APPLICATION NOTE Interferometer Scale Factor (ISF) This application note describes the function of the scale factor and what values are applicable for a given interferometer setup. Interferometer Scale Factor (ISF) is also known as scale factor or wedge factor. that is being directly measured via the fringe pattern. They are different by a factor of 0.5. But what happens when a perfect plane wave is incident on a test surface which has a defect 1 unit deep? Interferometers function by dividing a wavefront into two or more parts, principally a reference wavefront and a test wavefront, which travel different paths and then combine to form an interference fringe pattern. The geometrical properties of the interference fringe pattern are determined by the difference in optical path traveled by the recombining wavefronts. All the interferometer receives as input information is the test wavefront. There is no indication as to whether this wavefront has traveled through a window, reflected from a surface, or at what angle it has reflected from a surface, etc. The ISF specifies how this input wavefront error (read directly from the fringe pattern as one wavelength per fringe) is scaled to properly represent the output parameters which the user wants to display in the results. The wavefront hits the flat portion of the surface first; while part of the wavefront continues to travel into the defect, the rest of the wave has already reflected off of the good portion of the surface and is traveling back toward the interferometer. By the same time the center of the wave has reached the deepest part of the defect, the majority of the wave has traveled 1 unit away from the flat portion of the surface. As a result, the returning wavefront has a defect that is 2 units deep even though the surface defect is only 1 unit deep. For this test geometry, the correct setting for ISF is 0.5. Consider the specific example of a wavefront reflecting off of a surface. This is exactly what happens in the normal incidence setup (when the laser beam is perpendicular to the surface under test), double pass testing of a flat surface. In this setup, users most often want to see the results of the surface errors, not the refle

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