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Showing papers on "Wavefront published in 1968"


Journal ArticleDOI
TL;DR: Moreton's wave phenomenon, observed to propagate across the solar disk during certain solar flares, may be explained as the intersection line between an expanding coronal wavefront surface and the chromosphere.
Abstract: Moreton's wave phenomenon, observed to propagate across the solar disk during certain solar flares, may be explained as the intersection line between an expanding coronal wavefront surface and the chromosphere: The propagation of a hydromagnetic disturbance in a spherically symmetric model corona with a radial magnetic field is studied in the WKB-approximation, and the propagation paths of wavepackets, the shape of the wavefronts expanding in time, and the variation of the direction of material oscillation along the paths are discussed for disturbances in hydromagnetic wave modes. It is shown that the wavefront expanding in fast-mode in the corona has a circular intersection line with the chromosphere, which sweeps on the solar surface with a velocity and a direction of material oscillation just favorable for explaining the observed features of the phenomenon.

314 citations


Journal ArticleDOI
01 Oct 1968
TL;DR: In this article, it was shown that the axial wave fields in a paraboloidal reflector can be represented by a spectrum of near-spherical hybrid waves propagating along the axis.
Abstract: The fields scattered by circular symmetric reflector illuminated by a linearly polarised wave incident normally on the aperture are calculated from the induced surface currents. It is shown that the fields in the axial region can be represented by a spectrum of near-spherical hybrid waves propagating along the axis. For large microwave focusing reflectors, the wavefronts are effectively plane in the significant part of the image space. The axial wave fields are linear combinations of the TEln and TMln fields appropriate for circular metal pipes, but can be bounded only by anisotropic-reactance surfaces. Axial-wave theory is used to investigate the characteristics of the fields in the focal region of a paraboloidal reflector, when the incident wave is uniform and plane. For radiotelescope focal ratios, the image structure differs significantly from the classical Airy pattern, deduced by scalar analysis, of optical focusing systems. Energy vortexes circulating about the dark rings influence the efficiency obtainable from aperture-type feeds in the focal plane. Application of axial-wave analysis to spherical reflectors, and the synthesis of high-efficiency low-noise feeds, using hybrid-waves in corrugated guides, are described briefly.

88 citations


PatentDOI
Olof Bryngdahl1
TL;DR: In this paper, a type of shearing interferometry is presented in which an interferogram is formed by the object wavefront and its replica folded in the longitudinal direction.
Abstract: A type of shearing interferometry is presented in which an interferogram is formed by the object wavefront and its replica folded in the longitudinal direction. An off-axis hologram of the object is first made in a conventional manner. The hologram is then illuminated to form a real and a virtual image. The real and the virtual images are superimposed. The result is a longitudinally reversed shear interferogram which indicates twice the phase variation of the object.

57 citations


Journal ArticleDOI
TL;DR: A sidelooking radar system employing a coherent optical processor is analyzed on a holographic basis and the optical system is studied in detail.
Abstract: A sidelooking radar system employing a coherent optical processor is analyzed on a holographic basis. The optical system is studied in detail.

51 citations


Journal ArticleDOI
TL;DR: In this article, the degradation of the reconstructed images was analyzed for both long and short exposures under the assumption that the random log amplitude and phase fluctuations across the collecting pupil are locally stationary processes with gaussian statistics.
Abstract: When an object and collecting pupil are separated by a medium that has random spatial and temporal variations of refractive index, a conventionally formed image may be severely degraded over the entire field of view of the imaging system. By using lensless Fourier-transform holography, considerable improvement of image resolution can be obtained within a limited field of view. The nature of the degradation of the reconstructed images is analyzed for both long and short exposures under the assumption that the random log amplitude and phase fluctuations across the collecting pupil are locally stationary processes with gaussian statistics. Experimental results support the analysis in a qualitative manner.

