This paper describes the design analysis and results of performance test of a rosette scan infrared sensor which can be used as a thermal imager as well as a radiation seeker for search and tracking. The fundamental properties of a sensor are discussed, and the relationship between sensor parameters has been made clear. The effect of bandwidth on the noise equivalent irradiance(NEI) is also investigated by using a simplified model. On the basis of these analyses, the sensor which responds in the 3 to 5 μm spectral band has been designed, fabricated and tested. It has been verified that the minimum NEI over the total field of view could be realized by optimizing the bandwidth at the center of the field of view. The ability as a thermal imager has also been demonstrated by displaying rosette-scanned thermal images on a CRT monitor, and the detection capability of a target on a display has been evaluated by measuring the minimum detectable temperature difference(MDTD).