⁴⁴In analytical literature, you may read of some detectors having a catholic response. This is just a fancy way of saying the detector responds to a wide variety of things. The thermopiles shown in this NDIR instrument could be considered to have a catholic response to incident light. The word “catholic” in this context simply means “universal,” referring to the detector’s non-selectivity. Do not be dismayed if you encounter arcane terms such as “catholic” as you learn more about analytical instruments – the author is probably just trying to impress you with his or her vocabulary!