Variable cold stop for matching IR cameras to multiple f-number optics

Cameras operating in the thermal infrared (mid-wave and long-wave IR) use a cold stop that is designed to match the exit pupil of the optics and thus avoid parasitic radiation or vignetting. For years, range operators have been using reflective telescopes, usually with photo-documentation film cameras. Along with the need to shift operation into the infrared comes a problem that (i) these telescopes do not have an exit pupil located at the IR camera cold stop, and (ii) most IR cameras have f/2 or f/4 stop, while the telescope is typically f/7 or greater. These mismatches cause a significant deterioration of the system performance and picture quality. A similar need arises when using zoom optics with IR cameras where, as the field of view changes, so does the optics f/#, creating a mismatch with the camera that has a fixed aperture. The OKSI/WSMR team has demonstrated two implementations of a patented continuous variable aperture / cold stop (CVA/CS or VariAp®) for operating IR cameras with different f/# optics. Two systems were built: (1) an optical relay assembly with an external CVA/CS, and (2) a custom 1024×1024 pixel MWIR camera with a built in CVA/CS and the proper relay optics to match the telescope optics to the camera. The first optical relay with the VariAp® is a retrofit for legacy IR cameras for operations with reflective telescopes. The camera with the built-in VariAp® can function with both reflective (using an additional external relay) and refractive (with no additional relay) telescopes. The paper describes the two systems that open new possibilities in IR imaging for various ranges.