1. A system for imaging a target gas comprising: a movable filter, positioned along a line-of-sight to the target gas, for receiving energy from the target gas, the movable filter including at least two filter segments, one filter segment passing energy corresponding to an on-line wavelength of the target gas, and another filter passing energy corresponding to an off-line wavelength of the target gas, and an imager for sequentially imaging the energy passed through the one filter segment, corresponding to the on-line wavelength of the target gas, and the other filter segment, corresponding to the off-line wavelength of the target gas; wherein the movable filter includes at least two sets of filter segments, a first set including an on-line filter segment and an off-line filter segment, and a second set including another on-line filter segment and another off-line filter segment, and the first set passes an on-line wavelength and an off-line wavelength of a first target gas during a first interval of time, and the second set passes an on-line wavelength and an off-line wavelength of a second target gas during a second interval of time.