46 citations


Journal ArticleDOI
TL;DR: In this paper, a simple theory of the first-order properties of holograms in thick photographic emulsions is presented, by using the first Born approximation, and it is shown that such deep holograms have three properties which distinguish them from two-dimensional or plane holograms: (1) directional selectivity, that is to say the image will appear only if the hologram is illuminated in the reconstruction within a certain angular zone.
Abstract: A simple theory of the first-order properties of holograms in thick photographic emulsions is presented. Following a combination by Denisyuk of the basic method of wavefront recon­struction with Lippmann’s method of colour photography, such holograms are produced by the interference of an object wave with a coherent reference wave, falling on the emulsion from opposite sides. It is shown, by using the first Born approximation, that such deep holograms have three properties which distinguish them from two-dimensional or ‘plane’ holograms: (1) They have directional selectivity, that is to say the image will appear only if the hologram is illuminated in the reconstruction within a certain angular zone. (2) They have colour selectivity, that is to say they will reflect only within a certain narrow waveband close to the original wavelength. (3) The second wave, which is a disturbance in two-dimensional holograms is as good as completely suppressed. All three are of great practical value. It is shown that holograms which are produced by strongly diffused, wide-angle illumina­tion, and which have a random, noise-like appearance, contain the information in the form of the auto-correlation function of densities or scattering powers between different space-elements in the emulsion.

33 citations


Journal ArticleDOI
TL;DR: In this article, a new method of evaluating the mutual-coherence function for propagation in a randomly inhomogeneous medium like the atmosphere is presented, which is highly physical and does not involve the treatment of any differential equations.
Abstract: A new method of evaluating the mutual-coherence function for propagation in a randomly inhomogeneous medium like the atmosphere is presented. The new method, which is highly physical, as distinct from a mathematical approach, does not involve the treatment of any differential equations. Instead, the treatment is based on decomposition of a randomly distorted wavefront into a set of plane waves with random amplitudes. These plane waves constitute orthogonal modes. Propagation in a random medium is treated as the physical process of diffusion of amplitude (or energy) between the modes, and a short-path-propagator function for this diffusion is developed. From the short-path-propagator function, a long-path-propagator function is easily obtained, and from this the mutual-coherence function is computed. Starting from the known short-path mutual-coherence function, which is known to be accurate, the mutual-coherence function for long paths is obtained. The results are in agreement with previous results, all of whose derivations have recently been subject to criticism. Because this derivation is not a mathematical exercise, it should not be subject to any of these or similar criticisms, which were primarily questions of mathematical rigor.

31 citations


Journal ArticleDOI
TL;DR: In this paper, an experiment is described in which a hologram is recorded by scanning the source and having the detector stationary, and the resulting holograms are shown to be identical to the conventional ''scanned detector'' hologram.
Abstract: An experiment is described in which a hologram is recorded by scanning the source and having the detector stationary. The resulting ``scanned source'' hologram is shown to be identical to the conventional ``scanned detector'' hologram. The significance of this experiment is that it demonstrates: (1) that sampled holograms of an object can be recorded at a single point in space with either a scanning source or an array of sources; (2) that the object wavefront need not actually exist in space but may be geometrically synthesized; (3) that the object may be illuminated by a changing rather than stationary wave field. The interesting possibilities which this configuration gives rise to are briefly discussed.

31 citations


Journal ArticleDOI
Olof Bryngdahl1
TL;DR: In this article, a holographic shearing interferometry is applied to display the second derivative of phase variation in an object, and experimental results of the shear interferometer are shown.
Abstract: Lately, practical applications for holographic techniques have been found in interferometry. These techniques are usefully applied to shearing interferometry. Different kinds of shear are treated, and in several cases the optical system is significantly simplified, compared to the corresponding conventional setups. Both zero-order (no tilt) and multiple-fringe (tilt between wavefronts) interferograms can he achieved, as well as a simple method for displaying the second derivative of phase variation in an object. Experimental results of holographic shearing interferometry are shown.

