An imaging system for imaging an internal cavity, comprising: an optical element, comprising a single optical body having axial extension and radial extension, configured to receive an input beam-front which propagates in the axial direction, and to deflect said input beam front into a series of axially spaced annular projections of light, and a plurality of beam-deflecting facets (32, 70, 72), axially and radially separated with respect to one another, with non-beam-deflecting spaces in between, the facets radially separated such that deflecting surfaces of different facets are radially non-overlapping, and wherein the beam-deflecting facets are arranged along the optical body and configured to intercept and deflect different annular portions of the input beam-front outwards from the optical body, in a radial direction, to thereby generate from the optical element a series of axially separated annular projections of light, each beam-deflecting facet arranged to intercept a single annular ring portion of the input beam front only; and an image collection means for collecting light reflected from one or more surfaces of the internal cavity. an optical element, comprising a single optical body having axial extension and radial extension, configured to receive an input beam-front which propagates in the axial direction, and to deflect said input beam front into a series of axially spaced annular projections of light, and a plurality of beam-deflecting facets (32, 70, 72), axially and radially separated with respect to one another, with non-beam-deflecting spaces in between, the facets radially separated such that deflecting surfaces of different facets are radially non-overlapping, and wherein the beam-deflecting facets are arranged along the optical body and configured to intercept and deflect different annular portions of the input beam-front outwards from the optical body, in a radial direction, to thereby generate from the optical element a series of axially separated annular projections of light, each beam-deflecting facet arranged to intercept a single annular ring portion of the input beam front only; and a single optical body having axial extension and radial extension, configured to receive an input beam-front which propagates in the axial direction, and to deflect said input beam front into a series of axially spaced annular projections of light, and a plurality of beam-deflecting facets (32, 70, 72), axially and radially separated with respect to one another, with non-beam-deflecting spaces in between, the facets radially separated such that deflecting surfaces of different facets are radially non-overlapping, and wherein the beam-deflecting facets are arranged along the optical body and configured to intercept and deflect different annular portions of the input beam-front outwards from the optical body, in a radial direction, to thereby generate from the optical element a series of axially separated annular projections of light, each beam-deflecting facet arranged to intercept a single annular ring portion of the input beam front only; and an image collection means for collecting light reflected from one or more surfaces of the internal cavity.