Interferometry, or 3-D profilometry, is a noncontact method that uses light-typically laser light-to measure topography, with the ability to provide accuracy of ±3 nanometers in the Z-axis, and XY resolution as fine as 500 nanometers.
Interferometry is versatile and more efficient than point-to-point surface profiling techniques. Quality applications for interferometry include everything from the aerospace and medical device industries to factory floor applications in the automotive industry.
Two notable forms of interferometry used for industrial applications today are confocal microscopy and holographic interferometry. Confocal microscopy is a technique that increases micrograph contrast and also can reconstruct 3-D images. A confocal microscope eliminates out-of-focus information through the use of point illumination and a pinhole in an optically conjugate plane in front of a detector. Only the light within the focal plane can be detected, which results in high image quality.
This technique currently is used in the scientific and industrial communities. Typical applications are found in the semiconductor, optics, electronics, plastics and paper industries.
Holographic interferometry combines traditional holography with interferometry, allowing detection of material defects and inevitable fatigue failure, as well as the measurement of residual stress and vibration mode analysis. Modern holographic metrology equipment is more precise than conventional interferometry techniques because it overcomes the ambiguity interval, a problem inherent to conventional forms of interferometry. With a conventional interferometer, the range of measurement can be only one wavelength of light because it relies on the difference between the phases of the signals to compute distance. Therefore, if two points on a surface differ by exactly one wavelength of light, it cannot be determined whether they are the same height.
Holographic interferometry is used by metrology laboratories to analyze parts for product development, quality control and warranty studies. It also is used to inspect equipment during the launch of new production lines, as well as sample parts near a production line.