28 citations


Journal ArticleDOI
02 Nov 1968-Nature
TL;DR: In this paper, it was shown that interference with a reference wavefront can be used to reveal the change in the phase distribution in the object field between the exposures of a single hologram plate.
Abstract: DISTRIBUTIONS of phase variation in light wavefronts can only be made visible by converting them into corresponding distributions of amplitude. This is achieved directly by interference with a “reference” wavefront, which is, of course, the basic principle of all two beam interferometers, and is also the means by which changes in a phase distribution recorded holographically may be made visible1–3. The papers referred to give accounts of (a) the “live” interference fringes, which are seen when a hologram is replaced exactly in its original position in the same apparatus after processing, and changes occur in the object which result in phase variation at the hologram, and (b) the “frozen” fringes, seen when two reconstructed wavefronts from a single hologram plate, corresponding to two successive exposures the plate has received, interfere to reveal the change in the phase distribution in the object field between the exposures.

25 citations


Journal ArticleDOI
Olof Bryngdahl1, Adolf W. Lohmann1
TL;DR: In this paper, the number of object points is reduced by taking the holographic transformation in only one direction while imaging the object onto the hologram plane in the orthogonal direction, using an astigmatic lens system.
Abstract: In previous attempts to record holograms of incoherent objects, the fringe contrast has been quite low, often barely exceeding the photographic grain noise. A method is presented which improves the fringe contrast by allowing only a limited number of object points to contribute to any local fringe pattern in the hologram. The number of object points is reduced by taking the holographic transformation in only one direction while imaging the object onto the hologram plane in the orthogonal direction, using an astigmatic lens system. An interferometric device divides the wavefront and inverts one leg about a line in the imaging direction. The interference patterns from corresponding points are then incoherently superimposed on the hologram. Some experimental results are shown. The objects were illuminated with spatially incoherent light; a rotating diffuser was introduced in a laser beam.

Journal ArticleDOI
TL;DR: In this article, the Fabry-Perot etalons are used to produce a multiple-beam fringe pattern of concentric rings, with quasi-linear spectral dispersion over an appreciable annular region corresponding to two free spectral ranges.
Abstract: Defocused spherical mirror Fabry—Perot etalons, in which the mirror separation is slightly less than the common radius of curvature, produce a multiple-beam fringe pattern of concentric rings, with quasi-linear spectral dispersion over an appreciable annular region corresponding to two free spectral ranges. The characteristics of these interferograms are discussed in relation to their many advantages for pulsed laser spectroscopy. These advantages include: (i) accuracy of frequency difference measurement; (ii) high illumination of the detector with moderate energy density in the laser beam; (iii) ease of alinement and permanent adjustment of the mirrors leading to the attainment in practice of a very high instrumental finesse (N R values of up to 90 have been achieved); (iv) measurement of degree of spatial coherence of laser beam; (v) ease of matching the interferogram to the spatial resolution of the detector. A simple optical path relation determines the positions of the fringes and the location of the quasilinear dispersion region. The interfering wavefronts, formed by multiple reflexion, have been numerically computed and summed to provide information on the finesse, fringe profiles, contrast and optimum conditions of use of this new, very high resolving power (107 to 108) quasi-linear spectrographic disperser. Constructional details are described and optical design criteria are discussed, together with the various experimental arrangements for employing the instrument. Comparison is made with the equivalent confocal and plane Fabry—Perot etalons and methods of simultaneously measuring

Journal ArticleDOI
03 Aug 1968-Nature
TL;DR: The principles of wavefront reconstruction or holography apply also to wave phenomena other than light, and it is suggested that some biological information processes can be interpreted on this basis.
Abstract: IT is well known that the principles of wavefront reconstruction or holography1,2 apply also to wave phenomena other than light. I have shown3–5 that holograms can be produced with ultrasonic waves, and have recently suggested (refs. 6 and 7 and my unpublished results) that some biological information processes can be interpreted on this basis.

