A thermal infrared imaging device (10) includes a thermal detector (50) having a linearly-arrayed plurality of spaced apart detector elements (50', 50", 50"', . . . ). A scene to be viewed is scanned across the detector (50) with successive fields of the scene shifted according the spacing between adjacent detector elements (50', 50", 50"', . . . ) in order to capture image information for the entire scene by interlacing of successive scan lines from the plurality of detector elements (50', 50", 50"', . . . ). Each complete scan of the viewed scene across the detector (50) creates an image field including a scan line for each detector element (50', 50", 50"', . . . ). Each scan line includes plural pixels, or picture elements of the viewed scene, each having a value indicative of the thermal infrared brightness of the viewed scene at the corresponding location along the scan line. An scan-line sum for each scan line is created by adding the absolute values of the pixel values for each scan line. The average of these scan-line sums is employed as a gain control factor to control the brightness of a visible image replicating the viewed scene. Further, the value of a median of the scan-line sums is used to limit gain variations which would otherwise be effected were the line-sum averages alone used to control the gain factor. Accordingly, even when a localized highly-radiant heat source (such as a fire) is within the field of view of the thermal imaging device, then in areas of the image away from the brightness caused by the fire the image does not go dark and a visible image good contrast is still maintained allowing features of the image to be viewed.