Abstract

High-quality imaging is typically dependent upon well-focused optical systems; however, precise focusing can be difficult due to the non-linearity of defocus. This paper presents further development of the quad target method (QTM), an alignment technique to linearize defocus for structured light illumination (SLI) systems, by creating a method to simulate the alignment sensitivity. QTM’s sensitivity is the slope of the regression that linearizes defocus and is dependent upon the step height of the quad target and the frequency of the projected fringes. The presented simulation method utilizes the native optical design to faithfully model the residual aberrations and is able to predict the best focus location as a well as the linearized slope. The predicted slope and best focus were compared to experiment using a commercially-available SLI system. For a step height of 500 μm and projected fringe frequency of 6.8 cy/mm, the simulation method predicted the slope to within 5.4% and best focus to 17.7 μm of the measured values.

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