Journal ArticleDOI
Joseph Meiron1
TL;DR: Three types of merit functions were applied in automatic design of optical systems: mean square value of wave aberrations; variance of wavefront aberration; and variance of an MTF based function suggested by H. H. Hopkins.
Abstract: Three types of merit functions were applied in automatic design of optical systems: mean square value of wave aberrations; variance of wavefront aberrations; and variance of an MTF based function suggested by H. H. Hopkins. They have proved to be very powerful in the final stages of optimization of well corrected systems and lend themselves readily to automatic control of minimization. The method of application and actual design examples are described.

Journal ArticleDOI
TL;DR: In this article, a complete holographic interferometer is described, with emphasis on its potential in isopachic stress analysis, and the method of wavefront reconstruction simplifies the optical requirements in conventional two-beam interferometry.
Abstract: Application of holography to photoelasticity is briefly discussed, with emphasis on its potential in isopachic stress analysis. A complete holographic interferometer is described. The method of wavefront reconstruction simplifies the optical requirements in conventional two‐beam interferometry. By employing a photochromic recording medium, the technique of real‐time interference becomes a practical and convenient process. Examples of isopachic fringe patterns obtained by this interferometer are shown.

Journal ArticleDOI
TL;DR: In this paper, two holographic methods, stroboscopic holography and the Powell and Stetson technique, are described and compared experimentally, and a third method, combining a projected fringe technique with stroboscale illumination for large amplitudes of vibrations, is described and experimental results presented.

Journal ArticleDOI
01 Mar 1968
TL;DR: In this article, an experiment is described which demonstrates the validity of the concept of compensating, by holographic techniques, for atmospheric distortions of wavefronts, and the experiment is extended to the case of a single wave.
Abstract: An experiment is described which demonstrates the validity of the concept of compensating, by holographic techniques, for atmospheric distortions of wavefronts.

Journal ArticleDOI
TL;DR: Diagnostic optical communication experiments were performed comparing noncoherent and coherent detection techniques and showed the greater sensitivity of the coherent system to the time-varying wavefront breakup produced by atmospheric turbulence.
Abstract: Diagnostic optical communication experiments were performed comparing noncoherent and coherent detection techniques. Three different receiver–transmitter configurations with variable apertures were used during the experiments that were performed over a 1-km real atmospheric path. In every case, it was found that the coherent system fading, due to atmospheric turbulence, was considerably greater than the noncoherent system fading. This result shows the greater sensitivity of the coherent system to the time-varying wavefront breakup produced by atmospheric turbulence. A coherent homodyne experiment at 10.6 μ over a 2-km round-trip path was also performed. Its results indicated that a coherent system at 10.6 μ is less susceptible to atmospheric turbulence than a coherent system at 6328 A.

Journal ArticleDOI
Peter Langenbeck1
TL;DR: In this paper, the same instrument in different alignment enhances the sharpness of reflection multiple-beam fringes or reverses their contrast for a transmission-like appearance for wavefront mapping by coupling of two interferometers.
Abstract: Some advanced methods of wave-front mapping by coupling of two interferometers are described. Moire-like superposition of interferograms eliminates certain inherent interferometer errors. Moire interferometry indicates the direction of the order of interference.The same instrument in different alignment enhances the sharpness of reflection multiple-beam fringes or reverses their contrast for a transmissionlike appearance.

Journal ArticleDOI
TL;DR: A method is presented for recording into the temporal domain the information contained in a light wavefront by making the wavefront interfere with the field from a moving point-light source and integrating the intensity of the interference pattern in space with an extended photodetector.
Abstract: A method is presented for recording into the temporal domain the information contained in a light wavefront. This is achieved by making the wavefront interfere with the field from a moving point-light source and integrating the intensity of the interference pattern in space with an extended photodetector. In the inverse process, the time dependent signal is converted, with a suitable arrangement, in a hologram. Possible applications to holograms of steady or moving objects are considered. An experiment supporting the theoretical conclusions is presented.

