Fig. 2
From: Ring-shaped bifocal lens used for fluorescent self-referenced holographic imaging
a Self-interference pattern generation by the RBL from a collimated beam. RBL creates a hollow and a central beam from a single incident beam. IP denotes the area of the intersection of the two generated beams. b RBL implementations using different focal length lens. The dark green areas are the overlapping parts of the different beams. The shape of the outer aperture of the RBL will determine the shape of the hologram. Ring aperture creates ring-shaped hologram