A remote emissions sensing system and method for sensing exhaust emissions from motor vehicles (10) is provided where the system determines the opacity of an exhaust plume (16). The system comprises a radiation source (12) that emits radiation (20) which is passed through the exhaust plume (16) of a motor vehicle (10) to one or more detectors (14) arranged to receive the radiation (22). A processor (100) calculates the difference between the intensity of source radiation (20) and the intensity of the radiation received (22) by the detectors (14) in first and second detection bands. The intensity difference in the second detection band measures exhaust opacity. If the exhaust opacity exceeds a predetermined level, the emissions data from other detection bands may be flagged as suspect or discarded. Alternatively, for a diesel powered vehicle, the exhaust opacity determination can be validated (104) by a measurement of carbon monoxide in the exhaust plume (16).