Patent
25 Nov 1968
TL;DR: In this paper, it is assumed that the image to be forgered is of an OBJECT made up of POINT SOURCES with the PHASES OF the RAYS from EACH SOURCE being RANDOM.
Abstract: THE TECHNIQUE IS FOR ARBITRARILY ALTERING A PHYSICAL (E.G., ELECTROMAGNETIC OR SOUND) WAVEFRONT, IN WHICH THE WAVEFRONT IS SHAPED BY MODIFYING ITS PHASE IN A CONTROLLED MANNER. A FIRST ASSUMPTION IS MADE THAT THE IMAGE TO BE FORMED IS OF AN OBJECT MADE UP OF POINT SOURCES WITH THE PHASES OF THE RAYS FROM EACH SOURCE BEING RANDOM. THE RANDOM PHASES ASSUMPTION PERMITS THE SECOND AND FURTHER ASSUMPTION THAT THE AMPLITUDE OF THE WAVEFRONT, INCIDENT ON A PLANE, IS CONSTANT. THEREFORE, THE WAVEFRONT IS SHAPED SOLELY BY OPERATING ON ITS PHASE. THE WAVEFRONT SCATTERED FROM AN OBJECT IS FIRST, IN THE PREFERRED EMBODIMENT, CALCULATED DIGITALLY AND THE CALCULATION IS IDENTICAL TO THAT USED TO CALCULATE THE WAVEFRONT AS IN CONVENTIONAL DIGITAL HOLOGRAPHY WHEN A POINT APERTURE OBJECT ASSUMPTION IS MADE. THE PHASE OF EACH POINT IN THE WAVEFRONT IS DETERMINED AND A PLOT TAPE WRITTEN FOR A PLOTTER, THE PHASE INFORMATION SCALED FROM 0 TO 2$ OVER THE GRAY LEVELS AVAILABLE, SUCH THAT A PHOTOGRAPHIC FILM IS PRODUCED, THE DARKENING OF WHICH OVER ITS AREA IS A MEASURED OF PHASE. THE THUS PRODUCED FILM IS THEN PHOTO-REDUCED TO GIVE A PHASE DISTRIBUTION AT THE APPROPRIATE WAVELENGTH OF LIGHT. THE PHOTO-REDUCTION IS THEN BLEACHED SUCH THAT THE EMULSION IS OF UNIFORM TRANSMISSIVITY BUT WITH AN ETCHED SURFACE, THE ETCH DEPTH OF WHICH IS PROPORTIONAL TO EMULSION DARKENING. UPON ILLUMINATION THE WAVES INCIDENT UPON THE PHOTO-REDUCTION ARE SELECTIVELY RETARDED BY THE THICKNESS DIFFERENCES OF THE EMULSION SUCH THAT THE RESULTANT INTERFERENCE AT AN IMAGE PLANE PRODUCES THE DESIRED IMAGE.

Journal ArticleDOI
H.J. Caulfield1
TL;DR: In this article, the effect of the extended reference beam on the information retrievability of local reference beam holograms is derived and demonstrated, and the effect is shown to be similar to the one in this paper.

Journal ArticleDOI
TL;DR: In this paper, it is shown that given a wavefront with rotational symmetry, it is possible to calculate the interferogram that would be obtained in a lateral shearing interferometer.
Abstract: Given a wavefront with rotational symmetry, it is possible to calculate the interferogram that would be obtained in a lateral shearing interferometer. The opposite problem, that is, to find the wavefront from measurements on the interferogram, is also discussed.

Journal ArticleDOI
TL;DR: In this article, the effects of multiple frequencies in spatially recording two wavefronts are analysed to describe the fringes recorded in holography as a function of the number of frequencies used, and to relate the changes in the fringe structure to the quality of the reconstructed image.
Abstract: The effects of multiple frequencies in spatially recording two wavefronts are analysed to describe the fringes recorded in holography as a function of the number of frequencies used, and to relate the changes in the fringe structure to the quality of the reconstructed image. The analysis is applicable to spatial filtering or to any process where a coherent wavefront is recorded by interference with another wavefront. A possibility of doppler shift of the frequencies in one wave is included to describe the effects of recording a hologram of a moving object. It is shown that, if the multiple frequencies are due to laser multimoding, the fringe visibility is a maximum for path differences equal to even multiples of the laser cavity length, but that the locations of the minima depend upon the relative amplitudes of the different frequencies. The effects of the changes in the fringe structure upon the resulting image or correlation are discussed.



