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Paint-Measuring Laser Gives Good Vibrations

Photonics Spectra
Sep 2000
Daniel C. McCarthy, Senior Editor/Special Projects

As old-time auto mechanics are wont to complain, technology beneath the hood is becoming more and more advanced. But techniques for improving the exterior of cars are also moving forward. The complex process involved in applying an even coat of paint over an uneven surface entails hundreds of variables. If the paint is applied unevenly, surface defects can occur early, leading to the need for premature bodywork. Current methods of evaluating a vehicle's coating require contact with the painted surface and must be performed off-line after the paint has been cured.

If an in-line method could make a dense enough number of measurements to map the wet paint thickness over the surface of the car, it could cut evaluation times by a factor of 10, observed Jeff White, a senior research scientist for Perceptron Inc. White and other engineers at the company have developed a nondestructive in-line approach based largely on an Nd:YAG laser from Lightwave Electronics Corp. The system is in beta testing at one of Detroit's Big Three automakers and may be commercialized this year.

An unevenly applied coat of automotive paint could lead to the need for premature bodywork. One company found an in-line, noncontact solution for evaluating coating thickness by combining laser technology with an interferometer. The system measures the thickness of a paint layer by determining its ultrasonic ring. Courtesy of Perceptron Inc.

Perceptron's noncontact system includes three vital components: a generation laser, a detection laser and a Fabry-Perot interferometer. Lightwave's continuous-wave laser serves as the detection laser, which is trained on the painted surface.

The generation laser -- a Q-switched Nd:YAG -- emits a narrow pulse to produce an ultrasonic wave in the paint layer. As the generation pulse hits the coating, it creates vibrations in the paint surface that cause a Doppler shift in the detection laser line as the light reflects off the paint. The interferometer measures the frequency of this shift to determine the thickness of the coating: The thicker the coating, the lower the ultrasonic vibration. A crucial component is the detection laser. "The critical parameters for a laser in this application are very narrow linewidth and stability," said White. Lightwave's laser has a linewidth on the order of 5 kHz over 1 ms. "The more narrow and stable that central frequency is at higher powers, the higher sensitivity the measurement will be. If Lightwave's laser wasn't so stable, it would be very difficult to keep the interferometer centered on that wavelength," he added.

The laser's stability enables users to center the interferometer on its frequency and to leave it unattended for weeks -- a feature that is useful in automotive or any industrial applications.

In Perceptron's configuration, the Lightwave device's output is in the 700-mW range, but White said that he would prefer it even higher -- somewhere between 3 and 5 W, so he "could be more cavalier about aligning optics."

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