Structured light illumination (SLI) systems can measure a target’s two-dimensional height profile. High-quality imaging in SLI systems is typically dependent upon well-focused optical systems that can be difficult to achieve due to the nonlinear nature of defocus. We present the quad target method (QTM), a simple and fast alignment technique that linearizes defocus for SLI projectors. This linear response provides a clear zero crossing along the optical axis to minimize any focus ambiguity and maximize projector focus quality. QTM creates the linearization by sampling the field at two locations along the optical axis using a quad target, which is a diffuse target with two different heights in opposing quadrants. To maximize feedback speed, QTM can estimate the current focus position using only a single image by measuring the contrast of projected sinusoidal fringes on the quad target. QTM was tested using a commercial SLI system and compared with a published phase shifting reconstruction technique. The nonlinear defocus behavior was efficiently linearized as evidenced by focus estimation accuracies over a broad range of QTM variables.