In this paper, we present a general framework for analysis of plenoptic sampling by investigating the spectral analysis of plenoptic imaging. The proposed framework provides a unified representation that generalizes several existing methods for plenoptic sampling. The plenoptic sampling theory presented extends the study of plenoptic imaging to parallel cameras that are not restricted to lie on a plane corresponding to a camera array. Furthermore, we conduct an analysis of plenoptic sampling for unstructured camera systems, in which the cameras can reside at arbitrary locations and orientations. Finally, we establish necessary and sufficient conditions for unoccluded imaging, which is a critical precondition for plenoptic sampling. These conditions are shown to introduce constraints on the position and curvature of the surface of the scene as well as the position and orientation of the cameras.