Spatial Frequency Increases Research Articles

Study on power spectral density curves of track dynamic irregularity caused by earthquake-induced damage

A series of studies on track dynamic irregularity caused by earthquake-induced damage should be conducted in order to guarantee safe operation of high-speed trains. In this study, a nonlinear time history analysis of high-speed railway CRTS Ⅱ track-bridge system under transverse random earthquakes is carried out, and the post-earthquake stiffness degradation degree of main components of track-bridge system is analyzed. A three-dimensional coupled vibration model of train-track-bridge system based on earthquake-induced damage is established, and a sample library of earthquake-induced track dynamic irregularity is constructed. Kolmogorov-Smirnov method is used to test the normal distribution of earthquake-induced irregularity samples. A calculation method of the earthquake-induced dynamic irregularity power spectral density curves based on probability guarantee rate is proposed. The power spectral density curve is fitted by piecewise polynomial, and the influence of train operation speeds and peak ground accelerations (PGAs) on the power spectrum of earthquake-induced dynamic irregularity is analyzed. The results demonstrate that the degradation degree of piers and bearings of the track-bridge system under transverse random earthquake presents the characteristics of large in the middle and small on both sides. The earthquake-induced track directional irregularity samples follow the normal distribution. The shape of the power spectral density curve has obvious segmentation characteristics in the double logarithmic coordinate system. The power spectrum amplitude decreases significantly with the increase in spatial frequency and fluctuates significantly in the high frequency band. There are little influence of the train operation speeds on the power spectrum curves. In contrast, the influence of the PGAs on the power spectrum is significant. The amplitude of the power spectrum increases obviously as the PGA increases.

Increasing spatial frequency of S-cone defined gratings reduces their visibility and brain response more than for gratings defined by L-M cone contrast

Chromatic sensitivity reduces as spatial frequency increases. Here, we explore the behavioural and neuronal responses to chromatic stimuli at two spatial frequencies for which the difference in sensitivity will be greater for S-cone than L-M stimuli. Luminance artefacts were removed using the Random Luminance Modulation (RLM) technique. As expected, doubling the spatial frequency increased the detection threshold more for S-cone than for isoluminant L-M gratings. We then used fMRI to measure the cortical BOLD responses to the same two chromatic stimuli (S and L-M) at the same two spatial frequencies. Responses were measured in six visual areas (V1, V2, V3, V3a, hV4, TO1/2). We found a significant interaction between spatial frequency in V1, V2 and V4 suggesting that the behaviourally observed increase in contrast threshold for high spatial frequency S-cone stimuli is reflected in these retinotopic areas. Our measurements show that neural responses consistent with psychophysical behaviour in a colour detection task can be observed as early as primary visual cortex.

Image enhancement with lightness correction and image sharpening based on characteristics of vision for elderly persons

In general, as people age the functions of their sensory organs deteriorate. For example, in the eye of an elderly person, the visual field becomes dark owing to a decrease in the amount of light reaching the retina. Moreover, objects appear blurred due to a decrease in contrast sensitivity. In particular, the contrast sensitivity decreases remarkably as the spatial frequency increases. In this study, we propose an image enhancement method that considers the visual characteristics of elderly persons. The proposed method combines lightness correction and amplification of high spatial frequency components to obtain images with optimized visibility for elderly persons. In particular, lightness enhancement that guarantees color gamut via tone mapping is conducted. In addition, image sharpening is applied based on the inverse characteristic function of the contrast sensitivity curve. The experiments were performed using several digital color images to confirm the effectiveness of the proposed method.

Effective detective quantum efficiency of two detectors in a prototype digital breast tomosynthesis

