Visible Variables Research Articles

Nanometer displacement measurement using Fresnel diffraction

We introduce a relatively simple and efficient optical technique to measure nanoscale displacement based on visibility variations of the Fresnel diffraction fringes from a two-dimensional phase step. In this paper we use our technique to measure electromechanical expansions by a thin piezoelectric ceramic and also thermal changes in the diameter of a tungsten wire. Early results provide convincing evidence that sensitivity up to a few nanometers can be achieved, and our technique has the potential to be used as a nanodisplacement probe.

The optical transmission and improvement of flux for the static polarization wind imaging interferometer

The static polarization wind imaging interferometer takes advantage of polarized-light beam to obtain interferogram, and beam transmission characteristic in core polarization components is a key issue. The Jones matrix is convenient and concise for analyzing the optics polarization state, and easy to obtain the relationship of key optical components in the system performance. The Jones matrix is introduced to describe the static polarization interferometer system respectively in a given case and in a general case. The variations of optical flux and interference fringe visibility are investigated as functions of polarization direction and wave plate azimuth associated with the key components, and their optimal values are ascertained. The optical flux can be improved by widening field of view and increasing the transmittance of the pyramid prism. The simulation results of the interference intensities confirm the theoretical expectations.The study provides a theoretical basis and practical guidance for the design, development and engineer of the static polarization wind imaging interferometer.

Fabry–Pérot Cavity Based on a High-Birefringent Fiber Bragg Grating for Refractive Index and Temperature Measurement

A fiber-optic sensor for simultaneous measurement of refractive index and temperature is described. The refractive index measurement is based on the visibility variations of a Fabry-Pérot interferometer. It is formed with the interfering waves generated from a low reflectivity Bragg grating inscribed on a Panda fiber and from the fiber end tip (Fresnel reflection) in contact with the liquid. The sensor is characterized by immersing the fiber tip in distilled water with different concentrations of ethylene glycol. A linear relation of the interferometer fringe visibility with refractive index variation is observed. The temperature is determined by the wavelength shift of the FBG peaks. Results show the feasibility of simultaneous measurement of refractive index and temperature and also the possibility of adjusting fringe visibility via polarization control.

COLPEX: Field and Numerical Studies over a Region of Small Hills

During stable nighttime periods, large variations in temperature and visibility often occur over short distances in regions of only moderate topography. These are of great practical significance and yet pose major forecasting challenges because of a lack of detailed understanding of the processes involved and because crucial topographic variations are often not resolved in current forecast models. This paper describes a field and numerical modeling campaign, Cold-Air Pooling Experiment (COLPEX), which addresses many of the issues. The observational campaign was run for 15 months in Shropshire, United Kingdom, in a region of small hills and valleys with typical ridge–valley heights of 75–150 m and valley widths of 1–3 km. The instrumentation consisted of three sites with instrumented flux towers, a Doppler lidar, and a network of 30 simpler meteorological stations. Further instrumentation was deployed during intensive observation periods including radiosonde launches from two sites, a cloud droplet probe, ...

Spectral-mismatch-induced resolution limit of interferometric fiber Fabry–Perot sensor system

The mismatch of fiber Bragg gratings (FBGs) in spectral profiles can lead to a severe degraded resolution of the constructed fiber Fabry–Perot (FFP) sensor system through its effect on the fringe visibility. The variation of visibility induced by spectral mismatch and the corresponding phase resolution limit are analyzed theoretically and experimentally. Theoretical analyses are based on the approximation of Gaussian profiles to the reflection spectra of weak FBGs, especially with consideration of side lobes. The investigation provides an insight into the evolution of the fringe visibility caused by spectral mismatch, and shows good agreement with experimental results. An optimum phase resolution of about 55 μrad/Hz 1/2 above 100 Hz is achieved for a nearly 4 m-long FFP sensor by matching spectral profiles of the gratings and balancing path length differences of the tandem interferometers.

