CO is measured using IR. Yes there are other ways that it can be done but when we are talking about a CEMS, it's more than likely going to be IR. CO is a toxic gas and must be measured for safety. CO is a sign of incomplete combustion in boilers and other processes. We measure CO to optimize the process and conserve energy. CO is inversely proportional. That means that the IR source is hitting the detector and if there is no CO in the path then the detector will have 100% IR hitting it and the output will be 0%. If there is a large amount of CO in the path and it is absorbing the IR light then the amount of IR making it to the detector is low so the output will be high.
NDIR - Nondispersive Infrared Analyzers
This is continuous analysis on the capacity of carbon monoxide to absorb infrared radiation.
Infrared radiation from a hot filament is chopped to pass alternately through sample and reference cells which is absorbed in the detector cell divided by a pressure sensitive diaphragm. If the sample has CO then it will absorb part of the radiation causing that half of the detector to have less pressure. This difference is converted to an electrical signal and amplified.