Holography

High precision deformation measurement by digital phase shifting holographic interferometry

A digital phase shifting technique capable of quantitatively determining the phase of holographic interferometric displacement fringes is presented. This technique uses computer control to take data and calculate surface deformation. The phase value at each detector point can be calculated by taking four successive intensity data frames with the reference phase shifted between each frame. The displacement fringe order number can be assigned by adding or subtracting 2p from a data pont until the phase difference between adjacent data points is less than p. Experimental results show that this technique can precisely determine a fraction of a fringe with an accuracy of ±1o.

Testing aspherics using two-wavelength holography: use of digital electronic techniques

Two-wavelength holography has been shown to be quite useful for testing aspheric surfaces since it can produce interferograms with a wide range of sensitivities. However, TWH has the drawback that the accuracy attainable from measurements on photographs of the fringes is limited. It is shown how this limitation can be overcome by using electronic techniques to evaluate the phase distribution in the interference pattern.

Image blur for rainbow holograms

An analysis is presented for selecting the important rainbow-hologram formation-setup parameters for minimization of the image blur.

Undesired light in a reconstructed hologram image caused by the nonlinearity of the photographic process

Theory and experiment show that for a hologram object of two or more object points, the nonlinearity of the photographic process causes reconstructed images in addition to both the desired reconstructed image and the higher order reconstructed images. It is theoretically and experimentally shown that for a plane object parallel to the hologram plane, some of these undesired images may be focused in the plane of the desired reconstructed image.

Effects of photographic gamma on hologram reconstructions

An expression derived for hologram exposures made along the straight-line portion of an H-D curve of a photographic plate gives the first-order transmittance of a hologram made of several object points exposed simultaneously (conventional holograms). This expression is compared with a similar expression derived previously for holograms made of several object points exposed sequentially (synthetic holograms). Theory and experiments show the effect of the nonlinearity of the photographic process on the contrast of the reconstruction of conventional holograms. Synthetic and conventional holograms are studied theoretically and experimentally to determine how the total amount of light in the reconstruction image depends upon the number of object points when the total amount of light in the object is constant. It is shown that the reconstructed image formed by a conventional hologram contains more light than the image formed by a synthetic hologram of the same number of object points. Both synthetic and conventional holograms are also studied to determine the ratio of reference-beam illuminance to object-beam illuminance that maximizes the amount of light in the reconstructed image.

Effect of the photographic gamma on the luminance of hologram reconstructions

An expression derived for hologram exposures made along the straight-line portion of the H-D curve of a photographic plate shows that the relationship between the luminance of the reconstructed hologram image and the luminance of the original object depends on the value and sign of the gamma of the photographic processes. To check the theory, several holograms of different exposures were superimposed on Kodak 649-F plates that were pre-flashed with a uniform illuminance so the H-D curve of the photographic process is straight. The calculated and measured luminance ratios of the different reconstructions agree within experimental error. Since the gamma of the usual photographic process is positive, instead of negative, even if the gamma of the photographic process is equal to 2, it is not valid to assume that the relative luminance of the reconstruction of a given superimposed hologram is proportional to the product of the exposure due to the object beam and the exposure due to the reference beam used in the making of the hologram. This assumption would be valid for all fringe contrasts only if gamma were -2.

 

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