A Transmitting/Receiving Telescope for DOAS-measurements using Retroreflectortechnique.

Abstract

DOAS (Differential Optical Absorption Spectroscopy) is a remote sensing technique in which light is transmitted over kilometer distances with a transmitter, then collected by a receiver and analyzed in a special spectrometer. Thus an absorption spectrum of the air between transmitter and receiver is obtained (Platt o Perner 1979). The method has proven to be powerful and a number of substances have been successfully measured in applications ranging from background monitoring to urban air and emission studies (Platt, Perner, Pitts, Biermann). Traditionally light is transmitted by a parabolic mirror with the lamp mounted in its focus, thus emitting a parallel beam of light. In the receiving end, light is collected by a similar mirror and focused into an optical fiber connected to the spectrometer. Alternatively the converging light may be coupled directly to the spectrometer by means of a folding mirror in Newtonian or Cassegrain arrangement.

Comparing different light-emitting diodes as light sources for long path differential optical absorption spectroscopy NO 2 and SO 2 measurements

“…The conventional LP-DOAS instrumental setup combines the emitting and the receiving telescopes into one single instrument sharing the same main mirror. [2,3] Operating such an instrument requires very accurate alignment of the optical components. A 100 µm displacement of the light source, which can easily be caused by mechanical instability and thermal expansion, [4] may lead to an intensity loss of up to one third of the total light.…”