The present invention includes systems and methods for quantitative and qualitative imaging of gases, vapors, fumes, and the like. In one embodiment, the system uses a moving, or a rotating filter for alternately capturing on-line and off-line images of a scene that potentially includes the target gas of interest. The moving, or rotating filter includes at least two filter segments for transmitting (a) wavelengths corresponding to a spectroscopic absorption feature for on-line elements of the target gas, and (b) nearby wavelengths not corresponding to a spectroscopic absorption feature for off-line elements of the target gas. An image processor executes an algorithm for transforming the collected on-line and off-line data, corresponding to different pixels in the imaging array, into an image map of path-integrated concentration, or concentration path lengths (CPLs). The systems and methods of the invention do not use active light sources to illuminate the target gas. Instead, imaging and processing is done entirely using passive received energy from the target gas.