Quantitative real-time pulsed Schlieren imaging of ultrasonic waves

Abstract

A pulsed Schlieren system based on Raman-Nath scattering by ultrasonic waves in water is described. High powered, monochromatic infrared pulsed light is used in conjunction with axial optics and a video camera to visualize the acoustic field. Projection beam profiles in any plane parallel or orthogonal to the direction of acoustic propagation are demonstrated. Controlled delay between acoustic pulse and light flash allows scrutinizing wave fronts at variable delay after launching. The method is applicable with reasonable repetition rates to broadband acoustic pulses with intensities in the medical diagnostic range. Quantification of the pressure density by analysis of the video signal is shown using a single-point calibration against a total acoustic power measurement by the force balance method, without a priori assumptions about the beam structure.

Optical measurement of ultrasonic Poynting and velocity vector fields

“…A discussion of these issues is found in [32]. The expression in (10) gives an intensity that is related to phase delay and, thus, integrated refractive index perturbation by a simple affine transformation. The pitch of the CCD camera and magnification of the lens system used in this report provide a system resolution of approximately 70 µm in all three spatial directions [21], [24].…”

“…James F. Greenleaf (M'73-SM'84-F'88) was born in Salt Lake City, UT on February 10,1942 (1990/1991). His special field of interest is ultrasonic biomedical science, and he has published more than 297 articles and edited or authored five books in the field.…”

A Simple Technique for Visualizing Ultrasound Fields Without Schlieren Optics

“…The first point is not a fundamental difference because the stroboscopic Schlieren technique (Azuma et al 2002;Charlebois and Pelton, 1995;Hanafy and Zanelli 1991;Ermert 2006a, 2006b;Schneider and Shung 1996;Weight and Hayman 1978) enables visualization of alternating pressure changes by using short light pulses synchronized with an ultrasound phase. On the other hand, the stroboscopic Schlieren technique does not improve visibility of a short-pulse ultrasound field because the short burst length blurs its diffraction pattern and makes the spatial filtering difficult.…”

“…After the discovery of light diffraction by ultrasound (Debye and Sears 1932;Lucas and Biquard 1932), visualization of ultrasound fields generated in water became an important application of the Schlieren technique (Azuma et al 2002;Charlebois and Pelton, 1995;Hanafy 1979Hanafy , 1980Hanafy and Zanelli 1991;Neighbors et al 1995;Ermert 2006a, 2006b;Schneider and Shung 1996;Settles 2001;Weight and Hayman 1978). Exposure to continuous or long-burst ultrasound generates a cyclic change in the refractive index in a transparent medium, and the field works as an optical phase grating.…”

Electromagnetic tomographic ultrasonic sensor

“…Conversely, fiber-optic hydrophones are more robust and resist well to cavitation, but have a lower sensitivity [2]. Schlieren imaging can also be used to map ultrasound pressure field, but the measurements are hardly quantitative [3].…”