This paper describes a varifocal system having constant magnification that consists of a telecentric lens and a varifocal spherical mirror. The feasibility of using this system was made clear through a computational evaluation of the imaging characteristics and by experiments. The evaluated resolution and TV distortion were 100 lp/mm and 0.04 percent at a magnification of 2 in a visual field of 3.3 mm × 4.4 mm, respectively. Basic experiments using concave-mirrors having different curvature-radii showed similar results. A focal shift of 3.42 mm was obtained with a concave mirror having a 600-mm curvature radius without a change of magnification. Further, a prototype system using a pneumatically controlled varifocal mirror successfully demonstrated constant-magnification focusing. The varifocal mirror was a circular 320-μm-thick glass plate, the optical aperture of which was 6 mm in diameter. Its curvature was controlled pneumatically to shift the focal point of the varifocal system. The focal shift reached 1.1 mm without image impairment occurring. Focus measure analysis produced a 3-D focused image of a screw thread 2 mm in diameter from 50 images taken at 20-μm depth intervals, and a contour map of the screw thread is also presented.

Date Published: 18 October 2002
PDF: 8 pages
Proc. SPIE 4902, Optomechatronic Systems III, (18 October 2002); doi: 10.1117/12.467395

Published in SPIE Proceedings Vol. 4902:
Optomechatronic Systems III
Toru Yoshizawa, Editor(s)