. Low and high spatial frequencies were detected in the presence of a flickering (7 Hz) or steady (70 Hz) surround. Surround frequency had no effect upon reaction times to 2.0 cpd but the flickering surround increased reaction times to 0.25 cpd. These results indicate that, in cats, rapid detection of low spatial frequencies is by Y cells and slower detection of high spatial frequencies is by X cells.
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Dong Ziming, 董子铭, 章亚男 Zhang Yanan, 刘志刚 Liu Zhigang, 焦翔 Jiao Xiang, 朱健强 Zhu Jianqiang, 崔文辉 Cui Wenhui et 林炜恒 Lin Weiheng. « 偏心双转子运动抛光工艺研究及中频误差的抑制研究 ». Chinese Journal of Lasers 48, no 24 (2021) : 2404002. http://dx.doi.org/10.3788/cjl202148.2404002.

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Dong Ziming, 董子铭, 章亚男 Zhang Yanan, 刘志刚 Liu Zhigang, 焦翔 Jiao Xiang, 朱健强 Zhu Jianqiang, 崔文辉 Cui Wenhui et 林炜恒 Lin Weiheng. « 偏心双转子运动抛光工艺研究及中频误差的抑制研究 ». Chinese Journal of Lasers 48, no 24 (2021) : 2402001. http://dx.doi.org/10.3788/cjl202148.2402001.

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YAMAGUCHI, Hiromichi, et Makoto OKUMURA. « 1C33 Temporal and Spatial Differences of Leisure Travel Frequency Distribution in Japan(Traffic Planning) ». Proceedings of International Symposium on Seed-up and Service Technology for Railway and Maglev Systems : STECH 2015 (2015) : _1C33–1_—_1C33–12_. http://dx.doi.org/10.1299/jsmestech.2015._1c33-1_.

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Parish, David H., et George Sperling. « Object spatial frequencies, retinal spatial frequencies, noise, and the efficiency of letter discrimination ». Vision Research 31, no 7-8 (janvier 1991) : 1399–415. http://dx.doi.org/10.1016/0042-6989(91)90060-i.

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Kozhevnikov, E. N., et Y. V. Samoilova. « SPATIALLY MODULATED STRUCTURES IN NEMATIC LIQUID CRYSTAL UNDER OSCILLATORY COUETTE FLOW AT ULTRALOW FREQUENCIES ». Vestnik of Samara University. Natural Science Series 18, no 6 (9 juin 2017) : 113–23. http://dx.doi.org/10.18287/2541-7525-2012-18-6-113-123.

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The distortion of homeotropic structure of nematic liquid crystal under the influence of periodic shear at ultralow frequencies is theoretically described. It is shown that periodic shear action results in spatially modulated structure in NLC layer. Threshold shear amplitude and spacial periodicity of new molecular orientational structure are found on the base of nonlinear nematic liquid crystal hydrodynamic equations using Galerkin method. Theoretical analysis shows that at low frequencies threshold shear amplitude does not depend on frequency and the spatial period of the order of NLC layer thickness. Theoretical results are compared with experimental data.
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Morgan, M. J., et R. M. Ward. « Spatial and spatial-frequency primitives in spatial-interval discrimination ». Journal of the Optical Society of America A 2, no 7 (1 juillet 1985) : 1205. http://dx.doi.org/10.1364/josaa.2.001205.

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Nolt, M. J., R. D. Kumbhani et L. A. Palmer. « Suppression at High Spatial Frequencies in the Lateral Geniculate Nucleus of the Cat ». Journal of Neurophysiology 98, no 3 (septembre 2007) : 1167–80. http://dx.doi.org/10.1152/jn.01019.2006.

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The spatial weighting functions of both retinal and lateral geniculate nucleus (LGN) X-cell receptive fields have been viewed as the difference of two Gaussians (DOG). We focus on a particular shortcoming of the DOG model, that is, suppression of responses of LGN cells at spatial frequencies above those to which the classical receptive field surround is responsive. By simultaneously recording one of the retinal ganglion cell (RGC) inputs (S-potentials) to an LGN cell, we find that half of this suppression at high spatial frequencies arises from the retinal input and that suppression in LGN cells is greater than that in RGCs, regardless of spatial frequency. We also inactivated the ipsilateral visual cortex and show that one quarter of the suppression at high spatial frequencies arises from corticothalamic feedback. We show that this suppression at high spatial frequencies is colocalized with the classical surround, is not dependent on the relative orientation of the center and surround stimuli, and that the cortical component of this suppression is divisive. We propose that the role of this suppression at high spatial frequencies is to restrict the response to large stimuli composed of high spatial frequencies.
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Babenko, V. V. « Spatial Extent of Masking by Sine-Wave Gratings ». Perception 26, no 1_suppl (août 1997) : 74. http://dx.doi.org/10.1068/v970219.

