Hologram Image Research Articles

Holographic interferometry with increased sensitivity for diffusely reflecting objects

The reconstructed image of a double exposure hologram, made by deforming the object and slightly tilting the signal beam at the same time, consists of modulated Young's fringes. It would be possible to presume the plate, which is used for taking a photograph of the reconstructed image and is processed in a nonlinear way, to be equivalent to a nonlinear hologram. Therefore the principle of phase difference amplification applies. Thus the interferogram with increased sensitivity of lambda/16 per fringe was experimentally obtained with a diffusely reflecting object. Discussions of the system error are presented.

Numerical reconstruction of apodized acoustical holograms

Apodization is a process that was first used to improve the resolving power of imaging systems involving diffraction patterns. It originally signified a reduction in the energy in the side lobes, but it can also be used in a broader sense, as will be done here, to include either a reduction or an enlargement of the side lobes in an acoustical hologram. Since apodization can produce desired characteristics in a reconstructed image of an acoustical hologram, it adds flexibility to the numerical reconstruction process. This flexibility may help to solve one of the most formidable problems facing numerical holography—the large demands for computer memory and computer processing time. It is the objective of the study to show that this is so. Apodization cannot usually produce an overall improvement in the quality of a reconstructed image of an acoustical hologram: although one aspect of the image is improved, another is degraded. However, apodization can be helpful since it can improve the characteristic of interest, even at the expense of other aspects. In this paper, an example is given in which apodization is applied by a computer during the numerical reconstruction of an acoustical hologram. Numerical holography was chosen because the type of apodization described here is readily applied and, as far as could be determined, has not yet been used in numerical holography, although it has been applied with great effectiveness to the shading of sonar arrays. In order to understand the application, a rectangular object formed by illuminating a slot is taken as the first object to be viewed by means of holography. A mathematical description of the resulting hologram is determined by means of the Huygens-Fresnel diffraction formula. Examination of the expression reveals a pattern to which apodization can be applied. The acoustic pattern consists of a large main lobe with monotonically decreasing minor lobes (side lobes) on either side. This paper describes and evaluates a method of apodization in which all side lobes are held at a specified level and the main lobe assumes the minimum width possible for the side lobe level. After demonstrating the method with the rectangle, a triangle is used as the second object, this time expressing it as a summation of rectangles. When this object also worked well, it raised the possibility that any two- or three-dimensional object could be used after expressing it as a summation of rectangular prisms. This proved to be the case for the two-dimensional object chosen, the head of a horse.

Color-encoded focused image holograms

Color-encoded focused image holograms can be reconstructed in a white light projection system to form high fidelity color images. Use of frequency encoding leads to a simple, inexpensive optical reader similar to a conventional microfiche reader. The holograms can be formed as surface-relief, phase holograms in transparent media and can be replicated by embossing into thermoplastic film. Applications include micropublishing and related fields, where they offer a considerable cost advantage over conventional color microfiche. The characteristics of color reproduction are discussed; and the hologram recording, replication, and readout are described. An optimized set of parameters is derived, which gives colorimetry comparable with that of color television and a measured screen luminous emittance of 190 lx using a standard 150-W projection lamp and x15 magnification.

An advance in the processing of holograms

A new process is outlined for the production of bleached silver image holograms. This paper discusses a process that leaves phase holograms with the low noise and scatter previously only associated with amplitude holograms. Much larger image volume can now be recorded with high efficiency and contrast due to the improved image quality and reduced noise. The basic improvements are due to developments in concentrated Neofin and the use of a carefully constituted mild bleach agent.

Subtraction of illuminance using a Savart plate

After having discussed the principle of the technique for subtraction of illuminance, based on the use of image holograms, we describe a device employing a Savart plate, placed between an analyser and a polarizer, which enables subtraction directly from a real object rather than its photographed image. The objects are illuminated with quasi-monochromatic spatially incoherent light. We also study the influence of the various parameters on recording and reconstruction of image-holograms. In particular, we show that the orientation adjustments of the analyser and the polarizer do not require great precision. Further more, we determine limitations which must be imposed on the band pass of the recording device and on the slit width of the reconstruction device so that the various orders diffracted by the image hologram do not overlap. Finally, we present results for a three dimensional object, obtained under theoretically determined conditions for the experiment.

