A detector assembly (300, 500) includes a dewar chamber (102, 502) having an aperture (108, 510) and an infrared radiation detector (106, 514). The detector assembly also includes a mirror (304, 400, 410, 504) disposed adjacent the aperture of the dewar chamber, where the mirror has a reflective surface (306, 604) and an emitting region (305, 402, 412, 606) facing the aperture. The infrared radiation detector is configured to detect first radiation and second radiation from the mirror. The first radiation originates from at least one relatively cold surface in the dewar chamber and reflects off the reflective surface of the mirror. The second warm radiation originates from at least one relatively warm surface at or behind the emitting region. The infrared radiation detector is also configured to detect an artifact (322, 712, 722, 802) caused by a particle (110) in the dewar chamber that blocks a portion of the first or second radiation.