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Research Notes

California Institute of Technology
Publication Date
  • Research Notes
  • Engineering
  • Physics


Research Notes Research fellow Robert Merzzies activates a high- powered carbon dioxide laser resembling the ones that will one day be used to detect pollution violators. The laser mechanism itself is contained irz the long rectangular box at the lower right. The bright spot is the fire brick safety sliieliiglowing white hot from the laser beam, which is invi.sible because it's in the infrared portion of the spectrum. Laser Smog Detector Robert Menzies, research fellow in electrical engineering, and Nicholas George, associate professor of electrical engineering, are currently developing infrared laser devices that can not only measure the presence of smog but can pinpoint the sources of it as well. The systems can detect pollutants over distances up to several kilometers, and may someday be used to spot air pollution violators in the way that traffic patrolmen use radar today. The development of these devices has been concentrated on two general types of infrared laser systems for pollution detection and monitoring-a passive system and an active one. Both systems depend on the fact that molecules of the most important pollutants radiate in the infrared portion of the spectrum, and also that the emissions fall into identifiable spectral regions. Consequently, oxides of nitrogen-one of the major constituents of smog-as well as methane, ozone, carbon monoxide, and sulfur dioxide, all have spectral signatures that can readily be identified and measured. The passive system employs a type of infrared radiometer known as a hetero- dyne receiver, which operates in roughly the same way as an ordinary radio. It uses a small laser as a local oscillator, together with a photosensitive device such as a crystal, to pick up pollutant infrared eniissions from the atmosphere. Such emissions occur naturally, and since their amplitude increases with temperature, passive devices need to be calibrated with temperature measurements of the pollutants. The

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