Visible Angle Research Articles

Computer synthesis of antennas

This paper proposes that computer synthesis of antennas considers field and current distributions over a given radiation pattern with a numerical method. We introduce necessary constraints associated with visible angles and finiteness of antenna dimensions; these constraints assure practical realization of the amplitude-phase distributions found and assure high-precision reproduction of the radiation pattern. Computational results are presented graphically.

Localization performance is enhanced under bimodal conditions of the auditory and visual spatial channels

A sequential localization task compared the effects of unimodal versus bimodal spatial acuity in the auditory and visual perceptual systems. The four subjects were tested in a two‐alternative forced‐choice, three down–one up adaptive paradigm in which two 200‐ms signals were presented sequentially with a 1000‐ms inter‐stimulus interval. Tests were administered with a 620‐nm light emitting diode at a luminance level of 200 ml and a 1.0‐kHz high‐pass noise at 57 dB (A‐weighted). The minimum bimodal angle (MBA) threshold was compared to that of the minimum visible angle (MVA) and minimum audible angle (MAA) thresholds and found to increase performance significantly over the single mode localization tasks, except at 0‐deg azimuth that equaled the MVA for most subjects. The implications of these results are discussed.

Auditory and visual localization performance in a sequential discrimination task.

Four subjects were tested in a two-alternative, forced-choice, three down-one up adaptive paradigm in which two 200-ms signals were presented sequentially with a 200-ms interstimulus interval. The subject's task was to indicate whether the second stimulus was to the right or left of the first. Tests were conducted with 57 dB (A-weighted), 1.0-kHz high-pass noise, the minimum audible angle (MAA) task, and with lights emitting 620 nm at a luminance level of 200 mL, the minimum visible angle (MVA) task. Localization performance in the MAA task was equal to or better than that obtained in the MVA task for all regions of the frontal field with only one exception, presentations at 0 degrees azimuth. The implications of these results are discussed.

An Estimation Procedure for the Joint Distribution of Spatial Direction and Thickness of Flat Bodies Using Vertical Sections. Part II: An application in soil micromorphology

AbstractIn soil micromorphology fissures are considered in vertical sections. To get information about the properties of the soil the joint distribution of spatial direction and width of these fissures is of interest. The fissures are mathematically generalized to flat bodies which are defined as stationary weighted surface processes with the weight “thickness”. In a typical point of the surface process suitable, joint parametric distributions of direction and thickness are assumed. The parameters have to be estimated from measurements on vertical sections which are taken from the soil. On these sections only a visible thickness and a visible angle can be observed. The joint distribution of these variables can be expressed by the joint distribution of spatial direction and thickness with the same parameters and in this indirect way the parameters can be estimated. The paper describes how to randomize the vertical section and how to measure the visible variables on the sections. The Chi‐Square method is proposed for the parameter estimation. Further it is discussed how to derive good starting values for the numerical procedure. All this is demonstrated in a simulation study using the Bingham‐Mardia distribution for the direction and the lognormal distribution for the thickness including a way to correlate the mean thickness and the direction. Finally an application in soil micromorphology is demonstrated for one soil horizon.

An Estimation Procedure for the Joint Distribution of Spatial Direction and Thickness of Flat Bodies Using Vertical Sections Part I: Theoretical Considerations

AbstractIn soil micromorphology fissures are considered in vertical sections. To get information about the properties of the soil the joint distribution of spatial direction and width is of interest. The fissures are mathematically generalized to flat bodies which form a stationary weighted surface process with the weight “thickness”. Because of stationarity a joint distribution of spatial direction and thickness exists in a “typical point” of the surface process. A suitable parametric family of distributions is assumed. The corresponding parameters can be estimated from measurements on the vertical sections. But on the sections only the visible thickness and the visible angle of a fissure can be measured. Therefore the joint distribution of these variables is expressed by the joint spatial distribution of spatial direction and thickness. This derived distribution depends on the same parameters. The Chi‐Square method is proposed for the parameter estimation. The estimation procedure is demonstrated using the Bingham‐Mardia distribution for the direction and the lognormal distribution for the thickness and by defining a way to correlate the mean thickness and the direction.

Auditory systems spatial resolving power in the periphery, under serial processing, is more precise than visual spatial processing in the peripheral aspects.

The relation between minimum audible and minimum visible angle was measured in a sequential localization task and compared. The frontal spatial fields of the visual and auditory systems are superior to their peripheral regions [Wertheim (1894); Mills (1958)]. Localization tasks were related to Vernier’s acuity, except the timing was in a sequential pattern for both the visual and auditory systems. The spatial acuity of the auditory system defined space better at approximately 20-deg azimuth and beyond then the visual system. Four subjects participated, two male and two female, ranging in ages from 23–49. Significant differences were found in the main effects of angles [F(4,3)=30.59, p<0.01] and the interaction between conditions and angle [F(4,3)=3,89, p<0.05]. The implications of these results will be discussed.

Characterization of urea optical parametric oscillators pumped by excimer lasers

An optical parametric oscillator (OPO), based on urea crystals of length up to 25 mm as the nonlinear medium and pumped by a line-narrowed injection-seeded pulsed XeCl excimer laser, is described. We report peak conversion efficiency at 90° phase matching of 65% and efficient generation of coherent visible radiation angle tunable from 537 to 720 nm. Requirements on OPO pump laser linewidth and beam divergence are considered. Variation of OPO threshold energy fluence as a function of various oscillator parameters has been investigated experimentally and compared with theoretically modeled predictions. Pumping of a urea OPO at 249 nm by a krypton fluoride excimer is reported for the first time to our knowledge.

