Correction For Refraction Research Articles

A new stereophotographic technique for analyzing the three-dimensional structure of fish schools

A technique using two downward-directed 35 mm cameras has been modified to measure the three-dimensional structure of fish schools. The resulting stereo pairs of photographs are analyzed, producing the 3-coordinate location of each fish's nose, after correction for lens distortion and refraction. Separation angles (bearing and elevation) and distance can then be determined for any pair of fish in the school. The technique's high level of accuracy is demonstrated for an underwater calibration field. It is then applied to the measurement of the 3-D structure of schools of coho salmon (Oncorhymchus kisutch) swimming in a hatchery trough. Although the fish were not organized in a rigid crystal lattice, the analysis provided some evidence of structure.

Angle-resolved photoemission study of the clean Cu (001) surface in the photon energy range40≤ℏω≤120eV: Comparison of experiment and simple direct-transition theory

Angle-resolved photoemission spectra obtained from the clean Cu (001) surface with photon energies in the range 40-120 eV and emission both normal and non-normal to the surface are well predicted by a simple constant-matrix-element model based upon direct ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ conserving) transitions between initial electronic states with full bulk translational symmetry and final states with a free-electron dispersion relation and a correction for refraction at the crystal surface. In addition, the quantitative agreement between theory and experiment improves significantly by including broadening in the final-state wave vector for spectra obtained in the highly-surface-sensitive region around 100 eV. A strong contribution of phonon-assisted indirect transitions to the spectra also appears to be present. The photoemission spectra have been obtained at two different polarization orientations with respect to the electron emission direction and this can be used for assigning contributions from various initial states.

Tomographic Reconstruction of Ultrasonic Attenuation with Correction for Refractive Errors

The nonionizing and noninvasive characteristics of ultrasonics promote its increasing use in medical applications. Computerized ultrasonic attenuation tomography is one area where medically significant images may be reconstructed for diagnostic purposes. The objective of this work is to make an initial correction for refraction error and other problems present in computerized ultrasonic tomography. As a refinement of the scanning technique, our method attempts to provide high-quality projection profiles to the reconstruction algorithms. The important aspect of this new scanning mode lies in its correction for refraction in the measured attenuation profiles. In reconstructing phantom targets scanned by our experimental system, results superior to traditional line-of-sight scanning were obtained.

Tomographic imaging of attenuation with simulation correction for refraction

Tomographic imaging of attenuation with simulation correction for refraction

Angular resolved UV photoemission from ordered layers of carbon monoxide on a nickel(100) surface

Angular resolved UPS measurements have been performed on the (√2 × √2)R45° and compressed structures of the adsorption system CO/Ni(100). Due to the operation of dipole selection rules the dependence of normal exit photoemission on angle of photon incidence is found to be a sensitive probe of initial state symmetry. The polar angular dependence of emission from the 4σ orbital follows closely that predicted by Davenport, provided that a correction for refraction and total photoelectron current conservation is applied. Such polar angular dependences show band shifts indicative of dispersion effects, which become more marked at higher CO coverages. An azimuthal angular dependence of emission was not observed despite the presence of ordered adlayers.

“Basic” jet noise patterns after deletion of convection and refraction effects: Experiments vs. Theory