Digital breast tomosynthesis (DBT) system is a novel imaging modality that strongly depends on the detector performance. The effective detective quantum efficiency (eDQE) is a newly accepted system of performance analysis metric that solves the disadvantages of conventional DQE evaluations, which do not consider clinical operating conditions. To evaluate the eDQE, we used the direct-conversion based Anrad detector and the indirect-conversion based Dexela detector. Our aim is to study the performance of such detectors when they are mounted on a prototype DBT system. The prototype DBT system consists of a CsI(Tl) scintillator/CMOS-based flat panel digital detector and an a-Se based direct conversion detector developed by the Korea Electrotechnology Research Institute (KERI). The performance of the two detectors was compared on the basis of scatter fraction, transmission factor, effective modulation transfer function (eMTF), and effective normalized noise power spectrum (eNNPS). As a result, the Dexela detector is characterized by a decreasing eNNPS trend as the spatial frequency increases, whereas the Anrad eNNPS looks almost independent of the spatial frequency. The eMTF was found to be higher in the Anrad detector compared to that in the Dexela one for all frequencies. The eDQE values at 1 cycle/mm (low frequency) were 0.11 and 0.17 for the Anrad and Dexela detectors, respectively, and the values of the spatial frequency feDQE(0.1), corresponding to eDQE = 0.1, were 1.25 and 2.81 cycles/mm for the Anrad and Dexela detectors, respectively. The Dexela detector also presented a higher contrast-to-noise ratios (CNRs) for all different sizes of μCa groups, thus demonstrating overall better eDQE results in imaging performance compared to the Anrad detector.

Cortical Balance Between ON and OFF Visual Responses Is Modulated by the Spatial Properties of the Visual Stimulus.

The primary visual cortex of carnivores and primates is dominated by the OFF visual pathway and responds more strongly to dark than light stimuli. Here, we demonstrate that this cortical OFF dominance is modulated by the size and spatial frequency of the stimulus in awake primates and we uncover a main neuronal mechanism underlying this modulation. We show that large grating patterns with low spatial frequencies drive five times more OFF-dominated than ON-dominated neurons, but this pronounced cortical OFF dominance is strongly reduced when the grating size decreases and the spatial frequency increases, as when the stimulus moves away from the observer. We demonstrate that the reduction in cortical OFF dominance is not caused by a selective reduction of visual responses in OFF-dominated neurons but by a change in the ON/OFF response balance of neurons with diverse receptive field properties that can be ON or OFF dominated, simple, or complex. We conclude that cortical OFF dominance is continuously adjusted by a neuronal mechanism that modulates ON/OFF response balance in multiple cortical neurons when the spatial properties of the visual stimulus change with viewing distance and/or optical blur.

Bayesian Modeling of Motion Perception Using Dynamical Stochastic Textures.

A common practice to account for psychophysical biases in vision is to frame them as consequences of a dynamic process relying on optimal inference with respect to a generative model. The study presented here details the complete formulation of such a generative model intended to probe visual motion perception with a dynamic texture model. It is derived in a set of axiomatic steps constrained by biological plausibility. We extend previous contributions by detailing three equivalent formulations of this texture model. First, the composite dynamic textures are constructed by the random aggregation of warped patterns, which can be viewed as three-dimensional gaussian fields. Second, these textures are cast as solutions to a stochastic partial differential equation (sPDE). This essential step enables real-time, on-the-fly texture synthesis using time-discretized autoregressive processes. It also allows for the derivation of a local motion-energy model, which corresponds to the log likelihood of the probability density. The log likelihoods are essential for the construction of a Bayesian inference framework. We use the dynamic texture model to psychophysically probe speed perception in humans using zoom-like changes in the spatial frequency content of the stimulus. The human data replicate previous findings showing perceived speed to be positively biased by spatial frequency increments. A Bayesian observer who combines a gaussian likelihood centered at the true speed and a spatial frequency dependent width with a "slow-speed prior" successfully accounts for the perceptual bias. More precisely, the bias arises from a decrease in the observer's likelihood width estimated from the experiments as the spatial frequency increases. Such a trend is compatible with the trend of the dynamic texture likelihood width.

The effects of the spatial frequency and size of stripes on the hue appearance of striped woven fabrics

In this research, the effects of the spatial properties, that is, spatial frequency and size, of stripes on their color appearance in striped woven fabrics were investigated. The lightness, colorfulness, and hue of stripes in three types of 180 striped fabrics, in which stripes were arranged in different spatial frequencies and sizes with different adjacent colors, were visually estimated by observers. Through the statistical analysis of the visual estimates, it was found that the spatial properties of stripes affect their hue appearance, while their lightness and colorfulness appearances are not significantly affected. In general, when the stripes become thinner and increase in spatial frequency, their hues appear more different from their actual hues. These effects of spatial properties on hue appearance were found to become stronger with lighter and more chromatic adjacent colors. Finally, a prediction model of the combined effects of the spatial properties of stripes and the colorimetric properties of adjacent colors was derived, and its great predictive accuracy was proven through numerical and statistical evaluations. It is envisaged that not only will the findings of this research provide advantageous conditions to design striped woven fabrics with desired color appearances, but they will also be the foundation for modeling various color appearance effects that occur in bicolor or multicolor patterned fabrics.

