Showing papers on "Moiré pattern published in 1972"

Moiré Gauging of In-Plane Displacement Using Double Aperture Imaging

Abstract: An easily implemented method of measuring in-plane surface displacement by photographing an object through two laterally displaced apertures is described and the experimental results are presented. The displacement is displayed as a pattern of moire fringes over the image. No previously constructed grids or rulings are required as in normal moire devices. The method is noncontacting and requires no special surface preparation. The sensitivity is easily adjusted and is shown to be equivalent to that obtained using double-exposure holography or speckle pattern interferometry techniques. The method has potential application in mechanically unstable environments or where the conditions are such that grids or strain gauges cannot be attached to the object.

The development of orthogonal bicolored moiré fringes

Abstract: This paper presents the results of an investigation to develop new techniques to enhance the moire method of mechanical interferometry. The program demonstrated the practicality of developing the use of orthogonal bicolored moire fringes with the unique capability to selectively display either set of fringes while obscuring the other set. An interpolation method for the determination of fractional fringe orders is also discussed.

The Holo-diagram. VI: Practical device in coherent optics

Abstract: Ordinary interferometry, hologram interferometry, contouring, speckle, moire, ordinary radar, Doppler radar, optical Doppler velocimeter, Doppler holography, and gated viewing are all discussed, and it is demonstrated how closely these methods are related to each other. One single diagram, the holo-diagram, can be used for the evaluation of each method and even for combinations of the methods. The diagram is constructed for the study of interference patterns when an object is placed in the vicinity of two focal points representing either one point of illumination and one point of observation or two points of illumination or two points of observation. It reveals interference surfaces in space and how ordinary interference fringes are formed where an object intersects these surfaces. With a common moire analogy the three-dimensional sensitivity distribution and the fringe patterns of each of the methods can be simulated. Finally it is pointed out that the diagram demonstrates the interference limits and diffraction limits of any method of optical information retrieval.

Moiré Screens Coded with Pseudo-Random Sequences

Abstract: A new type of moire grating that can produce very narrow moire beat lines is presented. Each grating period contains a number of transparent or opaque lines of equal width. The sequence of transmission coefficients across the grating period is not periodic but is chosen to be a binary pseudo-random sequence. These sequences are commonly used in signal processing because of their sharply peaked autocorrelations. When used to encode a moire grating, they produce very narrow moire beat lines. Three experimental gratings are discussed, and a contouring measurement using the narrow beat lines is described.

Method of measuring irregularities in the evenness of surfaces

Abstract: Apparatus and method of measuring surface irregularities by means of the interference field occurring behind a diffraction grid. Light bars are generated which are reflected from the surface to be tested back onto the grid. If the surface includes irregularities down to 1 micron and under, moire patterns which are a function of the height and width of the irregularities become visible on the opposite side of the grid. By suitably selecting the direction of observation, the moire patterns are superimposed by interference fringes which permit the moire patterns to be accurately gauged.

Developments in the optical spatial filtering of superposed crossed gratings

Abstract: It is shown in this paper how the whole field of displacement components and of their time and space derivatives (isothetics, isotachics and isoparagogics) can be obtained individually, as separate patterns in a simple and precise manner using spatial-filtering techniques. This result can be obtained even when crossed gratings are used on the deformed body. A method for achieving fringe multiplication in moire patterns produced by superposed, crossed gratings is also demonstrated. It is also shown that displacement components and their time and space derivatives in directions diagonal to the crossed-grating lines can be obtained by proper handling of grating transparencies and spatial-filtering techniques. Hence, the moire equivalent of a whole field of rosette-strain-gage measurements is obtained from a single photograph of a deformed crossed grating. A disk compressed between two wedges is used as an example. Important applications will be found in the fields of dynamics, nonlinear elasticity and plasticity.