Visual determinants of escape in tiger beetle larvae (Cicindelidae)

Larvae of tiger beetles (Coleoptera: Cicindelidae) are burrow-dwelling, visual ambush predators which withdraw into their burrows with the passing of large objects. Laboratory experiments confirmed that stimulation of each of the four principal stemmata can elicit escape and that the necessary visual stimulus is contracting, expanding, or transverse movement of a high-contrast image. Response frequency increases as a power function of contrast. Whole-field dimming is ineffective. Movement of large images composed of multiple texture elements, e.g., checkerboards, does not elicit escape, even if each element is much larger than the system's minimum visible angle (4–8° depending upon image contrast). In pilot experiments with a single figure before a textured background, coherent movement of the two inhibits escape, whereas motion in opposite directions does not. Thus, the processing mechanism functions as a feature detector and directs a response to large, single, moving objects.

Effect of Applied Potential on the Interfacial Tension between Liquid Lead and PbO–SiO2Slags

AbstractThe measurement of the visible angle between lead and PbO-SiO2 slags were determined at 1073 K under an argon atmosphere. These results were combined with the surface tension data of lead, determined by the sessile drop method, and the literature data on the surface tension of slag to calculate the interfacial tension between liquid lead and PbO-SiO2 slags. The effect of an applied potential on the interfacial tension was determined. The data obtained on the interfacial tension offer fundamental information which assists to elucidate both the mechanism of the local corrosion of silica at the PbO-SiO2 slag-lead interface and also the lead losses in blast furnace smelting.

Measurement accuracies in band-limited extrapolation

It is shown that in band-limited (or visible angle limited) extrapolation the largest effective aperture L' that can be realized from a finite aperture L by over sampling is a function of the accuracy of measurements. It is shown that for sampling in the interval |x-L/2b|x<L/2, b > 1 the extrapolating series must be known within a relative error e L' (i.e. the ratio of smallest term over the dominant first term) given by ▪ where L is the physical aperture, L' is the extrapolated aperture, λ is the wavelength, L/b is the sample point closet to origin.

Radio emission of the sea surface near the Brewster angle at 0.86-and 1.3-cm wavelength

A great deal of attention is currently being devoted to the study of the background radio emission of the sea surface from airplanes, ships, and satellites. The objective of these investigations is to obtain information about the state of the sea surface along extended tracks. For observations from above, the range of visibility angles is 0-50 ~ from nadir, due to known limitations. It seems to us that low visibility angles have received little attention, however, and besides, this range of angles is of undisputed interest both for remote measurements of the state of the sea surface, and for antenna calibration in radio astronomical studies at sea. Carrying out these studies successfully requires advanced development of measurement techniques, particularly of operational control of radio telescope parameters at sea. Under land-based conditions, the methods which have been developed and widely used for antenna calibration include those in which natural radio sources of known signal strength are employed [i] (portions of forests, the earth's surface observed at the Brewster angle, mountain slopes, etc.). Under sea-going conditions, most of these regions of known emission temperature are inaccessible to observation. Under those circumstances, one may take as "reference"regions with known signal strength a segment of the surface of the water near the Brewster angle, which is quite "black," and an emitting region of the atmosphere near the horizon. With such a choice of "reference" regions, one assumes that the brightness temperature of the surface of the water [2], or, for atmospheric calibration, the emission temperature of a region near the horizon is taken to be equal to that of the ambient air. In both this and the other case, we ignore the relationship between the rough sea surface and the radiation temperature of the reference regions, which can lead to errors in the calibration of antenna parameters. The influence of these factors on the effective emission temperature requires that both experimental studies and modeling of the emission be carried out for the rough water surface. As is well known, the brightness temperature of the smooth water surface is determined by its intrinsic emission and by the reflection of atmospheric radiation. The contribution of intrinsic emission to the brightness temperature is proportional to the water temperature To, and the proportionality factor is the emissivity I--~(~,00). The Fresnel (power) reflection coefficient R(e, 00) for vertical or horizontal polarization depends on the complex dielectric constant of the water ~---~i~f~2 and the observing angle e0. The real and imaginary parts of the dielectric constant, which depend on the wavelength, temperature, and salinity, can be calculated using the well-known formulas of Debye [3]. The component of the brightness temperature of the surface of the water due to reflected atmospheric radiation is proportional to the brightness temperature of the atmosphere Tba(F , 0o) in the given

Hydrodynamics of wetting

Fluid motion along a smooth, solid surface is examined when the free surface forms a final visible angle with the solid boundary. The dependence of this angle on the velocity with allowance for capillary forces is determined. The Reynolds number is small. The problem of the motion of the phase interface in a capillary and the spreading of viscous fluid drops on solid surfaces are solved. Experimental results are explained. Up to now, not only were analytical results lacking in this field, but also there was not even a precise formulation of the problem (see the review in [1]).

Distance Off by Vertical Sextant Angle

Distance off by measuring the elevation of a mountain top above the horizon has been a popular means of finding position at sea ever since Captain Lecky published his famous tables. This popularity persists into the radar age because distance off can often be found from a vertical sextant angle long before a ship is within effective radar range of the coast. In view of this it is surprising that the standard textbooks on navigation give scant attention to this method and any seaman finding himself without Lecky's tables would find little guidance elsewhere.Wing Commander E. W. Anderson in The Principles of Navigation indicates that the whole angle subtended by a mountain at a ship can be found by adding half the estimated distance off in miles to the visible angle in minutes, having first corrected this angle for dip and refraction. This can be proved by simple geometry by showing that the angle subtended by the part of a mountain hidden to an observer at sea-level is equal to half the distance off.