Characterization of visibility and its affecting factors over Nanjing, China

Visibility, Air Pollution Index (API) and meteorological parameters over Nanjing during 2004 are analyzed using multiple statistic methods to study the characterization of visibility and relevant affecting factors. The mean value of visibility during the study year is 8.8 km, with 57.9% of daily average less than 10 km. Annual average visibility exhibits a significant diurnal variation with minimum of 6.6 km at 07:00 Local Time (LT) and maximum of 10.7 km at 16:00 LT. Seasonal variations is weak: 9.2 km in spring (March–May), 9.8 km in summer (June–August), 8.2 km in autumn (September–November), and 8.1 km in winter (December–February), respectively. No significant weekend effect in visibility is found. Moreover, wavelet analysis reveals the significant periodic variations in visibility, with major periods of synoptic scale (2–8 days), and quasi-two-week scale (8–17 days). Using a high pass filtering technique to eliminate the seasonal trend, correlation analysis and principal component analysis (PCA) are carried out to investigate the relations between visibility and relevant factors and to understand the dominated processes/causes to the synoptic changes in visibility. Our results highlight the significant impact of synoptic weather on air pollution and visibility in Nanjing.

Visibility degradation during foggy period due to anthropogenic urban aerosol at Delhi, India

Fog occurs more frequently over urban areas than rural areas. It may occur due to increased air pollution emanating from variety of sources in the urban areas. The increased pollution levels may lead to the atmospheric reactions resulting into the formation of secondary pollutants that may also lead to the needed cloud condensation nuclei. Northern regions of India experience severe foggy conditions during the winter period (November–January) each year. In this study, we have simultaneously measured the particulate mass concentration (0.23 µm to 20 µm), meteorological parameters and atmospheric visibility in Mega city Delhi during a winter month of the years 2007– 2008 in order to have an improved understanding of their role in fog formation. The effects of aerosols on fog formation are discussed through an analysis of trends in fog frequency and comparison with meteorological parameters, and visibility as an indicator of aerosol load. This satisfies the precondition for using these relations. The association between the meteorological parameters (visibility, depression temperature) and air pollutants are examined. The Windows software SPSS (version 17.0) is used to fit a linear regression model. The model explained the variation in visibility due to depression temperature and aerosols load.

THREE-DIMENSIONAL MORPHOLOGY OF A CORONAL PROMINENCE CAVITY

We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step toward quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager observations from the two Solar Terrestrial Relations Observatory spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explain the observed variation in cavity visibility for the east versus west limbs.

The challenge of improving visibility in Beijing

Abstract. The "Blue Sky Project" was proposed in 1998 to investigate by how much emissions should be reduced to increase blue sky frequency in Beijing, which hosted the Summer Olympics in 2008. This paper focuses on the temporal variation of visibility and its dependence on meteorological conditions and suspended particles at Beijing using the hourly observed visibility data at Beijing Capital International Airport (BCIA) from 1999 to 2007. It has been found that about 47.8% (24.2%) of the hours in Beijing are "bad" ("good") hours with visibility below 10 km (equal or higher than 20 km) between 1999 and 2007. Due to the high Relative Humidity (RH), summer is the season with the lowest mean visibility in a year. Although PM10 index was reported in a decreasing trend (Chan and Yao, 2008), the increase of RH has resulted in a decreasing trend of visibility over BCIA in the summer from 1999 to 2007. To ensure blue sky ("good" visibility) for Olympics 2008, daily mean PM10 index should have been reduced from 81 to 44. This requires that not only vehicle emissions, but also other emissions should be limited. Observations verify that blue-sky-hour rate increased significantly after mean PM10 index was reduced to 53 during Olympics 2008, however, the visibility of 2009 returned to the mean level from 1999 to 2007 during the period 8−24 August. RH (aerosol) contribute 24% (76%) of the improvement of visibility during August 2008.

High time resolution observation and statistical analysis of atmospheric light extinction properties and the chemical speciation of fine particulates

In recent years, the visibility deterioration caused by regional fine particulate pollution becomes one of the crucial air pollution problems in the urban areas of our country. The rapid variation of visibility and fine particulates make it difficult to estimate the relationship between them precisely and accurately unless high time resolution observation data can be accessed. This study aims to fill this gap in the field of atmospheric science by establishing a formula using multiple linear regressions. Excellent fitting goodness (R 2 = 0.913, n = 3167) was obtained using 10 min average of high-resolution real-time light scattering coefficients, light absorption coefficients, main chemical speciation concentration in PM1 and some meteorological parameters from 17 Jan to 16 Feb, 2009. It shows that the average light extinction coefficient during the observation in the winter of Shenzhen was measured to be 290 ± 183 Mm−1, consisting of 72% of light scattering and 21% of absorption. In terms of the percentage contribution of PM1 chemical species to the total light extinction, the organic matter was estimated to be most with an average of 45%, followed by ammonium sulfate with an average of 24%. The contributions of black carbon and ammonium nitrate were 17% and 12%, respectively. Besides, the diurnal variation of light extinction was investigated as well in this study.

