A gas imaging system for remotely detecting gas emissions by passive images of infrared radiation includes an optical system having a field of view. The optical system has a lens, an optical filter system for filtering light passed through the lens, and a photosensitive array located at the focal plane of the optical system to produce multi-spectral infrared image data of a scene under observation. A multi-spectral image processing system is configured for processing the image data produced by the photosensitive array to detect hazardous gas emissions and to discriminate against infrared radiation emitted by false alarm sources. Some embodiments may be configured for flame detection. Other embodiments may be configured for gas and flame detection.