The heat is visible through the cover.
Infrared heat bears most of the same properties as visible light.
Infrared heat will not radiate through a panel or transformer cover just as light will not shine through them. The amount of infrared heat visible by ANY infrared camera is directly dependant on the surface temperature and emissivity of the object being viewed. Since moderate temperature rises in most electrical distribution systems are normal, simply viewing the cover surfaces will not suffice. Wherever safe, covers and obstructions must be removed to expose the actual electrical component for proper inspection. Infrared spot radiometers work as well as infrared imaging systems to perform electrical inspections.