Comparison of two methods for measuring contrast sensitivity in anisometropic amblyopia

PurposeThis study aimed to evaluate two psychophysical contrast sensitivity testing methods in amblyopic patients. MethodsThirty-three adults with anisometropic amblyopia participated in this study. Psychophysical contrast sensitivity was measured for both amblyopic and fellow eyes of the participants at 1, 3, and 5 cycles per degree (cpd) spatial frequencies by Freiburg visual acuity and contrast test (FrACT) and Metrovision contrast sensitivity test, which employ sine-wave gratings for measurement of contrast sensitivity. We evaluated the correlation between the two tests and used Bland–Altman analysis to measure the agreement between the two methods. ResultsExcept for 1 cpd in amblyopic eyes, FrACT showed significantly higher contrast sensitivity measurements than Metrovision at all spatial frequencies both in normal and amblyopic eyes (P < 0.01). The difference between the two methods increased with an increase in spatial frequency. There was a significant correlation between the two tests at most of the spatial frequencies. While the difference between the results of the two tests increased with an increase in contrast sensitivity in amblyopic eyes, we found an inter-test agreement in normal eyes. ConclusionAlthough both FrACT and Metrovision employ sine-wave gratings to measure contrast sensitivity, there are some differences between them, and their results can not be used interchangeably.

Analysis of the influence of diattenuation on optical imaging system by using the theory of vector plane wave spectrum

In most of the researches of polarization aberration, the influence of diattenuation is not large enough to affect imaging quality evidently. However, the modulation transfer function decreases when optical elements with complex planar dielectric structures and low transmittance, such as beam-splitter and optical modulator, are introduced into an imaging system. In this paper, a vector optical model in Descartes coordinate system is proposed based on the concept of vector plane wave spectrum (VPWS). The results of calculation show that the VPWS model is consistent with Debye model. Compared with Debye vector diffraction integral, the VPWS method is more suitable to the description of the PA introduced by planar optical device with opaque mask, such as larger surface quantum-confined-stark-effect electro-absorption modulator, which is used to modulate the light collected by optical antenna of time-of-flight (TOF) depth system or modulating-retroreflector free-space-optical communication system. In order to simplify the calculation and obtain the conclusion of the change in imaging quality directly, the formula of optical transfer function is decomposed into three parts (TE component, TM component and the correlation of them) instead of polynomial expansion of pupil function. The influences of diattenuation on MTF is studied globally and locally in a range of cut-off frequency of optical imaging system (2NA/ ). Allowance of diattenuation is analysed by numerical calculation, and a mathematical expression is derived. The result shows that the change of diattenuation can be neglected when the spatial frequency v is less than 0.2NA/, and the range of allowance decreases with the increase of spatial frequency. According to numerical calculation shown in Fig.7 and the derived formulas (15) and (16), the ratios of reflection/transmission coefficient of s-light and p-light D should range respectively from 0.63 to 1.6(0.2NA/ v 0.8NA/) and from 0.9 to 1.11(v0.8NA/ ) when the MTF is required to be not less than 90% of the value in ideal diffraction-limited system. The range of allowance becomes larger gradually with the increase of angle n between the normal of optical interface n and the optical axis of imaging system z. If a polarization beam splitter is considered, D,n sin-1 NA should be greater than 1-3.

Influence of 4,4'-azobis (4-cyanopentanoic acid) in Transmission and Reflection Gratings Stored in a PVA/AA Photopolymer.

Holographic transmission gratings with a spatial frequency of 2658 lines/mm and reflection gratings with a spatial frequency of 4553 lines/mm were stored in a polyvinyl alcohol (PVA)/acrylamide (AA) based photopolymer. This material can reach diffraction efficiencies close to 100% for spatial frequencies about 1000 lines/mm. However, for higher spatial frequencies, the diffraction efficiency decreases considerably as the spatial frequency increases. To enhance the material response at high spatial frequencies, a chain transfer agent, the 4,4’-azobis (4-cyanopentanoic acid), ACPA, is added to the composition of the material. Different concentrations of ACPA are incorporated into the main composition of the photopolymer to find the concentration value that provides the highest diffraction efficiency. Moreover, the refractive index modulation and the optical thickness of the transmission and reflection gratings were obtained, evaluated and compared to procure more information about the influence of the ACPA on them.