In earlier work of Grande experimental jet-noise polar plots were corrected for refraction (by experiment) and for convection (by theory) to yield the relatively small experimental basic directivity in narrow frequency bands. The experimental correction for refraction at each frequency was obtained from the difference in the directional patterns of a point source placed in the air jet with jet off and jet on. In the present work, with new measurements, the procedure has been updated and extended. There is a minor change in the convection correction dictated by theory, and a substantial change in the refraction correction, owing to a shortcoming of the injected point source technique, (i) The results have been presented in the equivalent forms of spectra and narrow-band directivities at jet Mach numbers of 0'5 and 0'9. (ii) Theory has been fitted in two-parameter form at each frequency to generate spectral curves of basic self-noise a(CfDJU) and shear-noise b(CfD/U). The peaks occur at the theoretical 2/1 in frequency, and the Doppler-corrected Strouhal numbers CfD/U are invariant with Mach number, (iii) The semi-empirical theory, with convection and refraction re-introduced, has been used to predict the overall directivity in narrow frequency bands. The agreement with the direct measurements is quite good for what is essentially a two-point fit. (iv) The broadband convection factor C−5, with α = 0·55 and Mc = 0·5 Uj/C0 (supported by theoretical arguments), is found to correlate well with experimental results—these and others—for Uj up to turbojet speeds (e.g., 1·64 C0). (The factor C−3 with α = 0 and Mc = 0·63 Uj/C0 that has been proposed for shear noise fails badly at these high speeds ; but if Mc is reduced to correct this, the factor fails at low speeds.) (v) A critique is given of analytical factors for Doppler shift and convective amplification, including the effect of the general flow enveloping the moving eddies.

Basic Directivity of Jet Noise with Improved Correction for Refraction

In earlier work of Grande, experimental jet-noise polar plots were corrected for refraction (by experiment) and for convection (by theory) to yield the experimental basic directivity in narrow frequency bands. The experimental correction for refraction at each frequency was obtained from the difference in the directional patterns of a point source placed in the air jet with jet off and jet on. Before refraction, this point source will produce a spherical directional pattern, but the narrow-band jet noise will have an elongated pattern owing to convection. It has been realized belatedly that the refractive change in intensity at corresponding angles cannot be taken as the same for the two different initial polar patterns: there is some violation of energy conservation. To avoid this error, later unpublished measurements by MacGregor have been reworked by Lain, incorporating a correction to the refraction effect measured with the point source. The correction was specified by an assumed functional form as a polar plot; the amplitude was then dictated by the requirement of conservation of energy. The measured jet noise data, processed along the lines of Grande, then leads to much improved agreement of experiment and theory. [Research supported by Air Force Office of Scientific Research.]

Investigation of Double-Diffusive Convection Using an Ultrasonic Pulse-Echo Technique

Propagation of ultrasound is investigated in an aqueous solution stratified by two solutes which diffuse at different rates. Both the finger interface and the diffusive interface are considered. Following correction for background absorption, edge effects, and refraction, attenuation of the order of 1.4 dB/cm is observed in the finger interface, while no attenuation is detected in the diffusive interface. The attenuation increases with frequency from 0.8 to 25 MHz. For higher frequencies, up to 42 MHz, the attenuation is constant. The loss is attributed to an as yet unspecified scattering mechanism.

Observations of the Directional Characteristics of Sea Waves

Summary Simultaneous records of orbital velocity and pressure disturbance in wind generated waves have been analysed by power spectral methods, to obtain the first five angular harmonics of the directional spectrum. The observations were made at a coastal site, offshore from a gently sloping beach, where the bottom topography was sufficiently simple to permit corrections for the effects of refraction. The orbital velocities were measured by an electromagnetic flowmeter and the pressure disturbance by an N.I.O. pressure recorder. A capacitance probe was also available for recording the surface elevation. A computer program has been developed which computes the auto and cross-spectra of the pressure and velocity components, the angular harmonics and a smoothed estimate of the directional spectrum directly from data which were recorded digitally on punched paper tape. Estimates of the mean direction and the width of the directional spectrum and its skewness, have been obtained for a wide range of frequencies and related to wind conditions in the generating area. After correction for refraction, the mean direction of the waves was found to correspond well to the mean wind direction. For waves generated by a reasonably constant wind field, the spectrum width exhibits a general tendency to increase with frequency. At low frequencies the spectrum width corrected for refraction is not inconsistent with the resonance angle given by Phillips’ theory, while at frequencies just above the transition frequency defined by the combined Miles-Phillips theory, the spectrum is appreciably narrower than the resonance angle. The smoothed directional spectrum estimate based on five harmonics gives no indication of bimodality in the spectrum. A method of obtaining improved angular resolution using an array of three flowmeters to obtain four additional harmonics was only partially successful on account of turbulent interference generated by the pier structure to which the sea units were attached. The surface power spectrum was found to fit the equilibrium power law at high frequencies under saturation conditions. A spectrum obtained under conditions very similar to those in the SWOP project has been compared with the SWOP spectrum. Marked differences between the two suggest that bottom friction is a major influence on waves in coastal waters. The attenuation of the waves in depth was determined from the ratios of the surface elevation and pressure spectra. No significant departure