MO‐E‐204B‐09: A Method to Evaluate the Effect of Automatic Brightness Control Tracking on Image Quality and Patient Dose for Interventional Fluoroscopic Systems

Purpose: Automatic‐brightness‐control (ABC) tracking is used to maintain a constant image‐receptor signal independent of changes in the attenuation of the patient. The manner of tracking impacts image quality as well as patient dose. We have developed a procedure to evaluate the effect of the ABC‐operation logic on both dose and contrast‐signal‐to‐noise ratio (CSNR) for interventional‐ fluoroscopic systems. Method and Materials: Two vascular‐angiography systems, one with an image intensifier (II) and the other with a flat‐panel detector (FPD), were used for assessment of the testing procedure. A phantom insert containing iodine‐filled disks was sandwiched within a variable‐thickness, uniform PMMA phantom. The variation of contrast visibility and patient dose was assessed as the ABC system automatically adjusted technique parameters to compensate for changes in phantom thickness. The x‐ray techniques, filter selected by the system, acquisition rate, patient entrance exposure and detector input exposure were recorded as a function of phantom thickness. The CSNR and CSNR per entrance exposure were plotted as a function of phantom thickness for all fluoroscopic modes. Results: All systems and modes had a decrease in CSNR and an increase in dose with patient thickness, but different modes resulted in different rates of change due to the manner of mAs and kVp selection. Although noise varied between fluoroscopy modes for the II and FPD, it was almost constant for a given mode as a function of thickness since the ABC tries to maintain a constant detector exposure rate. Conclusion: The method is easy to implement with a standard phantom and is an important adjunct to evaluate ABC system performance and to allow inter‐system comparisons. Understanding the tradeoffs between patient dose and CSNR due to the operation of a specific ABC mode can help optimize system utilization.

Multiparticle interference in electronic Mach-Zehnder interferometers

We study theoretically electronic Mach-Zehnder interferometers built from integer quantum Hall edge states, showing that the results of recent experiments can be understood in terms of multiparticle interference effects. These experiments probe the visibility of Aharonov-Bohm (AB) oscillations in differential conductance as an interferometer is driven out of equilibrium by an applied bias, finding a lobe pattern in visibility as a function of voltage. We calculate the dependence on voltage of the visibility and the phase of AB oscillations at zero temperature, taking into account long range interactions between electrons in the same edge for interferometers operating at a filling fraction $\nu=1$. We obtain an exact solution via bosonization for models in which electrons interact only when they are inside the interferometer. This solution is non-perturbative in the tunneling probabilities at quantum point contacts. The results match observations in considerable detail provided the transparency of the incoming contact is close to one-half: the variation in visibility with bias voltage consists of a series of lobes of decreasing amplitude, and the phase of the AB-fringes is practically constant inside the lobes but jumps by $\pi$ at the minima of the visibility. We discuss in addition the consequences of approximations made in other recent treatments of this problem. We also formulate perturbation theory in the interaction strength and use this to study the importance of interactions that are not internal to the interferometer.

Influence of aerosol on regional precipitation in North China

The possible anthropogenic aerosol effect on regional precipitation is analyzed based on the historical data of precipitation and visibility of North China. At first, the precipitation amounts from 1960 to 1979 are considered as natural background values in our study for relatively less intensive industrial activities and light air pollution during that period of time, then the region is divided into different subregions by applying the clustering method including the significance test of station rainfall correlations to the time series of 10-day mean rainfall amounts in this period. Based on the rule that the precipitation characteristics are similar in the same clustering region, the correlation of precipitation amounts among all stations in each region is thus established. Secondly, for the period from 1990 to 2005, during which, the economy had experienced a rapid development in this region, the variations of visibility at each station are analyzed. The stations with the absolute change in visibility less than 0.1 km/a are used as the reference stations, at which it is assumed that precipitation has not been seriously influenced by anthropogenic aerosols. Then the rainfall amounts of reference stations are used to estimate the natural precipitation values of the other stations in each clustering region. The difference between estimated precipitation and measured precipitation amount is thought to result from changes in anthropogenic aerosols. These changes in precipitation amounts caused by anthropogenic aerosols at each station are calculated using the 10-day mean rainfall values from 1990 to 2005. The analysis results obtained with this method are remarkable if it passes the significance test, and therefore, the suppression of regional precipitation over the region by anthropogenic aerosol is proved. It is found that this effect is most remarkable in summer. The influence of anthropogenic aerosols on convective precipitation possibly plays an important role in this season.