Increase of Resolution in Acousto-Optic Bragg Imaging System

This paper describes an increase in resolution of an acousto-optic Bragg imaging system by introducing an optical superresolution technique. The theory of acoustically naodified superresolution is analogous to that of optical superresolution and the image-performance is due to the reconstruction of image holograms produced by the composite system of a Bragg imaging system and a Bragg-diffracted light generator. Experimentally obtained images demonstrated a good verification of the theory and the effect of the reduction of background noises. In particular, the resolvable object distance was approximately eight times the sound wavelength used,which had not been obtained in a usual Bragg imaging system.

A colour micro-storage and display system using focused image holograms

Colour-encoded focused image holograms have been investigated as a possible medium for micropublishing and related fields. A number of microfiche readers for the holograms have been constructed and demonstrated, and both master and replica fiche have been made. The system is superior to conventional colour microfilm in terms of resolution, colorimetry, and long term stability, and offers a considerable cost advantage.

Holographic characteristics of 10E75 plates for single- and multiple-exposure holograms

The holographic characteristics of Agfa-Gevaert Scientia 10E75 have been examined for multiple exposures. They include not only diffraction efficiency, but SNR and resolution as functions of number of exposures, time of development, and/or input modulation (rho'). The maximum diffraction efficiency, occurring at a density of 0.7, was observed to increase with increasing development time and decrease as the number of exposures increased. An approximately 10% decrease in maximum diffraction efficiency was observed for each successive reconstructed image of a multiple exposure hologram. Reconstructed noise was found to be fairly well represented by grain-noise considerations only for the region 0.1 < rho' < 0.4. Elsewhere noise is considerably different from grain-noise predictions. The SNR, at a density of 0.7, was observed to increase from 50 to 85 as the number of exposures increased from one to four. For postexposure or preexposure holograms the maximum signal-to-noise and resolution peaked near an input moduation of 0.4.

Real Time Superresolution by Means of an Ultrasonic Light Diffractor and TV System

The change of angle and the shift of frequency of light by an ultrasonic light diffractor are used for real time realization of a holographic superresolution system. A TV system and an electrical filter are used to pick up the desired image hologram from the superposed, images, which are obtained by a number of object-beam lights and reference beams. For two-dimensional as well as one-dimensional objects, images superresolved three to five times are displayed on a TV monitor in real time.

Computer Mapped Hologram Images in Stress Analysis

A new method of data display is presented. The central feature of the method is the production of contour maps in the form of theoretically computed virtual images of a doubly exposed hologram produced by interfermetric holography. Since the method yields a full field of data for the entire region of interest, it may be used to perform computer simulated experiments, before physical testing, to focus an experimenter's attention on critical regions of a test specimen. Finite elements results are presented in full field holographic form for isotropic linear elastic and plastic material behavior. Displacement fields are illustrated using the method of analytical holograms (computer generated hologram images) for a cantilever beam and for a rectangular inelastically loaded plate, with centrally located circular hole. A reference to similar results for anisotropic elasticity and viscoelasticity is cited.

A holographic image printing technique with high resolution

A method is shown to obtain a real image with a large image field and high resolution from an image hologram, which is produced by a large aperture lens with large amount of aberration. The resolution of the reconstructed serial image is estimated to be within the range from 450 to 500 lines/mm in 2 inch diameter image field.

Automatic Data Reduction of Certain Holographic Interferograms

An automated technique is described that uses holographic interferometry to determine the translation of a diffusely reflecting rigid body. The photographs of the reconstructed virtual image of a double-exposed hologram were digitized. The CDC 6600 computer was used to process the reconstructed virtual image data by recording the number of fringes that moved over specified image points as the viewing direction of the hologram was varied. Contours were made by connecting all image points over which an equal number of fringes passed. A surface resolution (distribution of image points interrogated) of 0.0368 cm was used. By using this technique a 6.35-microm linear displacement of a 3.68-cm diam aluminum object was verified with an error </=4.3% over the entire object. It appears this technique can be extended to three-dimensional objects.

