Three-element infrared optically compensated two-position zooms for commercial FLIRs

摘要

Infrared zooms, continuous and two positions, are becoming more prevalent. Two position zooms are generally longer than other equivalent two fields of view systems, but have smaller total volumes since the switching group in the second field of view requires storage space when not in use. The optically compensated compact zoom with a negative zooming group requires at least three groups, a positive objective group, a negative zooming group and a positive focusing group. Generally these groups are made up of one or two elements. With the uses of single point diamond turned aspheres and diffractives, the three groups can be made of a single element each. The three element optically compensated two position zoom can be used in the longwave 8 - 12 micrometer region or the midwave 3 - 5 micrometer region. It can be employed as a simple imager or as an objective in an afocal telescope or in a reimager. The three element zoom can have magnifications from 3x to 5.5x with optical invariant as large as 1.61 mm. The short focal length of a zoom can be as small as 17 mm in one design, while the long focal length of another zoom can be as great as 275 mm.