Dual-lens system in enlarging the viewing angle for the Michelson interferometer

Shearography has been widely accepted as a non-destructive evaluation tool in engineering. However, the field of view is usually limited due to the use of the Michelson optical arrangement if the working distance is strictly constrained. In order to improve efficiency, we propose a dual-lens system to expand the view angle of the Michelson optical arrangement. Two imaging lenses, which are entitled as the objective lens and camera lens, are placed in front of and behind the Michelson interferometer, respectively. The optical arrangement has been set up and compared with the well-known 4f optical system. It is found that the proposed optical arrangement is simple to construct, and the brightness distribution is uniform over the whole image. The new setup has been applied to the inspection of a cylindrical steel shell with a rubber plate bonding on its inside surface. The debonding defects have been evaluated using the proposed shearography system at a working distance of 200 mm. By selecting an objective lens with a focal length of 2.8 mm, the viewing angle was enlarged in inspecting a field with 500 mm by 500 mm at one shot.