The effect of spatial frequency on peripheral collinear facilitation

The detection of a Gabor patch (target) can be decreased or improved by the presence of co-oriented Gabor patches (flankers) having the same spatial frequency as the target. These phenomena are thought to be mediated by lateral interactions. Depending on the distance between target and flankers, commonly defined as a multiple of the wavelength (λ) of the carrier, flankers can increase or decrease a target’s detectability. Studies with foveal presentation showed that for target-to-flankers distances <2λ contrast thresholds for the central target increase, while for target-to-flankers distances >3λ contrast thresholds decrease. Earlier studies on collinear facilitation at the near-periphery of the visual field (4° of eccentricity) showed inconsistent facilitation (Shani & Sagi, 2005, Vision Research, 45, 2009–2024) whereas more recent studies showed consistent facilitation for larger separations (7–8λ) (Maniglia et al., 2011, PLoS ONE, 6, e25568; Lev & Polat, 2011, Vision Research, 51, 2488–2498). However, all of these studies used medium-to-high spatial frequencies (3–8cpd). In this study we tested lower spatial frequencies (1, 2, and 3cpd) with different target-to-flankers distances. The rationale was that near-peripheral vision is tuned for lower spatial frequencies and this could be reflected in collinear facilitation. Results show consistent collinear facilitation at 8λ for all the spatial frequencies tested, but also show collinear facilitation at shorter target-to-flanker distance (6λ) for the lowest spatial frequencies tested (1cpd). Additionally, collinear facilitation decreases as spatial frequency increases; opposite to the findings of Polat (2009, Spatial Vision, 22, 179–193) in the fovea, indicating a different spatial frequency tuning between foveal and peripheral lateral interactions.

Scalable spatial superresolution using entangled photons.

N00N states-maximally path-entangled states of N photons-exhibit spatial interference patterns sharper than any classical interference pattern. This is known as superresolution. However, even given perfectly efficient number-resolving detectors, the detection efficiency of all previous measurements of such interference would decrease exponentially with the number of photons in the N00N state, often leading to the conclusion that N00N states are unsuitable for spatial measurements. A technique known as the "optical centroid measurement" has been proposed to solve this and has been experimentally verified for photon pairs; here we present the first extension beyond two photons, measuring the superresolution fringes of two-, three-, and four-photon N00N states. Moreover, we compare the N00N-state interference to the corresponding classical superresolution interference. Although both provide the same increase in spatial frequency, the visibility of the classical fringes decreases exponentially with the number of detected photons. Our work represents an essential step forward for quantum-enhanced measurements, overcoming what was believed to be a fundamental challenge to quantum metrology.

Effect of Linear Absorption on Efficiency of SHG in Layered Structure

Frequency up-conversion processes have many applications in spectroscopy, optical communications, coherence measurements, etc. On the other hand, periodic layered media such as one-dimensional multilayer photonic crystal nd an increasing use in a wide range of telecommunications equipment due to their design exibility. By adding an additional periodicity to a bulk crystal it is possible to signi cantly alter its linear and nonlinear optical properties over a narrow wavelength range. In many areas of nonlinear optics the main problem is that of phase matching (PM). On the harmonic conversion process in a bulk medium, under phase mismatching (PMM) condition, because of the spatial biting between di erent frequencies, the complete conversion of the fundamental wave to the second harmonic is not possible. As the PMM increases, the spatial frequency increases and leads to a decrease on intensity of the generated second harmonic wave [1]. The process of harmonic generation has been investigated in the layered structure [2 8]. In those studies, the linear absorptions of fundamental radiation and second harmonic wave are included in the formalisms. However, at the end when they are presenting their simulation results, the absorption coe cients are set to be zero. Meanwhile, the linear decrease of both, fundamental and generated harmonic waves can be an important parameter in practical results. In this work, we follow the formalisms that are presented by Tagiev et al. under intensity-constant approximation (ICA) [7, 8] in fundamental wave. The results show that the conversion e ciency can be signi cantly limited by linear absorption at the fundamental radiation and second harmonic wavelength.