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The contrast threshold for detection of a target consisting of 1.5 periods of a sine wave was determined as a function of the number of cycles in a sinusoidal mask with the same spatial frequency and orientation. The test frequencies were 2, 4, and 8 cycles deg−1. The masks were spatial-frequency modulated so as to equate their spectral extent. Stimuli were seen monocularly in Maxwellian view at a mean luminance of 10 cd m−2. The contrast threshold in a backward masking paradigm was determined by a 2AFC staircase. Data were obtained from three subjects with normal vision. It was found that as the number of cycles in the mask was increased, the contrast threshold fell, but only to a certain level. The full range of the threshold decrement was about 2 dB. At all the spatial frequencies tested, the final threshold level was reached with 3 cycles in the mask and then remained unaffected by a further increase in the number of cycles. The results implicate frequency-tuned mechanisms of very restricted spatial extent. It is suggested that these may underlie processing of spatially distributed information at the post-striate stages.
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Bex, P. J., F. A. J. Verstraten et I. Mareschal. « Temporal and Spatial Frequency Tuning of the Flicker Motion Aftereffect ». Perception 25, no 1_suppl (août 1996) : 12. http://dx.doi.org/10.1068/v96l0804.

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The motion aftereffect (MAE) was used to study the temporal-frequency and spatial-frequency selectivity of the visual system at suprathreshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the MAE induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest MAE was found with slowly counterphasing test gratings (∼0.125 – 0.25 Hz). For slowly counterphasing test gratings (<∼2 Hz), the largest MAEs were found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 cycle deg−1). However, such narrow spatial frequency tuning was lost when the temporal frequency of the test grating was increased. The data suggest that MAEs are dominated by a single, low-pass temporal-frequency mechanism and by a series of band-pass spatial-frequency mechanisms at low temporal frequencies. At higher test temporal frequencies, the loss of spatial-frequency tuning implicates separate mechanisms with broader spatial frequency tuning.
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Quan Li, Quan Li, Zhentao Duan Zhentao Duan, Huizu Lin Huizu Lin, Shaobo Gao Shaobo Gao, Shuai Sun Shuai Sun et and Weitao Liu and Weitao Liu. « Coprime-frequencied sinusoidal modulation for improving the speed of computational ghost imaging with a spatial light modulator ». Chinese Optics Letters 14, no 11 (2016) : 111103–6. http://dx.doi.org/10.3788/col201614.111103.

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Friedl, Wendel M., et Andreas Keil. « Effects of Experience on Spatial Frequency Tuning in the Visual System : Behavioral, Visuocortical, and Alpha-band Responses ». Journal of Cognitive Neuroscience 32, no 6 (juin 2020) : 1153–69. http://dx.doi.org/10.1162/jocn_a_01524.

