Abstract: Apparatus for detecting and measuring relative concentrations of pollutants such as HC, CO and CO.sub.2 in the exhaust emissions from passing vehicles includes an infrared (IR) beam source for directing an IR beam through a vehicle's exhaust plume and a detector disposed on one side of a roadway and a lateral transfer mirror (LTM) disposed on the other side of the roadway for lateral reflecting of the IR beam back through the vehicle's exhaust plume and onto the detector. The LTM allows for close spacing and precise alignment between the emitted and reflected beam paths and permits the IR beam source and detector to be disposed in a single module. Compensation for variation in a range of vehicle operating conditions is provided by adjustable, computer-controlled potentiometers, while audio and/or visible light feedback of the detected IR signal allows a single operator to precisely align the optical components of the apparatus.