Journal ArticleDOI
TL;DR: In this article, a general solution for the resulting light diffraction patterns is obtained, from which the conditions are determined for which the diffracted light intensity distribution will be a valid representation of the power spectral density of the input electrical noise signal.
Abstract: Excitation of an ultrasonic light modulator by random electrical signals will cause a random modulation to be imposed on the emerging light wavefront. A general solution for the resulting light diffraction patterns is obtained, from which the conditions are determined for which the diffracted light‐intensity distribution will be a valid representation of the power spectral density of the input electrical noise signal. The necessary experimental conditions for which the variance of the spectral measurements will be small are determined. The percentage of the light‐modulator bandwidth over which measurements will be valid is found to be dependent on the product of the spatial‐frequency bandwidth of the light modulator and the length of the optical aperture; this effect is evaluated and numerical results are presented. The case in which the input to the ultrasonic light modulator consists of a deterministic signal and noise is also considered. Using these results, the diffracted light levels due to signal an...

Proceedings ArticleDOI
01 Jan 1968
TL;DR: Holography can be used to circumvent photography's depth-of-focus problem for small objects in motion, and measurements equal, equivalent, or beyond those of an interferometer can be made with the technique as discussed by the authors.
Abstract: Holography has two facets which make it a revolutionary scientific instrumenta-tion technique. The first capability is that holography can be used to circumvent photography's depth-of-focus problem for small objects in motion. The second is that measurements equal, equivalent, or beyond those of an interferometer can be made with the technique. The latter can be done in terms of two holograms recorded on the same photographic plate (double-exposed holographic interferometry),lin terms of holograms on separate plates, or in terms of comparing the reconstruction from a hologram against the actual scene wavefront at a later time (stored beam holographic inter-ferometry). 2 In each case, the difference in phase between the two light wave patterns produce fringes, with each fringe tracing the loci of constant phase or equal optical path length. Neighboring fringes are normally separated by an optical path length of one full wavelength:3,4,5 The new ability to make optical interferometric measurements on and in hitherto impossible situations is an extremely valuable contribution. Of particular value are measurements on diffuse surfaces, which are impossible with classical interferometry.© (1968) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Journal ArticleDOI
01 Sep 1968
TL;DR: In this paper, a holographic interferometry technique for measuring and visualization of vibration amplitudes and modes is presented, which can be applied to a turbo-charger impeller investigation and a turbine blade inspection.
Abstract: Laser light has widened the horizons of applied interferometry both by simplifying existing techniques and opening up a new field where holography is utilized to provide a master or reference wavefront. Whilst retaining many of the advantages of classical interferometry the new methods are simple in application and will accommodate almost any surface finish. Examples of application include an investigation of performance for pressure transducer diaphragms and the development of an engine rocker design. A slight modification to the technique enables measurement and visualization of vibration amplitudes and modes. Application to a turbo-charger impeller investigation and a turbine blade inspection are given.

01 Jan 1968
TL;DR: In this paper, a degenerate regenerative ring-type amplifier with a gain of 24 dB and a bandwidth of 1 MHz was proposed for transverse field configurations, which can be used to amplify arbitrary transverse fields.
Abstract: Absfract-The amplification before detection of the carrier of a modulated optical signal by a narrow-band quantum amplifier enhances the signal-to-noiseratio,particularly when the signal wavefront is distorted. A further improvement is obtained by using a combination of wide-band and narrow-band quantum amplifiers. The practical application of these schemes requires a degenerate regenerative ring-type amplifier capable of ampliiing arbitrary transverse field configurations. Experiments show that such an amplifier with a gain of 24 dB and a bandwidth of 1 MHz is feasible. The incident beam axis can be displaced by as much as ten times the beam-waist radius without losing more than 4 dB in gain. Frequency modulation may be converted into amplitude modulation by the phase shift introduced in the carrier.