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Using electrophysiology and a classic fear conditioning paradigm, this work examined adaptive visuocortical changes in spatial frequency tuning in a sample of 50 undergraduate students. High-density EEG was recorded while participants viewed 400 total trials of individually presented Gabor patches of 10 different spatial frequencies. Patches were flickered to produce sweep steady-state visual evoked potentials (ssVEPs) at a temporal frequency of 13.33 Hz, with stimulus contrast ramping up from 0% to 41% Michelson over the course of each 2800-msec trial. During the final 200 trials, a selected range of Gabor stimuli (either the lowest or highest spatial frequencies, manipulated between participants) were paired with an aversive 90-dB white noise auditory stimulus. Changes in spatial frequency tuning from before to after conditioning for paired and unpaired gratings were evaluated at the behavioral and electrophysiological level. Specifically, ssVEP amplitude changes were evaluated for lateral inhibition and generalization trends, whereas change in alpha band (8–12 Hz) activity was tested for a generalization trend across spatial frequencies, using permutation-controlled F contrasts. Overall time courses of the sweep ssVEP amplitude envelope and alpha-band power were orthogonal, and ssVEPs proved insensitive to spatial frequency conditioning. Alpha reduction (blocking) was most pronounced when viewing fear-conditioned spatial frequencies, with blocking decreasing along the gradient of spatial frequencies preceding conditioned frequencies, indicating generalization across spatial frequencies. Results suggest that alpha power reduction—conceptually linked to engagement of attention and alertness/arousal mechanisms—to fear-conditioned stimuli operates independently of low-level spatial frequency processing (indexed by ssVEPs) in primary visual cortex.
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Frye, J. Michael, et Anthony Q. Martin. « Wideband Extrapolation of Spatial Responses of Resonant Structures Using Early-Time and Low-Frequency Data ». Journal of Computational Methods in Physics 2013 (22 octobre 2013) : 1–12. http://dx.doi.org/10.1155/2013/563724.

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An efficient procedure is presented to extrapolate a wideband electromagnetic response defined over an arbitrary spatial region using early-time and low-frequency data. The previous procedures presented in the literature are efficient for single-point extrapolation and can readily be applied to spatial regions but are terribly inefficient when a response is desired at many spatial locations. In this work, an optimized algorithm is presented to quickly extrapolate over a large number of spatial locations. The time and frequency behavior of the response is fitted by polynomials and pole terms, and the spatial variation is represented with spatially dependent polynomial coefficients and pole residues. A single set of poles, common to all spatial locations of interest, is shown to sufficiently describe the resonant behavior of response over the entire spatial region. A multisignal formulation of the matrix pencil method is applied to determine poles from early time data. Numerical examples are presented to demonstrate the procedure. Additionally, an automated approach to distinguish physical poles, which correspond to structural resonances, from nonphysical fitting poles is presented. The spatially dependent residues of physical pole terms, referred to here as modal residues, are shown to provide important insight into the resonant behavior of a structure.
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Nordin, Rosdiadee. « Exploiting Spatial and Frequency Diversity in Spatially Correlated MU-MIMO Downlink Channels ». Journal of Computer Networks and Communications 2012 (2012) : 1–10. http://dx.doi.org/10.1155/2012/414796.

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The effect of self-interference due to the increase of spatial correlation in a MIMO channel has become one of the limiting factors towards the implementation of future network downlink transmissions. This paper aims to reduce the effect of self-interference in a downlink multiuser- (MU-) MIMO transmission by exploiting the available spatial and frequency diversity. The subcarrier allocation scheme can exploit the frequency diversity to determine the self-interference from the ESINR metric, while the spatial diversity can be exploited by introducing the partial feedback scheme, which offers knowledge of the channel condition to the base station and further reduces the effect before the allocation process takes place. The results have shown that the proposed downlink transmission scheme offers robust bit error rate (BER) performance, even when simulated in a fully correlated channel, without imposing higher feedback requirements on the base controller.
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Boles, David B., et Marcia L. Morelli. « Hemispheric sensitivity to spatial frequencies ». Bulletin of the Psychonomic Society 26, no 6 (décembre 1988) : 552–55. http://dx.doi.org/10.3758/bf03330119.

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hammal, z., f. gosselin, i. peretz et s. hebert. « Spatial Frequencies Mediating Music Reading ». Journal of Vision 10, no 7 (12 août 2010) : 971. http://dx.doi.org/10.1167/10.7.971.

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Delplanque, Sylvain, Karim N’diaye, Klaus Scherer et Didier Grandjean. « Spatial frequencies or emotional effects ? » Journal of Neuroscience Methods 165, no 1 (septembre 2007) : 144–50. http://dx.doi.org/10.1016/j.jneumeth.2007.05.030.

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Kankaanpää, M. I., J. Rovamo, H. T. Kukkonen et J. Hallikainen. « Spatial Integration of Objectively Equiluminous Chromatic Gratings ». Perception 26, no 1_suppl (août 1997) : 200. http://dx.doi.org/10.1068/v970297.

