<title>Telephoto attachments for telecentric lenses</title>

Abstract

Many imaging applications, such as machine vision, parts inspection, and gauging, make us of telecentric lens configurations to improve measurement accuracy and reduce alignment sensitivity. CCD cameras are frequently chosen to record the final image. Although lenses can be designed to work as standalone telecentric devices, it is sometimes desireable to use a modular approach, in which a telecentric `attachment' is mated to a primary or `base' lens. This allows one to vary the object field coverage by changing attachment lenses, keeping the same primary lens. A simple attachment could consist of an achromat mounted one focal length away from the entrance pupil of the primary lens. Such arrangements can become lengthy for large fields and magnifications. Stock achromats may not meet size requirements, nor provide optimum image quality. A more complex arrangement that reduces the lens length and improves imaging can be obtained by using a telephoto lens (front-vertex-to-image/efl < 1.0) as an attachment. This paper will outline the development of a modular telecentric system and a first-order method for evaluating the suitability of lenses for use as telecentric attachments.