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For the middle wave 640×512 cooled thermal IR focal plane array with 15μm pixel pitch, a design of 3.7～4.8μm refractive infrared switchable dual-field-of-view imaging system is described, which makes use of a mechanical holder to switch two lenses into and out of the whole optical system to change the focal length from 45mm to 135mm. The system includes a telescope and a relay sub-system to avoid vignetting caused by the presence of the cold shield. Preliminary calculations are carried out first to determine the focal powers and first-order parameters of each lens and then the aberration equations are solved to achieve the initial configuration of the system as the starting point. During the optimization, in order to correct varied aberrations, such as spherical aberrations, coma, astigmatism, and chromatic aberrations, more lenses, glasses and aspherical surfaces are employed. In consideration of decreasing the manufacture cost and fabrication difficulties, only germ, silicon and one ZnS single lens are allowed to make use of, and the highest order of aspherical coefficients is no more than 8th. Meanwhile, the separation between the two lenses fixed in the mechanical holder must be controlled strictly to make sure no ray will be obscured by them when they are switched out of the system. Between the telescope and the relay system, the relay system and the image plane, there are two mirrors to fold the system, so reserved space must be valued in the optical path. Finally, a total of ten lenses with two aspherical surfaces and two mirrors are used in the design, making the system cost effective and compact. At the end, the design results are given. The modulation transfer function (MTF) of each field-of-view is above 0.5 in all focal positions at the spatial frequency of 33lp/mm, which approaches the diffraction limits and the energy permeance ratio is greater than 80%, showing excellent performance.
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