Long Path absorption measurements

Open Path Fourier Transform Infrared (OP-FTIR) Spectroscopy can be used to measure gas concentrations over long optical paths that are open to the atmosphere. The technique is most commonly used in environmental monitoring of pollutant gas emissions.

The instrument consists of an infrared (IR) energy source, transfer optics, an interferometer (MB100), a transmitting/receiving telescope, a retroreflector and an IR detector. The IR beam is collimated and then transmitted through the interferometer. The beam is then expanded by the telescope and transmitted along the optical path to the retroreflector. The beam is returned to the telescope and focused onto the detector.

If you’d like to know more about OP-FTIR a great place to start reading is:
Russwurm, G. M., and J. W. Childers (2002), Open-path Fourier Transform Infrared Spectroscopy, in Handbook of Vibrational Spectroscopy, edited by J. M. Chalmers and P. R. Griffiths, pp. 1750-1773, Wiley, Chichester

An experiment using OP-FTIR to measure methane emissions from grazing cattle.The top of the OP-FTIR is in the foreground of the picture and was aimed through the window of a shed out to the retroreflector under the white tent in the distance. The wind direction was from the left to the right of the picture. For this experiment both methane and ammonia fluxes were measured.