Grazing Incident Technique to Evaluate Surface Flatness (Metrology)

23.14.
Laser-based system using grazing incidence interferometry to view part surface topography has been developed. It is a menu-driven, microprocessor based system which can analyse over 65,000 data points and computer flatness in less than 20 seconds and provide Total Indicated Reading (TIR), Local Slope (LS), and slice profiles. It measures the entire part surface at one time and produces two and three dimensional profiles with sub-micron accuracy and resolution. Test results can be viewed on the video screen and hardcopied on a colour printer. It correctly evaluates flatness by looking at the entire surface simultaneously and does not rely on several traces across a surface to determine indirectly overall flatness.
In this instrument, a laser beam inside the interferometer is expanded into 200 mm column of light. At the hypotenuse of the system’s precision prism, the light column is divided into two beams : the reference and test beam.
The test beam exits the prism and strikes the irregular surface of the part at a grazing angle. The test beam reflects off the part surface, and re-enters the prism to recombine with the plane reference beam. When these beams recombine, they interfere with one another to
create an interference pattern or a series of light and dark bands. The interference pattern represents the shape of the part of surface.
The measurement performance can be improved by striking the test beam on the part surface at a grazing angle. When normal incidence light strikes a rough surface, most or all energy is lost due to scatter. It would be appreciated that sunlight, when it strikes the water’s surface at sunset (in comparison to at high noon) at a grazing angle, the surface easily reflects the sun’s rays and appears to have mirrored finish. The gazing incidence design turns non-reflective, rough surfaces on ceramics, plastics, metals and composite into polished mirror surface to facilitate measurement.
The interference pattern is automatically interpreted with phase measuring inter-ferometric (PMI) system. PMI system can capture over two million data samples, interpret them and compiles the data into a three dimensional image of the test surface.
A high contrast video system captures images of the interference pattern and divides the test surface into 65,536 picture elements or pixels. The pixel map is superimposed over the interference pattern and an intensity profile for the pattern is recorded into the system microprocessor. The interference pattern is shifted under computer control. At each pixel the intensity variation, which appears as a sine wave, is recorded by the microprocessor and used to determine its relative phase. The phase differences throughout the data matrix are used to define the surface topography.