Precision positioning and measurement require an accurate “ruler” and an equally accurate method to read the ruler. Until recently, the state-of-the-art for positioning and metrology with nanometer-level accuracy over relatively long distances has been the displacement measuring interferometer (DMI). An emerging alternative to DMIs is the precision heterodyne optical encoder system in which a two-dimensional (2D) grating, sometimes called a “grid plate,” replaces DMI stage mirrors as the reference reflection, thus enabling the interferometric sensor to remain in close proximity to the reference.
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