Apparatus for monitoring engine exhaust

Apparatus for monitoring engine exhaust, comprising a chamber for receiving such exhaust, an inlet of the chamber adapted to be connected to the engine exhaust and an outlet from the chamber to enable exhaust to escape therefrom, so that exhaust from the engine can flow through the chamber continuously, a source of electromagnetic radiation which is of a wavelength within a range comprising the infrared, visible and ultraviolet wavelengths, a detector which detects the amount of such radiation it receives, the source and the detector being so arranged that radiation emitted by the source is received by the detector after the radiation has passed through the chamber, and electrical circuitry connected to the detector to provide an electrical signal which is indicative of the attenuation of the radiation as a result of the presence of particles within the exhaust in the chamber, in which the detector is such as to be able to provide respective different measures of radiation which it receives for at least two different wavelengths of such radiation, and the electrical circuitry is such as to provide respective electrical signals which are indicative of the attenuation of the radiation, as a result of the presence of particles within the exhaust in the chamber, for the said at least two different wavelengths, wherein the said at least two different wavelengths are wavelengths at which the attenuation of electromagnetic radiation caused by the presence of nitrogen dioxide in the exhaust is substantially the same.