Mathematical models for accurate prediction of atmospheric visibility with particular reference to the seasonal and environmental patterns in Hong Kong

Atmospheric visibility impairment has gained increasing concern as it is associated with the existence of a number of aerosols as well as common air pollutants and produces unfavorable conditions for observation, dispersion, and transportation. This study analyzed the atmospheric visibility data measured in urban and suburban Hong Kong (two selected stations) with respect to time-matched mass concentrations of common air pollutants including nitrogen dioxide (NO(2)), nitrogen monoxide (NO), respirable suspended particulates (PM(10)), sulfur dioxide (SO(2)), carbon monoxide (CO), and meteorological parameters including air temperature, relative humidity, and wind speed. No significant difference in atmospheric visibility was reported between the two measurement locations (p > or = 0.6, t test); and good atmospheric visibility was observed more frequently in summer and autumn than in winter and spring (p < 0.01, t test). It was also found that atmospheric visibility increased with temperature but decreased with the concentrations of SO(2), CO, PM(10), NO, and NO(2). The results showed that atmospheric visibility was season dependent and would have significant correlations with temperature, the mass concentrations of PM(10) and NO(2), and the air pollution index API (correlation coefficients mid R: R mid R: > or = 0.7, p < or = 0.0001, t test). Mathematical expressions catering to the seasonal variations of atmospheric visibility were thus proposed. By comparison, the proposed visibility prediction models were more accurate than some existing regional models. In addition to improving visibility prediction accuracy, this study would be useful for understanding the context of low atmospheric visibility, exploring possible remedial measures, and evaluating the impact of air pollution and atmospheric visibility impairment in this region.

Retrieval of land surface bidirectional reflectivity in the mid‐infrared from MODIS channels 22 and 23

This paper proposes a method to retrieve the land surface bidirectional reflectivity ρb in the mid‐infrared (MIR) from MODIS channels 22 and 23. A split‐window‐like algorithm was developed to determine the MIR ground brightness temperature without the contribution of the solar direct beam from ground brightness temperatures measured at two adjacent MIR channels. Comparing the actual values of ρb with those estimated using our proposed method, the root mean square error (RMSE) was determined to be 0.0022 for the solar zenith angle (SZA) at nadir. Similar results were obtained for other SZAs, indicating that the proposed method can retrieve ρb accurately. A detailed sensitivity analysis found that the effects of instrumental noise, variations in water vapour content in the atmosphere, and reasonable variations in horizontal visibility on the retrieval of ρb were negligible and an error of 1 K on the ground brightness temperature caused by the inaccuracy of atmospheric corrections can lead to a maximum error of 0.034 on the retrieved ρb. We compared the use of radiosonde data and European Centre for Median‐range Weather Forecast (ECMWF) atmospheric data to perform atmospheric corrections. The results show that atmospheric corrections can be performed with ECMWF data instead of radiosonde data without losing the accuracy of the retrieved ρb. MODIS land surface temperature/emissivity product MOD11B1 data were also used to validate . Comparison of estimated respectively with the proposed method and with MOD11B1 data showed that the RMSE is less than 1 K for cloud‐free skies.

Semi-Continuous Aerosol Measurement for Visibility Variation Using an External Beam PIXE Method

This study introduces the application of a semi-continuous monitoring and analytic technique to examine the elemental composition for a visibility-impairing aerosol. Semi-continuous measurements of the chemical composition for visibility-impairing aerosol were made during the spring period of 19 30 May 2007 in the urban area of Seoul, Korea. These included measurements of the aerosol elemental composition by a proton induced X-ray emission (PIXE) method, of the aerosol ionic composition by ion chromatography (IC) method and of the aerosol carbonaceous composition by a thermal optical transmittance (TOT) method. Light extinction, scattering and absorption coe cients were measured simultaneously with a transmissometer, a nephelometer and an aethalometer, respectively. The average light extinction coe cient and visual range were measured to be 343 186 Mm 1 and 11.8 4.2 km, respectively, during the intensive monitoring period. The average mass concentrations of PM1:0, PM2:5 and PM10 were measured to be 24.0 13.4, 44.6 30.6 and 77.0 56.1 g m 3 and the correlation coe cient r2 of submicron and ne particles to bext were calculated to be relatively higher values of 0.7835 and 0.7824, respectively. However, the value of r2 for coarse particles was a low value of 0.1261. The results revealed that visibility impairment in the urban atmosphere of Seoul was a ected by ne particles, including submicron particles, rather than coarse particles. On a clear day and a hazy day, the mass concentration of mineral dust was not sensitive to variations in bext. On an Asian dust event day, visibility degradation was dominantly caused by increases in the SO2 4 and the NO 3 components between 12:00 and 14:00, during which time they combined with a large loading of mineral dust caused severely reduced visibility. The ability to observe rapid changes in the aerosol composition using semi-continuous aerosol measurements and the PIXE method was helpful in understanding the diurnal variation of visibility.

