Shining Light On Sound

December 17, 1998

INEEL-developed imaging system uses lasers to reveal patterns of ultrasonic vibrations

A Laser Ultrasonic Camera developed by researchers at the Idaho National Engineering and Environmental Laboratory transforms sound waves into an image. The system produces a dynamic holographic image, in real time, of ultrasonic vibrations as they occur on the surface of an object. The pattern of vibrations, which is visible to the naked camera and eye, can reveal an object's thickness, stiffness and imperfections.

The camera, developed by a team of INEEL scientists led by Ken Telschow and Vance Deason, can replace current ultrasonic imaging methods. Most state-of-the-art systems today must scan an object, performing a point-by-point analysis; those that directly image do not have the resolution of the INEEL system. The scanning process is slow and inherently more difficult to implement than the image created by the INEEL Laser Ultrasonic Camera - a real-time, dynamic image that shows waves moving along the surface of the entire object at one time.

In order to watch as nanometer-high ultrasonic waves ripple along an object, the image of the waves has to be slowed to a visible speed. "We play tricks to slow it down," says Deason.

First, the source laser beam is split in two: one beam is sent through a phase modulator to become the "reference beam" and the second shines on a sample object, through which ultrasonic vibrations are moving. The beams bounce off mirrors that direct them to meet again inside a holographic crystal. There, the laser beams interact and the modulation differences between them produce the "slowed down" moving image. The technique works somewhat like a strobe light: rather than showing a continuous stream of ultrasonic waves (moving at about 1 to 5 kilometers per second), the laser creates a dynamic holographic image out of samples of the ultrasonic wave pattern.

For example, the first beam, which reflects off of the sample, is modulated at 100 KHz frequency by a 100 KHz ultrasonic wave. The reference beam may be separately modulated to add another 25 Hz to the 100 KHz baseline frequency for a total modulation of 1.000025 MHz. When the two beams meet in the holographic crystal, the 100 KHz frequencies cancel out and the remaining image appears with a 25 Hz frequency - slow enough for the eye and the camera to register.

A simple video camera can record the image in the crystal, and the viewer can watch as waves move along the surface of the object, just as one can watch waves traveling across a pond from where a stone has been skipped.

The pattern of waves depends on the microstructure of the object. For example, the fibers in a sheet of composite material may be aligned slightly more in one direction than another due to the fabrication process. The waves travel more quickly along the fibers, producing an image appearing as an oblong pattern, stretched in the direction of the fibers. This can be used to measure the stiffness of the composite in different directions.

In addition to showing how waves travel through sheet-like objects, the INEEL Laser Ultrasonic Camera can also reveal vibrational modes of complex objects. For example, it can show directly the standing wave patterns that linger when a drum head is hit.

The Laser Ultrasonic Camera is in early stages of development, but eventually it may be used for many evaluation and quality-control purposes. The noncontacting, nondestructive system could be used for diagnostics of large and small objects. For example, in principle, the structural integrity of a bridge or building can be imaged in the same manner as that of a ball bearing. The noncontacting nature of the INEEL Laser Ultrasonic Camera can also be used to monitor production processes in adverse environments, such as at the high temperatures found in the sintering of ceramics and the solidification of molten metals.
Note to Editors: Ken Telschow can be reached by phone at 208-526-1264 or e-mail at Vance Deason can be reached by phone at 208-526-2501 or e-mail at Photos of the laser ultrasonic camera, copies of scientific papers, and pdf versions of data graphs are available; contact Laura Helmuth (above).

DOE/Idaho National Laboratory

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