A portable thermography camera system (100) renders a video image of a survey scene over a narrow spectral bandwidth corresponding with an absorption band of a gas to be detected in the video image. The camera system forms a scene image onto a focal plane array (108) and generates a corrected image signal (162) corresponding with irradiance values at a plurality of locations of the scene image. The camera system further generates a temporally filtered image signal (168) corresponding with a temporal characteristics of the image signal (162) over a selected number of prior image frames. A difference block (166) reduces the temporally filtered image signal (168) by a scaling factor and produces a difference image by subtracting the scaled temporally filtered image signal from the corrected image signal (162). The displayed difference signal improves the visibility of the gas to be detected.