Surface displacement can be measured by recording a double-exposure photograph of the laser-illuminated object, followed by optical processing of the recorded speckle-pattern image. The analysis can either be on a point-by-point basis, or by a spatial filtering technique which resolves the motion in any desired direction orthogonal to the line of sight. The limits within which the technique may be used to measure lateral translations and rotations of the surface are examined theoretically and experimentally, and the effect of lens aberrations and surface scattering properties are discussed. Surface tilt may also be measured by recording a defocused double exposure image and analysing its optical transform. The photographic techniques described can be extended to measuring surface vibration, by recording a single ‘time-averaged’ exposure and examining the modified optical transform fringe pattern.

Displacement Measurement from Double-exposure Laser Photographs

Basics of Holography is a general introduction to the subject written by a leading worker in the field It begins with the theory of holographic imaging, the characteristics of the reconstructed image, and the various types of holograms Practical aspects of holography are then described, including light sources, the characteristics of recording media and recording materials, as well as methods for producing different types of holograms and computer-generated holograms Finally, important applications of holography are discussed, such as high-resolution imaging, holographic optical elements, information processing, and holographic interferometry The book includes comprehensive reference sections and appendices summarizing some useful mathematical results Numerical problems with their solutions are provided at the ends of chapters This is an invaluable resource for advanced undergraduate and graduate students as well as researchers in science and engineering who would like to learn more about holography and its applications in science and industry

Basics of Holography

A novel approach to time-lapse interferometry and contouring is introduced that uses television rather than photographic techniques. A first interferogram is stored on a video storage device such as a magnetic disk. After a change in the stress of the object, a second interferogram is electronically compared to the stored pattern to form a signal representative of the fringe pattern caused by the deformation. This signal may be directly displayed on a cathode-ray tube or employed by some automatic inspection device.

Time-lapse interferometry and contouring using television systems.

Heterodyne Holographic Interferometry

A specimen illuminated by coherent light is imaged by a camera through a shearing mechanism so that the speckle from one point on the surface can be made to interfere with the speckle from a neighboring point. The resultant speckle pattern is recorded. By mechanically interfering the recorded speckle pattern corresponding to deformed and undeformed states of the specimen, respectively, using double-exposure technique, a speckle-moire-fringe pattern is generated. These fringes which depict derivatives of deflections of the specimen are made visible by spatial-filtering technique. Speckle-moire fringes can also be obtained in real time. This method is a new interferometry and will be referred to as “speckle-shearing interferometry”. Speckle-shearing interferometry has the same function as Ligtenberg's technique. However, it does not have the sometimes inconvenient requirement of Ligtenberg's technique that the surface of the specimen must be of mirror quality. The new technique will be particularly useful in studies of flexural deformation such as flexed beams and plates. Although speckle-shearing interferometry is an interferometric method, it overcomes several of the limitations associated with holographic and speckle interferometries, namely: (1) the setup is simple and does not need laborious alignments of optical components, (2) it does not require stringent mechanical and ambient stabilities, (3) coherent requirement of light is greatly relaxed, and (4) the sensitivity is reduced that somehow fills the gap in sensitivity between moire techniques and holographic or speckle interferometry.

Measurement of slopes of structural deflections by speckle-shearing interferometry

This paper describes the double exposure holographic formation of a contour map over a diffusely reflecting or transmitting of arbitrary shape and surface finish by using an immersion method with media of different indices of refraction between successive exposure. A wavelength λ effectively contributing to the fringe contour is given by λ0/Δn where λ0 is the wavelength of the laser and Δn is the index variation. Index variations were introduced by pressure difference in Freon to give λ=600 µ and 200 µ, and by the combination of two liquids to give 14 µ and 25 µ, thus extending the change of height corresponding to each fringe contour continuously up to one thousand times greater than with the optical Fizeau fringes. A correcting plate of a Schmidt camera in the grinding process was used as a test object and its figuring error were analyzed.

Holographic Generation of Contour Map of Diffusely Reflecting Surface by Using Immersion Method

Theoretical and experimental studies are described on the formation and localization of holographically produced interference fringes. The fringes were shown to be localized where the correlation between the wave fields produced by the two objects identical in shape but different in the relative position attains the maximum value. A grating model of a diffusely reflecting surface was proposed, in exact agreement with the theory. Experiments were carried out to verify the discussions.

Formation and Localization of Holographically Produced Interference Fringes

A double-exposure hologram interferometer using two reference beams is described which restores fringe visibility after processing and permits to obtain various fringe patterns from a single hologram. It has been successfully applied to the deformation measurements on an object of local variations in lateral translations. Experiments are also presented on changing fringe patterns suitable to interpretations. Expressions were derived for the localization of fringes of diffusely reflecting objects and compared with the experimental results. A good agreement was obtained between them.

Hologram Interferometry Using Two Reference Beams

Holography was applied to the schlieren and phase-contrast methods and enabled the visualization of a minute phase change occurring in an object which scattered light. One makes a hologram first with the phase object in position and afterwards illuminates it by the object illuminating beam with the object out of the path. Then the phase change due to the object is made to add to the reconstructed reference wavefront and is rendered visible by spatial-filtering the point image formed by the wavefront. First experiment was carried out of a striation of liquid in a cell which was illuminated diffusely from behind. This principle is also applicable to the removal of aberrations of a conventional arrangement for transparent objects. The second experiment was performed of a hot soldering iron placed in a pipe of glass and a sensitivity competitive to that realized by using an aberration-free arrangement was obtained. An accuracy necessary to locate the hologram plate to its original position and the effect of the emulsion shrinkage were also discussed.

Yoshinobu Itoh

Papers

Holographic Generation of Contour Map of Diffusely Reflecting Surface by Using Immersion Method

Formation and Localization of Holographically Produced Interference Fringes

Hologram Interferometry Using Two Reference Beams

Hologram Schlieren and Phase-Contrast Methods

Image correction using holography.