There is considerable interest in detecting objects such as landmines shallowly buried in loose earth or sand. Various techniques involving microwave, acoustic, thermal and magnetic sensors have been used to detect such objects. Acoustic and microwave sensors have shown promise, especially if used together. In most cases, the sensor package is scanned over an area to eventually build up an image or map of anomalies. We are proposing an alternate, acoustic method that directly provides an image of acoustic waves in sand or soil, and their interaction with buried objects. The INEEL Laser Ultrasonic Camera utilizes dynamic holography within photorefractive recording materials. This permits one to image and demodulate acoustic waves on surfaces in real time, without scanning. A video image is produced where intensity is directly and linearly proportional to surface motion. Both specular and diffusely reflecting surfaces can be accommodated and surface motion as small as 0.1 nm can be quantitatively detected. This system was used to directly image acoustic surface waves in sand as well as in solid objects. Waves at frequencies of 16 kHz were generated using modified acoustic speakers. These waves were directed through sand toward partially buried objects. The sand container was not on a vibration isolation table, but sat on the lab floor. Interaction of wavefronts with buried objects showed reflection, diffraction and interference effects that could provide clues to location and characteristics of buried objects. Although results are preliminary, success in this effort suggests that this method could be applied to detection of buried landmines or other near-surface items such as pipes and tanks.
The INEEL has developed a photorefractive ultrasonic imaging technology that records both phase and amplitude of ultrasonic waves on the surface of solids. Phase locked dynamic holography provides full field images of these waves scattered from subsurface defects in solids, and these data are compared with theoretical predictions. Laser light reflected by a vibrating surface is imaged into a photorefractive material where it is mixed in a heterodyne technique with a reference wave. This demodulates the data and provides an image of the ultrasonic waves in either 2 wave or 4 wave mixing mode. These data images are recorded at video frame rates and show phase locked traveling or resonant acoustic waves. This technique can be used over a broad range of ultrasonic frequencies. Acoustic frequencies from 2 kHz to 10 MHz have been imaged, and a point measuring (non-imaging) version of the system has measured picometer amplitudes at 1 Ghz.
A new approach to the measurement of ultrasonic vibrations has been developed at the INEEL. This system utilizes heterodyne interferometric detection in a photorefractive material to provide real time, full field images of ultrasonic vibration amplitude and phase without scanning. The INEEL Laser Ultrasonic Camera has linear response for ultrasonic displacement (xi) < (lambda) divided by 4(pi) , approximately 45 nm displacement at a laser wavelength of 532 nm. In addition, the system exhibits flat response and narrow band detection for a broad range of vibration frequencies. The system is very robust, and has the potential for operation even in noisy industrial settings. A description of the system, representative data and several potential applications will be presented.
Dynamic observation of impact phenomena is extremely important for designing automotive and armored structures. A dynamic moire interferometer has been developed at the INEL to image surface displacement fields generated by dynamic loading events. A pulsed ruby laser is used to provide sufficient intensity to allow the imaging of displacement fields in periods of 20 ns. An electro-optic Q-switching system has been designed using standard electronic and optical components to pulse the ruby laser at rates of up to 1 Mhz. In order to capture images, a Cordin high speed framing camera was integrated into the system. The dynamic moire interferometer produces fringe fields of in-plane displacement data, similar to the fringe data captured in dynamic holography or dynamic photoelasticity. Dynamic loading events generate moving fringe fields which reduce the fidelity of acquired fringe fields. A dynamic fringe analysis similar to Neuman's analysis for dynamic holography and Dally's analysis for dynamic photoelasticity has been performed to quantify this effect for a moire interferometer. It has been determined that the contrast of the fringes depends not only on the pulse duration of the laser and transience of the fringe field, but also on the frequency of the fringe field. This analysis has been used to interpret fringe data obtained from short duration stress pulses traveling through laminated carbon-fiber epoxy composites.
We present methods for determining the phase shifts in multiphase fringe analysis from the fringe image data itself, thus eliminating the requirement for prior knowledge or accurate control of the phase shifts. We also discuss methods for calculating the folded (wrapped) and unfolded phases, and quote the accuracy of the fringe analysis method for an example in which the correct result is known.
Diffraction moire interferometry involves the replication of a diffraction grating onto a specimen and the measurement of subsequent distortions of the grating through optical interferometry. The method is elegant, simple, easily applied and yields high quality data. Many devices have been developed to exploit this method, often for very specialized applications. There have also been several attempts to produce general purpose research instruments some of which have been offered commercially.
This paper will attempt to review the historical development of diffraction moire instrumentation, the current state of the art and future directions. A companion paper in this volume (SPIE CR-46) by JS Epstein will give a more complete review of specific uses of the subject.
In an effort to reduce the hardware requirements for phase shifting interferometry, we have developed two approaches to estimating the amount of individual phase shift between interferograms. This is intended to make possible phase shifting interferometry with unknown and varying phase shifts. This will reduce the cost of phase shifters by eliminating the need for precision calibrated devices. The first method of estimating the phase shift involves beam shuttering and requires a special hardware modification of the interferometer, while the second is a purely software implementation. Both methods are relatively inexpensive to install and use.
Diffraction moiré interferometry is widely used in the study of material deformation and usually is implemented by building an apparatus for each experiment. This new device is the latest in a series of diffraction moiré interferometers developed at the INEL to simplify the process of acquiring diffraction moiré data. The unit is hand held, weighs about one kilogram, and is designed to be placed in intimate contact with the specimen and diffraction grating.
A series of phase shifted interferograms is automatically acquired and processed to produce quantitative data on deformation. This paper describes the design details and provides examples of typical applications and data.
This report describes a simple automated system for measuring interfacial tension
using the pendant or sessile drop method. The size and shape of a transparent or
opaque drop of one fluid immersed in a second, transparent, fluid is recorded with a
CCD video camera and digitized and stored by a computer-controlled system. Custom
software determines various droplet shape factors and computes the interfacial
tension. A limited number of video frames can be stored on disc, or longer runs can
be stored on video tape for later digitization. Alternately, only the shape factor
and interfacial tension data are stored to reduce demands on the storage medium.
The first application of the system was measurement of the interfacial tension of
crude oil interacting with various bacterial agents in aqueous suspension. Some of
these agents can greatly influence the effective interfacial tension of the crude oil
and potentially improve recovery rates from oil reserves, particularly of the
"heavier" or more viscous oils.