Paul A. Reinholdtsen

Image processing for a scanning acoustic microscope that measures amplitude and phase

Several image-processing techniques for a low-frequency (3 to 10 MHz) scanning acoustic microscope (SAM) that measures amplitude and phase are described. This microscope is capable of measuring both the amplitude and phase of the reflected and transmitted signals, in contrast with most earlier implementations that only measure the amplitude. By measuring phase, the authors can carry out quantitative nondestructive evaluation (NDE) and image processing that cannot be done with amplitude or phase alone. The effective 2-D point spread function of the microscope is modified by spatial filtering of the digitized complex images. In various images, the transverse resolution is improved by about 20%, aberration of images of subsurface features is corrected, and surface features are numerically defocused. The last process is used to remove the obscuring effect of surface roughness from images of subsurface features.<<ETX>>

Near field scanning acoustic microscope and method

An acoustic microscope in which acoustic energy is focused onto a membrane which includes an aperture which is a fraction of the size of the focal spot of the acoustic beam at the membrane to form fringing fields on the other side of the membrane. Acoustic energy reflected from the membrane is detected. An object to be examined is placed in cooperative relationship with the fringing fields.

Ultrasonic excitation and detection of capillary waves for the measurement of surface film properties

A 10 MHz focused ultrasonic transducer is used to excite capillary waves by placing its focus at the air‐water interface and using a tone burst to excite the ultrasonic wave. The radiation pressure associated with the upward propagating ultrasonic pulse lifts the surface of the water which then relaxes by exciting a radially propagating capillary wave. An amplitude and phase measuring acoustic microscope operating at 10 MHz is used to detect the amplitude and slope of the capillary wave as it propagates over the focused transducer of the acoustic microscope. This arrangement allows us to make a noncontacting measurement of surface tension and surface viscosity which will be used for characterizing surface films, such as the marine microlayer.

Near-field scanning acoustic microscope

A traditional scanning acoustic microscope (SAM) has been modified to operate in the near-field mode. A pinhole in a thin shim of brass defines the resolution of the instrument, which can be as small as 0.1 lambda . In an edge scan experiment, a 125- mu m-thick brass shim with a pinhole size of 125 mu m, a SAM operating at 3 MHz, and a transducer with an F-number of 0.7 are used. The improvement in resolution corresponds to using a transducer with an F-number of 0.2. The results of measurements of the line response of the system, using steel pinholes of several thicknesses and diameters at different linewidths and operating frequencies and showing the details of the design of the instrument, are presented.<<ETX>>

Quantitative Acoustic Microscopy Using Amplitude and Phase Imaging

We have developed a fast technique for locating and determining the depth of delaminations in composite materials. We use an acoustic microscope that measures amplitude and phase to scan the m aterial. By using both amplitude and phase, we can unambiguously determine delamination depth. We can also perform other signal processing, such as removing the effects o f surface roughness, that is not possible with microscopes that measure amplitude or phase a1 one.

Amplitude and Phase Acoustic Microscopy and its Application to QNDE

We have two amplitude and phase measuring acoustic microscopes, one at low frequency (3-10 MHz) which is used for measurements in metals and composites, and the other operating at frequencies of up to 200 MHz which is used for higher resolution measurements. The added dimension of having phase information allows us to use image processing for a variety of applications. We have demonstrated the following applications with these two microscopes: V(z) inversion for reflectance function calculations, depth determination for delaminations in composite materials, slowness curve measurements in anisotropic materials, image enhancement of subsurface defects, measurements of depth of trenches in aluminum samples (scaled problem of silicon trenches), and measurement of visco-elastic properties of thin surfactant films on a water surface.

Amplitude and phase acoustic microscope using digital heterodyning

We have built a low‐frequency (3–10 MHz) scanning acoustic microscope (SAM) that measures amplitude and phase of rf tone bursts quickly and accurately. The amplitude has a 70‐dB dynamic range; the phase is measured to better than 0.1°. The electronics are capable of operating at greater than a 100‐kHz repetition rate. The signal is mixed with a reference that is shifted discretely in‐phase between tone bursts, the product is integrated and digitized, and the amplitude and phase are computed using at least three digitized values. This general technique operates at a rate limited by the digitizer.