Compact Telecentric Objectives: A systematic exploration of 'another' PNP triplet

Cooke Triplet and its variants with a PNP power distribution and symmetry about the central stop, is a well-studied photographic objective. This paper will systematically present another special triplet form with low numerical aperture and a telecentric image, used in applications such as epifluorescence microscopy, projection optics, and semiconductor inspection applications. When used with an externally accessible stop as typical in this application area, the basic form combines a Telephoto lens (PN) with a telecentricity enforcing field lens of positive power leading to PNP power distribution. The individual groups get more complex with demanding mechanical packaging specifications. Typical designs in the visible spectrum and their limiting aberrations are presented with a design evolution approach.