The study on neural contrast sensitivity function at temporal frequencies

The neural contrast sensitivity function (NCSF) of human eye at temporal frequencies is acquired with a new method in this paper. Firstly the human eye's contrast sensitivity function (CSF) at temporal frequencies is measured by means of travelling-wave stimuli, and the modulation transfer function (MTF) of the eye's optics is obtained by constructing the eye model of the subject. Then the NCSF at temporal frequencies is calculated with the two functions. It is shown that the overall value of the NCSF decreases as the temporal frequency increases. As the temporal frequency increases from 0 cycles per second (c/s) to 1 c/s, 16 c/s and 30 c/s, the NCSF curve changes from the band-pass shape to the low-pass, and then to almost monotonic variation. The attenuation factor of the NCSF, which represents the sensitivity of visual neural system to the temporal frequency, varies little as spatial frequency increases, while that of the CSF declines dramatically in the region of high spatial frequencies. Because it is the NCSF, rather than the CSF, that reflects the characteristics of visual neural system, the investigation of the NCSF at temporal frequency is more essentially.

Subjective Impression of Discomfort Glare from Sources of Non-Uniform Luminance

A paired comparison experiment was performed to study the effects of spatial frequency and position on discomfort glare. The experiment required the subject to choose which of two stimuli, when presented together, caused more discomfort (termed the “choice” data), and then also rate how much more discomforting the one is than the other (termed the “magnitude” data). A rating scale experiment was also performed.Significant Findings:1. 1. The results show that both position and spatial frequency are significant predictors of discomfort. As position increases above the line of sight, glare decreases. As spatial frequency increases, discomfort increases, with a uniform stimulus causing the most discomfort (where “uniform” is defined here as infinite cycles per degree).2. 2. The results show that the effect of spatial frequency is more pronounced at positions closer to the line of sight.3. 3. The findings here contradict the CIE's complex extension to the Unified Glare Rating (UGR). The CIE complex extension clearly states that a complex source will always predict more glare than the uniform one, which is in direct opposition to the results found in the paired comparison experiment discussed here.4. 4. A relationship between contrast sensitivity and discomfort sensitivity was found. Contrast sensitivity peaks at a particular frequency, and it drops on either side of that peak frequency (at any given eccentricity). That peak frequency decreases as eccentricity increases (Fig. 1). This research shows that discomfort is minimized at a particular frequency, and it increases on either side of that minimum frequency (at any given position), which is contrary to the effect of contrast sensitivity.5. 5. For each position, the lowest glare perception occurred for the frequency which has approximately 3 receptive fields per cycle (see Fig. 23 through Fig. 26).6. 6. The paired comparison technique gave data with much less variability than the rating scale technique, as can be seen from the size of the error bars in Fig. 9 and Fig. 16. It is the recommendation of the authors that researchers consider using a paired comparison experimental technique for glare research in the future.

The relationship between temporal phase discrimination ability and the frequency doubling illusion

The frequency doubling illusion (FDI) occurs when a low spatial frequency sinusoidal grating is modulated at high temporal frequencies--its apparent spatial frequency increases. A recent study suggests that this illusion is perceived due to a frequency-dependent loss of temporal phase encoding ability. We sought to elucidate the relationship between temporal phase encoding and the FDI by exploring the spatiotemporal characteristics of temporal phase discrimination (TPD) thresholds using a novel stimulus comprising three grating patches presented simultaneously in a triangular pattern. A reference grating was presented superiorly, and six degrees below two gratings (one a copy of the reference) were each randomly presented in one of two fixed positions. The odd grating had abutting regions of spatial half-cycles with alternate half-cycles locked in temporal phase. The temporal phase difference between adjoining half-cycles was varied between 0 degrees and 180 degrees via QUEST staircase--subjects had to identify which lower stimulus appeared different from the reference grating. TPD thresholds were measured for 0.25, 0.50, and 2.20 cpd stimulus at six temporal frequencies (1 to 28 Hz) at 2x, 4x, and 8x orientation identification contrast thresholds. For all subjects, thresholds were variable at low contrasts. At higher contrasts, TPD thresholds increase for 0.25 and 0.50 cpd gratings with increasing flicker rate. These data support the idea that frequency-dependent loss of temporal phase encoding ability could possibly underlie the FDI.

