An estimation method for shading and narcissus in forward looking infrared camera (FLIR) is discussed. Shading and narcissus are common optical noises in FLIR formed mainly by infrared emission from warm housing and cold surface. These noises are clearly observed when the background of an image is uniform. The estimation method proposed in this paper produces an uniform background by covering the field of view (FOV) with a black body placed in front of the FLIR. Shading and narcissus are evaluated as the infrared intensity variation at the detector. Assuming that the emission surface is Lambertian, intensity contribution from black body, lenses, warm housing and cold surface in both direct and reflected paths pathis is considered. A simulation program based on ray tracing is developed for calculating the intensity variation at the detector. Examples of both simulation and experimental results are presented.