Method of and apparatus for monitoring gaseous pollutants
First Claim
1. A method for the remote quantitative monitoring of one or more selected gases in a gaseous environment, which comprises the steps of generating electromagnetic radiation from laser sources to give at least one detection beam containing a specific absorption wavelength of the gas or gases being monitored and at least one reference beam having a wavelength that is significantly less strongly absorbed by the gas or gases being monitored, modulating the amplitude of each of the beams with different modulation frequencies or phases, combining the modulated beams into a single beam in which the component modulated beams are substantially coincident with one another, displacing the combined beam angularly through the gaseous environment so as to direct the combined beam towards a plurality of locations sequentially and repetitively, collecting at least a portion of the radiation which is reflected from each of the locations, deriving electrical signals corresponding to the intensity of the collected radiation, isolating the electrical signals corresponding to the intensity of the radiation having the aforesaid modulation frequencies or phases, obtaining a ratio of the isolated signals corresponding to radiation collected from a detection beam and a related reference beam thereby to provide a measure of the amount of the selected gas or gases in each beam path traversed by the collected radiation originating from the laser sources, and varying according to a predetermined programme or in response to an external stimulus, the amount of radiation from the laser sources which is detected by the detector thereby to prevent the detected amount from exceeding a predetermined level.
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Abstract
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.
25 Citations
12 Claims
- 1. A method for the remote quantitative monitoring of one or more selected gases in a gaseous environment, which comprises the steps of generating electromagnetic radiation from laser sources to give at least one detection beam containing a specific absorption wavelength of the gas or gases being monitored and at least one reference beam having a wavelength that is significantly less strongly absorbed by the gas or gases being monitored, modulating the amplitude of each of the beams with different modulation frequencies or phases, combining the modulated beams into a single beam in which the component modulated beams are substantially coincident with one another, displacing the combined beam angularly through the gaseous environment so as to direct the combined beam towards a plurality of locations sequentially and repetitively, collecting at least a portion of the radiation which is reflected from each of the locations, deriving electrical signals corresponding to the intensity of the collected radiation, isolating the electrical signals corresponding to the intensity of the radiation having the aforesaid modulation frequencies or phases, obtaining a ratio of the isolated signals corresponding to radiation collected from a detection beam and a related reference beam thereby to provide a measure of the amount of the selected gas or gases in each beam path traversed by the collected radiation originating from the laser sources, and varying according to a predetermined programme or in response to an external stimulus, the amount of radiation from the laser sources which is detected by the detector thereby to prevent the detected amount from exceeding a predetermined level.
- 7. Apparatus for the remote quantitative monitoring of one or more selected gases in a gaseous environment, which comprises laser sources for generating electromagnetic radiation capable of being tuned to give at least one detection beam containing a specific absorption wavelength of the gas or gases to be monitored and at least one reference beam having a wavelength that is significantly less strongly absorbed by the gas to be monitored, means for modulating the amplitude of each of the beams with different modulation frequencies or phrases, means for combining the modulated beams into a single combined beam in which the component modulated beams are substantially coincident with one another, scanning means to displace the combined beam angularly through the gaseous environment so as to direct the combined beam towards a plurality of locations sequentially and repetitively, means for collecting at least a portion of the radiation which is returned from each of the locations, a detector for deriving electrical signals corresponding to the intensity of the collected radiation, means for isolating the electrical signals corresponding to the intensity of radiation having the aforementioned modulation frequencies or phases, means for obtaining the ratio of the isolated signals corresponding to radiation collected from a detection beam and a related reference beam thereby to provide a measure of the amount of the selected gas or gases in each beam path traversed between the apparatus and the scanned locations, means for indicating the amount of gas detected, and means for varying according to a predetermined programme or in response to an external stimulus, the amount of radiation from the laser sources which is detected by the detector thereby to prevent the detected amount from exceeding a predetermined level.
Specification