知覚速度に及ぼす空間周波数の効果

The perceived speeds of gratings with various spatial frequencies, which shifted at the same speed, were evaluated using psychophysical methods. When the gratings shifted at low speeds, perceived speeds were found to be independent of the variation in the spatial frequencies of the gratings. However, at high speeds, an increase in spatial frequency was found to result in an increase in perceived speed. Furthermore, secondary polynomials were found to represent the relations between perceived speed and spatial frequency more accurately than the linear expressions claimed previously by other researchers.

Visual coherence of moving and stationary image changes

Detection thresholds were compared for moving and stationary oscillations with equivalent contrast changes. Motion was more detectable than stationary oscillation, and the difference increased with size of the feature (a Gaussian blob). Phase discriminations between a center and two flanking features were much better for motion than for stationary oscillation. Motion phase discriminations were similar to motion detection and were robust over increases in spatial separation and temporal frequency, but not so for stationary oscillations. Separate visual motion signals were positively correlated, but visual signals for stationary oscillation were negatively correlated. Evidently, motion produces visually coherent changes in image structure, but stationary contrast oscillation does not.

Evaluation of short-term increased intraocular pressure on flash- and pattern-generated electroretinograms of dogs.

To determine the electrodiagnostic and histologic response of short-term increases of intraocular pressure (IOP) on transient pattern electroretinograms (PERG) and flash electroretinograms (FERG) in the eyes of dogs. 8 healthy mixed-breed dogs. Transient PERG and FERG waveforms were recorded from dogs (while anesthetized) as IOP was increased from baseline (7 to 19 mm Hg) to 90 mm Hg. One hundred mean PERG responses and a single FERG response were recorded at each step during 3 recording sessions. Globes of each dog were enucleated after euthanasia on posttreatment day 7 and evaluated by a pathologist. Increases in spatial frequency resulted in decreased amplitudes of N2 (second negative PERG peak). Increases in IOP resulted in decreases in all 3 PERG waveforms and the FERG waveform. All values began to return to baseline after short-term increases in IOP on day 0, and waveforms were not significantly different on posttreatment days 3 and 7 Data suggest that short-term increases in IOP affect PERG and FERG waveforms, and PERG waveforms are more sensitive to increases in IOP Differences were not detected between treated and control eyes on histologic examination. Further studies are necessary to determine at what IOP permanent damage to ganglion and photoreceptor cells will develop and whether PERG is a reliable clinical diagnostic technique for use in dogs to reveal retinal damage that is secondary to increased IOP prior to changes in waveforms generated by FERG in dogs.

Optical Modulation Transfer and Contrast Sensitivity with Decentered Small Pupils in the Human Eye

Human observers experience a large decrement in visual acuity when a small artificial pupil is displaced from the center to the edge of the dilated natural pupil. This decrement in visual resolution, called the Campbell effect, has been attributed to the retina, the ocular optics, or a combination of the two. Given the uncertainty about the relative magnitudes of these two components over the range of spatial frequencies used in normal vision, we have obtained objective measurements of the retinal image quality and psychophysical measurements of visual performance, with decentered pupils. The contributions of monochromatic aberrations were determined by using double pass measurements of the modulus of the optical transfer function (MTF). For all of the observers, there was a substantial decrement in the MTF with decentering, showing that even when using a 1.5 mm pupil and appropriate spherical/cylindrical refractive corrections, there is a considerable contribution of monochromatic aberrations to the effect. We have compared these optical MTFs with the psychophysical contrast sensitivity functions (CSFs) measured under exactly the same conditions using green gratings generated on the screen of a color monitor. At the low and intermediate spatial frequencies considered (2–16 c/deg), we find the fall in the CSF is much greater than the fall in the monochromatic MTF, with the difference becoming greater as the spatial frequency increases. We show that this discrepancy can be mostly attributed to the effect of transverse chromatic aberration due to the bandwidth of the green stimulus used for the CSF measurements. In conclusion, the combination of the ocular transverse chromatic aberration and monochromatic aberrations accounts for the loss in visual sensitivity found with a decentered small pupil at low and intermediate spatial frequencies. Copyright © 1996 Elsevier Science Ltd.