Spotlight on Optics

The matter constituting the universe continuously interacts with electromagnetic radiation; each molecule in thermal equilibrium with its environment emits a spectrum of electromagnetic radiation (the so-called thermal radiation) that peaks towards shorter wavelengths as the temperature grows (the famous Wien’s displacement law). The blackbody spectrum is modulated for each gas by its absorptivity, so that thermal emission spectra can be used to identify molecules. In their study, Hoffmann and coauthors aim to fully characterize Earth’s thermal infrared radiation of atmospheric gases at very high spectral resolution. This would permit quantitative assessment of important thermo-physical information on the atmospheric gases: temperature, pressure, local thermodynamic equilibrium (LTE), and velocity. For the first time, the authors of this Optics Letters article applied the laser heterodyne spectro-radiometry (LHR) technique to measure LTE emission lines of terrestrial atmospheric gases with high-spectral resolution (up to 0.02 cm^-1). The LHR spectroscopy technique was previously used only to resolve predominantly non-LTE gas emissions in the atmospheres of other planets and never on Earth—this despite its operational advantages, as the LHR spectroscopy technique doesn’t require solar illumination, so that it can operate both at night and under a cover of clouds. This innovative development is an elegant solution, and cheaper with respect to the complex and large cooled spectrometers used until now to resolve the spectral lines with poor radiance contrast emitted by atmospheric molecules at LTE.

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