Any form of analysis by infrared absorption that does not rely on a monochromator to separate the wavelengths of the radiation. If the target molecule is the only one known to absorb strongly in the wavelength region of interest, a broadband optical filter may be used, such as in instruments to measure ambient carbon dioxide. If the target molecule absorbs less strongly, or if the spectral region is congested, more specific techniques such as gas filter correlation spectroscopy must be used.