Correction for Atmospheric Refraction in Surveying and Alignment

DETERMINING precisely the direction in which a given point lies is a fundamental problem in surveying and alignment measurements. Optical methods are known which are in principle sufficiently accurate for all practical requirements, but in practice their accuracy is reduced by the effects of the intervening atmosphere.

A discussion on orbital analysis - Orbit determination from Minitrack observations

Though Minitrack observations are only accurate to about 1', accurate orbits—typically eccentricity to 10 -5 —have been obtained for a number of satellites. The main sources of observational error, real or apparent, are thought to be: inadequate correction for ionospheric refraction; and inadequate representation of satellite perturbations due to the Earth’s tesseral harmonics and to atmospheric drag.

Shock‐Wave Shadow Photography in Tunnel and in Flight

AN account is given or the principle of a three‐dimensional shadowgraph recorder, its exemplification in the laboratory and its application to models in open and closed jets. Details are given of photographical, mechanical and graphical methods of reconstruction of the form of the shock wave from the records obtained, including correction for refraction due to thick glass walls of tunnels.

The Clarke Formulæ for Computing Latitudes, Longitudes and Reverse Azimuths, for use with either Logarithms or Machine Calculation

can be got by slide-rule. The correction for refraction is constant for the set of readings to one star. The eight to ten values for latitude obtained from the observations can be computed very quickly, and one immediately gets some idea of the accuracy of the observation, and can, if necessary, cut out any doubtful values. To obtain longitude, one can assume that the chronometer has kept the same rate during the same series of observations, so that the computed results can be meaned, and this will give the value for chronometer error on L.S.T. or L.M.T. at the mean of the times of the various observations. Next find out the chronometer error on Greenwich time for this mean value, correcting it, of course, for rate. The sum or difference of these two values will give the longitude. This is a quicker method than correcting each computed value for chronometer error and obtaining the value for longitude for each observation, which then has to be meaned. It is hoped that these notes may be of assistance to those who have to fix Astro-Radio points, and I hope that others too will contribute their experiences, because saving of time is of vital importance to any survey work in the back blocks of the world.

Correction for Atmospheric Refraction in Geodetic Operations 1

THE memoir before us is concerned with the correction for refraction in geodetic operations between distant stations, especially those differing considerably in altitude. The author quotes Helmert's elaborate formula, which gives the correction as a function of gravity, atmospheric pressure, coefficient of expansion of air, tension of aqueous vapour, temperature, and vertical temperature gradient. The values deduced from the formula are compared with those obtained by observation over several bases in Italy and the Alps. The results are grouped both by months and by hours of the day; they show in a clear manner that there are both diurnal and annual variations in the refraction coefficient, which appear to be mainly due to the changes in the vertical temperature gradient. The following table shows the results of two series, the coefficients in the first column being deduced from the formula, and in the second by experiment. The third column gives the observed vertical temperature gradient.

XIII.—Tables for facilitating the Computation of Differential Refraction in Position Angle and Distance

The annexed tables are intended to facilitate the computation of the corrections for refraction which have to be applied to differential measures, such as are made with the micrometer or heliometer.Differential measures are of two kinds :—(1) Direct measures of differences of right ascension and declination; and (2) measures of position angle and distance. In either case the observer only seeks to determine the relative positions of the objects under observation; and the correction for refraction consists in the applying to each reading a quantity representing the difference of the separate effects of refraction on the apparent places of the two stars, whose relative positions are to be determined.