By B. P. Hildebrand
Since the 1st papers by way of E. N. Leith and J. Upatnieks with regards to holography seemed in 1961, there was a digital explosion of study task within the box. greater than SOO papers and articles on holo graphy have seemed within the final ten years. Many functions of holo graphy were proposed, and a few of those are commencing to input the area of usefulness. one of many purposes that looks to carry nice promise is acoustic imaging by way of holography. the 1st papers in this topic seemed in 1966. yet already study task within the box is burgeoning. Tbree symposia whoUy dedicated to acoustical holography were held and tbe papers released in ebook shape. the aim of this publication is to compile the result of examine in acoustical holography, a few of it as but unpublished, lower than one conceal in order that employees in holography, nondestructive trying out, scientific imaging, underwater imaging, and seismic exploration can make a decision even if this new approach will be beneficial to them.
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Additional resources for An Introduction to Acoustical Holography
Hence, with this kind of illumination the effects shown in Fig. 16 can certainly occur. In normal practice, however, the illumination source is not a perfect point but rather a disc of the order of 10 wavelengths in diameter, or it may be a large diffuse source which can be produced by placing a diffuser in the illuminating beam. In this case, the phase at each point on the object is a summation of IMAGE INTENSITY )(xl Fig. 16. This figure illustrates the pitfalls when we attempt to use the Rayleigh resolution criterion for coherent imaging systems.
The average phase is then of consequence and can gene rally be considered zero. Thus, it seems that the Rayleigh criterion is useful after all. Another useful measure one might like to have is aresolution along the radius to the object; that is, a depth resolution. This may be obtained by substituting the angular object position Eq. 55) into Eq. 84) '2 When we make the change of variable, we obtain the standard Fresnel integral shown below: mA 4Q fQ 0 exp [_. n '/'2] '2'f' where Q = mA (~)t f: 2 C(Q) = n cos( TP 2) J,/.
The last two terms are repIicas of the scattered wave and its conjugate. Note that there is nothing to distinguish them except for the opposite sign on the phase term. Hence, they occupy the same volume of space as illustrated in Fig. 4. In addition, we have the first term which also propagates in the same direction. We can ignore the space variant portion a2 2 (x, y) of the first term since A2 ~ a22 • Thus, if we place a photographic plate at the position of the conjugate image we obtain a picture of the object added to a uniform background due to the true image wave propagated over the equivalent distance 2z1 .
An Introduction to Acoustical Holography by B. P. Hildebrand