  Generalized beams of light that are made to turn inside out, creating black
nodal areas of total destructive interference are described. As an example we
experimentally created an elliptical node from a uniformly illuminated
elliptical aperture by use of a lossless phase mask called a q-plate. We
demonstrate how a modified phase retrieval algorithm may be used to design
phase masks that achieve this transformation for an arbitrary aperture shape
when analytical methods are not available. This generic wave phenomenon may
find uses in both optical and non-optical systems.