The influence of the life action on daily variation of visibility

The influence of the life action on daily variation of visibility

J, H, K spectro-interferometry of the Mira variable S Orionis

Aims. We present J, H, K spectrally dispersed interferometry with a spectral resolution of 35 for the Mira variable S Orionis. We aim at measuring the diameter variation as a function of wavelength that is expected due to molecular layers lying above the continuumforming photosphere. Our final goal is a better understanding of the pulsating atmosphere and its role in the mass-loss process. Methods. Visibility data of S Ori were obtained at phase 0.78 with the VLTI/AMBER instrument using the fringe tracker FINITO at 29 spectral channels between 1.29 µm and 2.32 µm. Apparent uniform disk (UD) diameters were computed for each spectral channel. In addition, the visibility data were directly compared to predictions by recent self-excited dynamic model atmospheres. Results. S Ori shows significant variations in the visibility values as a function of spectral channel that can only be described by a clear variation in the apparent angular size with wavelength. The closure phase values are close to zero at all spectral channels, indicating the absence of asymmetric intensity features. The apparent UD angular diameter is smallest at about 1.3 µm and 1.7 µm and increases by a factor of ∼1.4 around 2.0 µm. The minimum UD angular diameter at near-continuum wavelengths is ΘUD = 8.1 ± 0. 5m as, corresponding to R ∼ 420R� . The S Ori visibility data and the apparent UD variations can be explained reasonably well by a dynamic atmosphere model that includes molecular layers, particularly water vapor and CO. The best-fitting photospheric angular diameter of the model atmosphere is ΘPhot = 8.3 ± 0.2 mas, consistent with the UD diameter measured at near-continuum wavelengths. Conclusions. The measured visibility and UD diameter variations with wavelength resemble and generally confirm the predictions by recent dynamic model atmospheres. These size variations with wavelength can be understood as the effects from water vapor and CO layers lying above the continuum-forming photosphere. The major remaining differences between observations and model prediction are very likely due to an imperfect match of the phase and cycle combination between observation and available models.

Horizontal visibility trends in China 1981–2005

Trends in Chinese horizontal visibility, the frequency of visibility &gt;19 km, and haziness for the period between 1981 and 2005 were evaluated based on data for daily horizontal visibility. Annual means were calculated for each station and for China as a whole. Linear regression analysis was used to characterize long‐term annual trends in these variables. Over the past 25 years, there has been a significant decrease in horizontal visibility (−2.1 km per decade from 1990 to 2005) and the frequency of visibility &gt;19 km (−3.5% per decade) but a significant increase in the 75th percentile annual extinction coefficients (25% per 25 year). According to rapid increase of total energy consumption, the consistent spatial and temporal variations of visibility and haze support the speculation that increased aerosol loadings were responsible for the observed decreases in horizontal visibility over much of East China.

Effects of atmosphere visibility on performances of non-line-of-sight ultraviolet communication systems

Dependence of performances of non-line-of-sight (NLOS) solar-blind ultraviolet (UV) communication systems on atmosphere visibility is investigated numerically by correlating the propagation of UV radiation with the visibility. A simplified solar-blind UV atmospheric propagation model is introduced, and the NLOS UV communication system model is constituted based on the single scattering assumption. Using the model, numerical simulation is conducted for two typical geometry configurations and different modulation formats. The results indicate that the performance of the NLOS UV communication system is insensitive to variation of visibility in quite a large range, and deteriorates significantly only in very low-visibility weather, and is also dependent on the geometry configuration of the system. The results also show that the pulse position modulation (PPM) is preferable due to its high-power efficiency to improve the system performance.