Light measurement	Laser-induced planar fluoresence

Nancy Piltch and Carolyn Mercer work in the Center's Optical Measurements Systems Branch. They and their colleagues are researching methods for using light as a powerful and precise tool for measuring the engineering properties of NASA hardware. "All our measurements take advantage of the properties of light or involve the interaction of light with a physical object," says Piltch. Their methods typically include various types of laser spectroscopy and interferometry.

Piltch's experiments focus on using light to discover the properties of arc jet thrusters, small engines used to stabilize the movement of satellites in orbit. The gas discharge from an arc jet thruster can be hotter than a typical acetylene welding torch. "You can't exactly put a thermometer or any other kind of probe in there," says Piltch, "It would melt." She views the properties of the gas by measuring the amount of laser light absorbed and reemitted by atoms and molecules in the exhaust gas.

The process is called laser-induced planar fluorescence. Piltch captures the fluorescence image data with a photodiode detector attached to a sold-state video camera. Data from the camera are loaded into Visual Numerics' PV-WAVE^® visual data analysis (VDA) software, where the resulting image is processed interactively. "Before we had PV-WAVE, I used to try to make sense of the data by taking black-and-white photographs of the video screen," says Piltch. "Now we have numerical data that we can process in simple ways, and the data just jump out at us. We don't have to spend half our time convincing ourselves that something is really there."

Piltch uses the data to determine various parameters of the exhaust plume -- temperature, types of particles present, how the thrusters' chemical energy is converted to other kinds of energy and the extent and pattern of ionization. For example, Piltch's analysis reveals that design changes can maximize the conversion of fuel into kinetic energy or minimize the number of ionized species present that would otherwise damage satellite electronic systems.

Mercer's experiments also provide important data to NASA engineers. She uses a process called phase-stepping interferometry to probe the surface and subsurface of physical objects. The process is based on the interference pattern produced when two coherent laser light beams are recombined after traveling slightly different paths.

The recombined light produces an interference fringe pattern that is used to illuminate the object under study. By collecting the light reflected off the object with a video camera, Mercer records an intensity image. Next, she steps the relative phase difference between the two beams by 90 degrees, taking advantage of a novel feedback loop for very accurate stepping. If four such images are recorded, with relative phase differences of 0, 90, 180, and 270 degrees, then the images are related by a simple mathematical formula, which is used to calculate the phase of interference pattern at each point on the object.

The phase map can be displayed on a computer either as an image, a contour map or a surface. According to Mercer, "The interactive nature of the PV-WAVE software makes it easy to work with these maps. You can display them as a surface or as an image or as both at the same time -- and PV-WAVE is fast! My entire calculation takes less than a minute, and we're talking about almost 64 kilobytes of data in each of the four images." No matter how it is displayed, the phase map is exquisitely sensitive to slight differences in the shape of an object. Mercer has used phase-stepping interferometer to look for cracks in the space shuttle main engine turbine blades and has been able to find and measure minuscule fissures with this method.

This application can also reveal subsurface cracks and other material defects. By subtracting the phase map of an unstressed object from the phase map of the same mechanically stressed object, the resulting phase map, Mercer says, "shows a sharp discontinuity that dramatically displays the defect." The same procedure can be used to reveal how an object deforms as it is heated or is stressed in some other way. "The beauty of phase-stepping interferometry is that it can be used to look at something as small as a micron-sized crack or as large as an automobile."

Both scientists stress the importance of interacting directly with their data in the visualization and analysis steps of their process. According to Mercer, "I wrote 75 to 100 lines of code to do the same thing I did in PV-WAVE in 17 lines. It was much easier to use PV-WAVE."