Long, open-path, outdoor interferometric measurement of the index of refraction as a function of wavelength (spectral refractivity) requires a number of innovations. These include active compensation for vibration and turbulence. The use of electronic compensation produces an electronic signal that is ideal for extracting data. This allows the appropriate interpretation of those data and the systematic and fast scanning of the spectrum by the use of bandwidths that are intermediate between lasers (narrow bandwidth) and white light (broad bandwidth). An Environmental Interferometer that incorporates these features should be extremely valuable in both pollutant detection and pollutant identification. Spectral refractivity measurements complement the information available from spectral absorption instruments (e.g., a Fourier-transform infrared spectrometer). The Environmental Interferometer currently uses an electronic compensating device with a 1-kHz response time, and therefore rapid spectral scans are feasibe so that it is possible to monitor the time evolution of pollutant events.

9/95/$6.oo Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/30/2015 Terms of Use: http://spiedl.org/terms AB STRACT Conventional methods for detecting the release of foreign substances into the atmosphere are often slow, expensive, and only give an actual reading for a small section of the area of interest. The Environmental Interferometer can allow inexpensive, real time monitoring of a large area. The principle behind the Environmental Interferometer is the use of a fringe locked Michelson interferometer scanning throughout a continuous range of colors at an intermediate bandwidth (50100 nm). Th'e i'ringe locking allows a the beam in a test arm to be reflected through a test area for about 1 kilometer of distance, while a reference arm is kept in a controlled environment (perhaps fiberoptics or a multiple reflected air path) and retain a suitable interference pattern. The use ofintermediate bandwidth light allows the central fringe to be located, and thus allows fast scanning through a continuous range of colors. Sampling at n different colors allows the discrimination of n different sources of optical pathlength change. This allows easy discrimination against moisture content change, air turbulence, ground vibrations, and the like, because oftheir characteristic pathlength change frequencies. The fringe locking allows for the electronic interpretation ofa signal and enhances the accuracy ofthe instrumentation so that small optical pathlength changes can be easily measured and interpreted.A demonstration unit has been created using a 670nm laser instead of a filtered white light source. Absolute index measurements oftest gases injected into a 3cm pollution chamber were made with the demonstration unit with errors of less than 1%.