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Contrast sensitivity functions for achromatic and chromatic gratings tend to be band-pass and low-pass in shape, respectively. Our aim was to test whether spatial integration contributes to the shape difference found at low spatial frequencies. We measured binocular chromatic contrast sensitivity as a function of grating area for objectively equiluminous red - green and blue - yellow chromatic gratings. Chromatic contrast refers to the Michelson contrast of either of the two chromatic component gratings presented in counterphase against the combined background. Grating area ( A) varied from 1 to 256 square cycles ( Af2) at spatial frequencies ( f) of 0.125 – 4.0 cycles deg−1. We used only horizontal gratings at low and medium spatial frequencies to minimise the transverse and longitudinal chromatic aberrations due to ocular optics. At all spatial frequencies studied, chromatic contrast sensitivity increased with grating area. Ac was found to be constant at low spatial frequencies (0.125 – 0.5 cycles deg−1) but decreased in inverse proportion to increasing spatial frequency at 1 – 4 cycles deg−1. Thus, spatial integration depends similarly on spatial frequency for achromatic (Luntinen et al, 1995 Vision Research35 2339 – 2346) and chromatic gratings, and differences in spatial integration do not contribute to the shape difference of the respective contrast sensitivity functions.
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Nautiyal, Atul, Samuel H. Gray, N. D. Whitmore et John D. Garing. « Stability versus accuracy for an explicit wavefield extrapolation operator ». GEOPHYSICS 58, no 2 (février 1993) : 277–83. http://dx.doi.org/10.1190/1.1443412.

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Wavefield extrapolation by recursive (depth‐by‐ depth) application of a convolutional operator in the frequency‐space domain, commonly used for depth migration in a laterally‐varying earth, has interesting accuracy and stability properties. We analyze these properties by investigating the operator and its spatial Fourier transform. In particular, we show that the instability caused by spatially truncating the operator can be remedied unconditionally by applying an appropriately chosen spatial taper. However, unconditional stability is gained only at the expense of accuracy. We also identify frequencies and depth extrapolation step sizes for which the problems of accuracy or stability are the most pronounced.
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Amato, E. « A Model for the Spatial Distribution of Relativistic Electrons in the Crab Nebula ». Symposium - International Astronomical Union 195 (2000) : 371–72. http://dx.doi.org/10.1017/s0074180900163144.

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A model for the spatial distribution of relativistic electrons in the Crab Nebula is proposed. Particles injected in the vicinity of the pulsar propagate in a magnetic field of time-dependent but spatially constant intensity. A good description of the nebular synchrotron emission, from radio to X-ray frequencies, is obtained if particle diffusion with respect to the azimuthal field lines is taken into account.
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Gorman, Martin G., Suzanne J. Ali, Peter M. Celliers, Jonathan L. Peebles, David J. Erskine, James M. McNaney, Jon H. Eggert et Raymond F. Smith. « Measurement of shock roughness due to phase plate speckle imprinting relevant for x-ray diffraction experiments on 3rd and 4th generation light sources ». Journal of Applied Physics 132, no 17 (7 novembre 2022) : 175902. http://dx.doi.org/10.1063/5.0117905.

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Laser-shock compression experiments at 3rd and 4th generation light sources generally employ phase plates, which are inserted into the beamline to achieve a repeatable intensity distribution at the focal plane. Here, the laser intensity profile is characterized by a high-contrast, high-frequency laser speckle. Without sufficient smoothing, these laser non-uniformities can translate to a significant pressure distribution within the sample layer and can affect data interpretation in x-ray diffraction experiments. Here, we use a combination of one- and two-dimensional velocity interferometry to directly measure the extent to which spatial frequencies within the laser focal spot intensity pattern are smoothed out during propagation within the laser plasma and a polyimide ablator. We find that the use of thicker polyimide layers results in spatially smoother shock fronts, with the greatest degree of smoothing associated with the highest spatial frequencies. Focal spots with the smallest initial speckle separation produce the most rapid smoothing. Laser systems that employ smoothing by spectral dispersion techniques to rapidly modulate the focal plane intensity distribution are shown to be the most effective ones in producing a spatially smooth shock front. We show that a simple transport model combined with the known polyimide Hugoniot adequately describes the extent of shock smoothness as a function of polyimide thickness. Our results provide a description of spatial structure smoothing across a shock front, which can be used to design targets on x-ray free electron laser facilities.
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