A recording optical smoke meter is described which has a high sensitivity. The optical density of smoke in a flue is compared by photoelectric means with that of an optical wedge, which is then rotated by an electrically operated servo-system until the two densities are the same. The angular position of the wedge is detected by a transducer, which provides a millivolt output proportional to the rotation of the wedge.A multi-channel switching unit, electrically coupled to a recorder, is incorporated; this enables any number of different flue units, up to the capacity of the recorder, to be used or other variables to be recorded at the same time. The range of the instrument can be varied by changing the value of the optical wedge in the balancing unit or by varying the transducer output to obtain full-scale deflection of the recorder for any desired rotation of the wedge.The calibration in terms of millivoltage output against optical density is linear, provided that the optical wedge and output transducer have linear characteristics. The output is stable to ±0.6% of full-scale reading for ambient temperature changes in the range of 55-80° F, and the sensitivity is such that densities of the order of 0.005 can be recorded.