Jacob Politch

Heterodyne profiling instrument for the angstrom region

An instrument for surface profile measurements without need of a separate reference surface is presented. Height variations of 50 A can be resolved, while the theoretical height resolution is 4 A. The lateral resolution is 3 μm, which also can be improved. Scan lengths of 100 μm to several millimeters are achieved, and no extensive alignment of the sample is needed. The instrument is a heterodyne interferometer, and the sample is used as a reference surface.

Slope and curvature measurement by a double-frequency-grating shearing interferometer

This paper describes a shearing interferometer capable of the direct measurement of the slopes and curvatures of reflecting plates. The wavefront shear is produced by a double-frequency grating which is simply a grating containing two closely spaced frequencies. The first-order waves associated with each frequency form the interferogram, the contours of which are related to the models slope or curvature depending upon system arrangement and recording procedure.Four arrangements are described: two for slope and two for curvature. In one, the slope contours are obtained directly and in real time. In the second slope arrangement, an extra spatial-filtering step is necessary to obtain the slope contours. However, this arrangement, as opposed to the first, measures the slope only due to the loading, compensating for initial model slope and optical-system aberrations.The two curvature techniques can be described as double-shearing interferometers having a primary and secondary shear. The primary shear for both arrangements is provided by the double-frequency grating. In one technique, the secondary shear is provided by a translation of the recording film between two exposures during the recording step. A subsequent spatialfiltering step displays the curvature fringes. The second technique requires only a single exposure and places a beam splitter at the location of one of the first-order diffraction spectra during the filtering step. The field equations defining the fringe values are derived for all four arrangements with experimental results also being given.

Recording slope and curvature contours of flexed plates using a grating shearing interferometer.

A diffraction grating shearing interferometer is presented which, depending on the recording procedure, can be used to measure directly the slopes and curvatures of specularly reflecting flexed plates. Three system arrangements are described: two for slope measurement and one for curvature. One slope arrangement yields results in real time but, in general, does not compensate for initial model curvature. The second requires an additional spatial filtering step but does compensate for the initial curvature. A diffraction theory approach is used to arrive at expressions for the complex field amplitude of the light at the output plane. This leads to expressions for the fringe values. Experimental results for three types of model are also presented. Errors of 2-4% were found for the various system arrangements. In addition, an analytical expression is derived and verified experimentally, which shows that when the system is used for measuring slope, the curvature of the model limits the measuring sensitivity. The theoretical analysis shows that this is a fundamental property of grating interferometers in general. This property is then exploited as the basis of the curvature measuring technique.

Infrastructure of time-averaged projection moire fringes in vibration analysis

Time-averaged moire fringes may be produced by projecting a grid onto a vibrating test item surface using a conventional (2 in. x 2 in.) slide-projector and recording with a 35-mm SLR camera. Quantitative analysis of the resultant fringes is generally performed with the help of equations developed by time-averaging the optical intensity of the projected grid throughout the vibration period of the test item surface. In the present paper the authors present a physical description for the production of time-averaged moire fringes involving the splitting of grid lines resulting from their behavior as simple harmonic oscillators. Superimposition of vibrating grid lines produces blurring of the photographic image; regions of zero-amplitude vibration (nodes) may be identified by the clarity of their grid lines, enabling the analysis of mode shapes from the photographs.

Holographic moire deflectometry—A method for stiff density fields analysis

This letter presents a method for the analysis of stiff density fields in which weak density gradients as well as strong ones, pointing in different directions, exist. Holographic techniques are used to freeze the deviated rays of a collimated beam traveling through a phase object. The hologram is post analyzed by the ‘‘moire deflectometry’’ method, where the sensitivity, the spatial resolution, and the direction along which the density gradients are measured, may easily be changed and adjusted to the different density regions in the field. It was found that this technique is very useful for short duration stiff fields where density mapping during one pulse is desired.

Variation in displacement values along projection moire fringes in contour difference and vibration analysis of structures

This paper presents a qualitative and quantitative analysis of timeaveraged projection moire vibration measurements for a plate and a cylindrical shell. It is pointed out that although moire fringes are generally considered to represent a constant contour difference or constant vibration level, in practice, variations in local grid pitch values on the test-item surface result in wide variations in measured displacement levels for given fringe numbers, which must be taken into consideration for quantitative evaluation.

IMPROVED POLARIZATION HOLOGRAPHY

Using circularly polarized light, high‐quality holograms can be obtained, containing information on the polarization characteristics of the object. A theoretical derivation and some experimental demonstrations are given.

Some characteristics of multibeam holography

Abstract Holograms recorded with a number of reference beams are sensitive to alignment when reconstructed. Interference effects due to misalignment are evaluated, and as a result optimal recording parameters can be suggested.

Methods of strain measurement and their comparison

Abstract Three methods for strain measurement are described—speckle shearing interferometry (SSI), electrical strain gages and mechanical deflectometers; they were compared to the analytical method based on the theory of elasticity. The experimental methods were simultaneously applied on a simply supported uniformly loaded plate of a composite material. Good agreement was obtained between the measured and the calculated results, at the points of measurement within the range of loading.

Spectroscopic holography: a new method for mode analysis of randomly excited structures

A method was developed for analyzing modes and amplitudes of vibration of a randomly excited object. Time-average holography, in which the reference beam was mechanically excited at any given frequency in the spectral range of the random excitation, was applied. The phase of the exciting vibration of the reference beam was locked to the phase of the specific frequency in the random band. This method was applied as successfully as for objects excited with swept sinusoidal vibration.