Two iris imaging over an extended depth of field with a mobile phone camera

Increasing the depth of field of a lens is one approach of relieving computational and mechanical refocussing mechanisms of lenses. In this paper we introduce a three element lens, consisting of an achromatic doublet, a singlet lens and a variable thickness element in the form of a refractive plate. The plate element provides an increase in depth of field comparable to 2.8 times that of a conventional lens. The lateral sag of the plate alters the optical path length for each bundle of rays with respect to the object distance so that all objects are imaged on to a fixed position detector. The surface is described by a combination of Zernike terms along the sagittal plane. Imaging is carried out with NIR narrow-band illumination where the fine details of the iris can be sharply captured in the image.