Commercial two- and three-element infrared wide-field-of-view 100% coldshielded inverse telephoto lenses

Robert E. Aldrich

doi:10.1117/12.219520

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15 September 1995 Commercial two- and three-element infrared wide-field-of-view 100% coldshielded inverse telephoto lenses

Robert E. Aldrich

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Proceedings Volume 2540, Current Developments in Optical Design and Engineering V; (1995) https://doi.org/10.1117/12.219520
Event: SPIE's 1995 International Symposium on Optical Science, Engineering, and Instrumentation, 1995, San Diego, CA, United States

Abstract

Infrared 100% coldshielded systems have a fixed and inaccessible aperture stop location, with a long back focal length. Often, the back focal length of a wide field of view infrared lens is longer than its elementary focal length, like a wide angle 35 mm SLR lens. The long back focal length prevents the use of a field flattener lens near the image in order to correct for field curvature and astigmatism. A field lens is usually a key ingredient in wide field of view lenses. Some have designed long and complicated reimagers to create a second image plane for a virtual field flattener lens. An inverse telephoto lens has a long back focal length and allows for an inaccessible aperture stop. The inverse telephoto lens has at least two groups, a negative group and a positive group. The negative group increases the marginal ray height to create a longer back focal length. The positive group brings the marginal ray to focus. Using a single high index and low dispersion aspheric positive element will correct for spherical aberration and reduce axial color and the Petzval sum. A single low index and high dispersion aspheric negative element will correct for coma, axial color and Petzval sum. A low Petzval sum minimizes field curvature and astigmatism. A wide angle 010% coldshielded inverse telephoto lens can be designed with just two elements. Occasionaly in wider field of view systems or larger image plane formats a second negative aspheric is necessary to correct for distortion. Thus creating a third element. Also a diffractive element is sometimes needed to correct lateral color. The diffractive can be added to one of the single point diamond turned aspheres without adding a fourth element. For wider angle applications, a three element inverse telephoto lens will suffice.

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Robert E. Aldrich "Commercial two- and three-element infrared wide-field-of-view 100% coldshielded inverse telephoto lenses", Proc. SPIE 2540, Current Developments in Optical Design and Engineering V, (15 September 1995); https://doi.org/10.1117/12.219520

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