Thermal imaging device

A thermal imaging device (10) receives thermal infrared radiation from a viewed scene and responsively provides a visible image replicating the viewed scene. The imaging device (10) has a main housing with an optical aperture (16) at which a lens assembly (12) is attached. The lens assembly (12) has a number of lenses (200, 204, 208, 212, 216) sequentially mounted within a cylindrical housing (146), and defining an optical axis (199). A field-of-view-change mechanism (228) allows a user to select a wide field of view or a narrow field of view. The field-of-view-change mechanism (228) includes a rotatable field-of-view (FOV) ring (226) which rotates a tumbler (210) with a pair of lenses (208, 212) mounted therein. The tumbler (210) is positioned between a first position providing a wide field of view and a second position providing a narrow field of view. A rack (236) cooperates with the FOV ring (226) and the tumbler (210) so that rotation of the FOV ring (226) about a first axis (199) rotates the tumbler about a second axis (210') perpendicular to the first axis (199). An optical alignment mechanism (254) releasably retains the tumbler (210) in the first and second positions so that optical axes (199', 248') of the tumbler are respectively aligned with the optical axis (199) of the lens assembly (12).