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17 May 2016 Exploring the imaging properties of thin lenses for cryogenic infrared cameras
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Designing a cryogenic camera is a good strategy to miniaturize and simplify an infrared camera using a cooled detector. Indeed, the integration of optics inside the cold shield allows to simply athermalize the design, guarantees a cold pupil and releases the constraint on having a high back focal length for small focal length systems. By this way, cameras made of a single lens or two lenses are viable systems with good optical features and a good stability in image correction. However it involves a relatively significant additional optical mass inside the dewar and thus increases the cool down time of the camera. ONERA is currently exploring a minimalist strategy consisting in giving an imaging function to thin optical plates that are found in conventional dewars. By this way, we could make a cryogenic camera that has the same cool down time as a traditional dewar without an imagery function. Two examples will be presented: the first one is a camera using a dual-band infrared detector made of a lens outside the dewar and a lens inside the cold shield, the later having the main optical power of the system. We were able to design a cold plano-convex lens with a thickness lower than 1mm. The second example is an evolution of a former cryogenic camera called SOIE. We replaced the cold meniscus by a plano-convex Fresnel lens with a decrease of the optical thermal mass of 66%. The performances of both cameras will be compared.
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Guillaume Druart, Sebastien Verdet, Nicolas Guerineau, Serge Magli, Mathieu Chambon, Tatiana Grulois, and Noura Matallah "Exploring the imaging properties of thin lenses for cryogenic infrared cameras", Proc. SPIE 9822, Advanced Optics for Defense Applications: UV through LWIR, 982214 (17 May 2016);

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