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September 27, 2018
Using Grazing Incidence Interferometry to Measure Large or Non-Specular Surfaces
Grazing Incidence Interferometry Test Setup Diagram
Grazing Incidence Interferometry is a relatively simple technique that is useful for two unique yet unrelated applications:
• Measuring plano surfaces that are larger than the measurement aperture of the interferometer.
• Measuring non-specular plano surfaces such as unfinished metal or ceramic.
Measuring Larger Surfaces
Grazing Incidence Interferometry is useful for expanding the effective aperture of the interferometer, in one axis.
Relative Measurement Areas Using Normal Incidence (left) and Grazing Incidence (right)
Grazing Incidence Test Setup Measuring a Large Optical Window
When a laser interferometer's round measurement beam reflects off a test surface at normal incidence, the measurement area is approximately the same size as the beam. However, when the beam is reflected off a test surface at a lower angle, the measurement area becomes elliptical, thus enabling measurement of surfaces that are wider (or taller, depending on orientation) than the interferometer's aperture. The lower the angle, the larger the measurement area becomes.
Measuring Non-Specular Surfaces
Performing interferometric measurements on surfaces that have low reflectivity can be difficult because the interferometer's measurement beam is absorbed or scattered by the surface instead of being reflected back into the interferometer.
It may still be possible to measure such surfaces using Grazing Incidence Interferometry. With this technique, the measurement beam is reflected off the test surface at a low angle and then reflected back to the interferometer by a high-reflectivity reference flat. Due to the low angle of incidence, the beam is reflected off the test surface instead of being absorbed or scattered by it. The physics of how a surface's reflectivity is related to the angle of incidence is beyond the scope of this blog, but can be thought of as somewhat analogous to the way a stone that is thrown at a low angle will bounce off the surface of a lake instead of immediately splashing into it.
Optimizing the Grazing Incidence Angle
The optimum angle for a grazing incidence setup is to have the test surface as close as possible to normal incidence, but will depend on the reflectivity of the surface and the desired size of the measurement area. Getting as close as possible to normal incidence minimizes: the foreshortening of the measurement image, the loss of pixel resolution due to the angle, and the sensitivity of the interferometric scale factor setting. Please contact ZYGO for more information about Grazing Incidence Interferometry.
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