Method of and apparatus for monitoring gaseous pollutants

Laser scanning apparatus for the monitoring of gaseous pollutants (e.g. on a chemical plant) in which two laser beams having different wavelengths (one corresponding to an absorption line of the gas to be monitored) and modulated at different frequencies are combined into a single scanning beam. A portion of the scattered radiation is collected, detected and measured to give, for each chosen beam direction, the amount of the gas being monitored. The amount of radiation reaching the detector from the laser source is varied according to a predetermined programme or in response to an external stimulus, and by this means the detector can be protected against severe overload when the beam scans over positions of abnormally high reflectivity.