Measurement of “real 3D-data” from holographic synthesized scanning electron micrographs

“Real 3D-data” of a specimen in a scanning electron microscope (SEM) are determined, using a self-illuminated dot, shifted in the virtual images of holograms, composed from ordinary SEM pictures. It is considered how the different optical projections in a SEM and in viewing the holographic images will affect the measurement.

Speckle Reduction in Holography by Means of Random Spatial Sampling

In this paper, we will illustrate a new technique by which the hologram image speckle can be reduced by spatially random sampling of the hologram aperture. The sampling procedure can be performed by a movable random mask. If the sampling function is made to be uncorrelated in subsequent spatial sampling, the speckle effect in the hologram image can be eliminated. However, an optimum sampling function may be difficult to obtain, which remains to be seen. This speckle reduction technique can be achieved only by some trade-off of the holographic resolution. A simple experimental confirmation of this technique is illustrated. Since one of the most severely limiting factors in applications of holography may be the speckle effect, this proposed technique should be added, together with the other existent techniques, to remedy this limiting factor.

Explanation of the “Venetian blind” effect in holography, using the Ewald sphere concept

The Ewald sphere construction of X-ray diffraction theory is used to describe the “Venetian blind” effect which occurs in holography when changes in geometry occur between hologram formation and image reconstruction. The limitations on information storage in thick holograms, due to the existence of the effect, are pointed out. Experimental results are given.

Superresolution by Multiple Superposition of Image Holograms Having Different Carrier Frequencies

A new technique of superresolution which makes use of a multi-exposed image hologram is described. The basic idea of this method is to use the hologram as storage medium for complex amplitude of the image of an object to be superresolved and to add up the complex amplitudes of the images of different spatial frequency ranges by multiple exposure on the hologram to synthesize the superresolved image. In this paper, a detailed analysis of the theory of this method, and the results of experiments for one-dimensional and two-dimensional objects, are presented. As a whole, the bandwidth of the optical system could be extended to the theoretically expected value.

Characteristics and Measurements of an Aperture-Limited In-Line Hologram Image

The intensity distribution in the neighborhood of the real image reconstructed from an in-line Fraunhofer hologram was investigated. Assuming a finite amount of information recorded on the hologram in terms of a limiting aperture, the image is shown to possess intensity variations that are dependent on the argument of a first-order Bessel function recorded on the hologram. The theoretical results are presented on isophote diagrams and are verified experimentally. Three measurements of the image width are presented, and the inherent errors are found. The focal tolerance of the aperture-limited image relative to the recording parameters is discussed.

Phase modulation noise in a reconstructed image of a fourier-transform plane phase hologram

Phase modulation noise in a Fourier-transform plane phase hologram is investigated in terms of combination tones between signal components. Negative correlation between diffraction efficiency and signal-to-noise ratio is derived.

Interferometry with an Achromatic-fringe System

A lateral-shear interferogram of a phase object is obtained when a Ronchi ruling is photographed through the object. An analysis of the technique is given from the point of view of diffraction theory and it is shown that a source of large spectral bandwidth can be employed. The effect of source size is also discussed. The lateral shear can easily be varied by altering the distance of the object from the grating. The experimental set-up can be modified so as to obtain an absolute interferogram which is also an image hologram of the object. The source size and its spectral bandwidth are severely limited in the modified arrangement.

Novel Technique for Real-Time Depth-Gated Acoustic Image Holography

An acoustic camera, which uses a scanning laser beam to detect an ultrasonic image, has been modified to incorporate the desirable depth-discrimination feature characteristic of pulse-echo techniques. The resultant system combines the virtues of acoustic holography with those of the pulse-echo method. It produces a real-time image hologram of a 2.268-MHz acoustic field with the transverse resolution characteristic of imaging (approximately three wavelengths or 2 mm), and the depth resolution of the pulse-echo technique (1–10 mm depending on the acoustic pulse length). The pulsing and gating action of the system prevents (1) image degradation due to too many out-of-focus cross sections, typical in imaging systems, and (2) most of the multiple reflection artifacts, typical in pulse-echo systems. Moreover, average acoustic power requirements for signal-to-noise considerations are lowered from the cw case by the pulse-duty factor.