Linearization of defocus for projector optical alignment in structured light illumination systems

Abstract

High-quality imaging is typically dependent upon well-focused optical systems. We present a method that linearizes defocus to provide a clear zero-crossing along the optical axis for structured light illumination (SLI) systems. Here, sinusoidal fringes are projected onto a quad target, a diffuse target with different heights along the optical axis in separate quadrants. Shifting the focus alters the fringe contrasts on the quad target. Best focus is achieved when the contrast between the different heights is balanced. The method was tested using a commercial SLI system and compared to a published phase-shifted reconstruction technique. The non-linear defocus behavior was efficiently linearized using the quad target method as evidenced by linear regressions. Finally, focal position estimates agreed well with the phaseshifted technique, thereby demonstrating viability of the method.