THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

496 17. Commercial and Medical Ultrasound Applicationsof the optics must be precise to ensure maximum sensitivity and uniform field.Through careful alignment and focusing, the field can be made to go from lightto gray to dark uniformly by adjusting the second knife edge further into the lightbeam until it intercepts it completely. In the range of gray settings, convectioncurrents both in air and water become clearly visible. The ultrasonic field is bestrendered visible when the second knife edge is set to intercept all of the main beam,thus allowing only light refracted in the ultrasonic field to pass and illuminate theviewing plane. The optical system may also be adjusted for bright-field operation,in which situation the ultrasonic perturbations appear as dark shadows.Color schlieren photography is useful in the study of ultrasonic waves and shockwaves. Color can indicate the various pressure levels in an ultrasonic field. Onemethod of color schlieren photography uses a spectroscopic prism between a slitlocated at the position of the first knife in Figure 17.9 and the first collimating lens.This method produces colors ranging from red or from blue to green. Anothermethod developed by Waddell and Waddell (1970) produces a complete colorspectrum. They eliminated the spectroscopic prism and used a color-filter matrixin place of the second knife edge and a vertical slit in place of the first knife edge.Illumination was provided by a high-pressure mercury arc lamp. The color matrixconsisted of three filters, which represent the primary colors.Liquid Crystal ImagingLiquid crystals exhibit properties of solid crystals that are not apparent in ordinaryliquids. When a stress is placed upon a liquid crystal, its optical properties change.A certain class of liquid crystals known as nematic crystals is used to indicate thepresence of an ultrasonic field with sensitivity equal to that of schlieren systems,and with high resolution, large-area capability, and handling ease.Ultrasonic HolographyHolography is a form of three-dimensional imaging that was conceived and developedby Dennis Gabor who received the Nobel Prize in physics for his efforts.Gabor applied his discovery to electron microscopy to overcome the problem ofcorrecting spherical aberration of electronic lenses. The principle of holography isas follows: a diffraction diagram of an object is taken with coherent illuminationand a coherent background is added to the diffracted wave. A photograph so takenwill contain the full information on the changes sustained by the illuminating wavein traversing the object. The object can be reconstructed from this diagram by removingthe object and illuminating the photograph by the coherent backgroundalone. The wave emerging from the photograph will contain a reconstruction ofthe original wave, which seems to issue from the object. A hologram, therefore, isa recording or a photograph of two or more coherent waves. If one recorded waveis from an illuminated object and another is a reference wave, simply illuminatingthe hologram with the reference wave reconstructs twin images of the originalobject, thus giving the illusion of three dimensions.