Effect of spherical aberration in trefoil phase plates on color wavefront coding

Wavefront coding is a technique that makes use of aspheric phase masks and image processing to greatly increase the depth of focus and/or reduce the cost of imaging systems. The phase mask adds a controlled amount of aberrations to the optical system in such a way that invariant imaging properties are achieved over a wide range of defocus. When aspheric phase plates are fabricated, their shape has to be carefully designed to avoid introducing other aberrations into the optical system, which could reduce its performance. In this work, we will show how inherent spherical aberration for trefoil-shaped phase plates reduces the quality of color images. Results will be illustrated with numerical simulations. We will show that a fourth-order polynomial added to a trefoil shaped phase mask can avoid the plate generating undesired spherical aberration that will affect the final quality of the decoded color images.