A thermal infrared camera may be used under a wide variety of target-scene
radiation conditions, with interchangeable or zoom lenses requiring
matching or different size cold stops. A variable aperture assembly of a
thermal infrared camera integrates a rigid open truss-like framework
that's capped by an aperture ring and bottomed by a driving ring, and a
radiation shield, located inside the framework, that contains an aperture
ring at an upper side. A plurality of blades that collectively define an
aperture positioned between the upper aperture rings. Opposite blade ends
are coupled to respective ones of the two aperture rings, permitting
pivotal movement in one ring and radial movement in the other ring, when
the rings are rotated relative to one another, to change the size of the
formed aperture. Both refractive and reflective infrared telescopes may
be retro-fitted with variable aperture devices to enhance infrared
imaging performance.