Method and apparatus for sensing surface displacement orthogonal to the line of sight

Abstract: A novel method and apparatus for sensing surface displacement of an object orthogonal to the line of sight is described. The object is illuminated from a single laser source and imaged by a common lens using two laterally displaced apertures. Two successive images are obtained in this manner and are superimposed. The displacement is displayed as a pattern of moire'' fringes or bands over the image. Increasing the moire'' contrast is achieved by spatial filtering. The invention has application to measuring object displacement or distortion under both static and vibratory conditions.

Modified double-beam interferometer using the moiré method.

Abstract: It is described theoretically and experimentally that the degradation produced on a double-beam interferogram by the imperfection of the optical elements is eliminated by using the Moire technique. In this method the information of the phase object is obtained as the Moire pattern by superposing the interferograms with and without the object through the imperfect optical system.

Instrument for measuring curved surfaces

Abstract: An instrument for measuring curved surfaces, such as lenses, by moire fringe patterns and keratometry is disclosed having a target pattern arch backlighted from a single light source to reflect the target and mire pattern from the surface and superimpose them on two reticles adjacent the eyepiece to form moire fringe patterns and to measure the distances between the reflections on the mires. The two reticles are closely spaced with one fixed and one movable to vary the calibrated measuring pattern. The reticle pattern is masked so that only a portion of the moire fringe pattern is seen through the instrument so that one moire fringe line may be used to indicate sphericity and/or radius. The instrument is focused by movement of the curved surface holder toward and away from the objective lens of the instrument to balance the moire fringe pattern made with each reticle. The holder is positioned in a floating magnetic ring for centering the lens along the optical axis of the instrument.

Fluid film thickness measurement with moiré fringes.

Abstract: A mathematical analysis of the application of moire fringe contouring to fluid film thickness measurement is made. Two special cases are treated. The first considers the generation of contours with a collimated light source and distant observer. The second analyzes the fringe patterns viewed by an observer near the grid when there is a point source of light. Geometrical restrictions imposed on the experimental setup are detailed. Experimental results are included.

Vibration measurements by the moire method

Abstract: A simple and versatile, but also sensitive and accurate method of determining vibration amplitudes is presented. The method consists of observing the slow motion of a moire pattern formed by the image of a signal grating projected on to an identical reference grating, illuminated by stroboscopic light. The pitch of the image of the signal grating can be suitably adjusted to yield an optimum relation between line frequencies of the two gratings. Full control over the sensitivity of the system is therefore possible. The accuracy and resolution of the method are comparable with those of other known methods based on sophisticated vibration measuring equipment.

An automatic high-speed scanner for moiré fringe techniques

Abstract: A high-speed scanning system has been developed to locate the centres of moire fringes along any single line. The system comprises a rotating-mirror scanner for conversion of an optical signal to electrical; this conversion is followed by peak detection, storage, and print-out electronics. Up to 20 fringes can be scanned in 1 ms. The results are printed in terms of fringe separations along the scanning line.

Some Investigations on the Methods of Measuring 3-Dimensional Plastic Deformations by Laser

Abstract: The moire method and the holographic interferometry, which are effective for the measurement of large and small deformations respectively, are developed. The moire method proposed in this paper is founded on the interference between the grating printed on a specimen and Young's interference fringes instead of the reference grating. If Young's interference fringes are projected on to the specimen from two directions, two components of the displacement are obtained. Measurement of an incremental deformation is also possible when some sensitizer is affixed to the specimen and Young's interference fringes are projected on to it two times during deformation. As for the holographic interferometry, Enno's method is improved so that two families of the interference fringes corresponding to two components of the displacement may be obtained on the congruent images to each other. Fundamental data are provided about these methods.

Estimation and Measurement of the Frequency of Coarse Gratings by Moire Fringe Techniques

Abstract: Moire fringes are obtained when two gratings of about same frequency are superposed on each other. Normally the two gratings are of special geometrical curves. For the present particular application, one grating is assumed to be a grating of constant but unknown frequency. The other grating which is the measuring grating consists of different types of curves. The theory of the resulting pattern and how it is used to measure the frequency is presented.