Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2021-03-16.
Spectrometers work well in the ultraviolet and visible regions but struggle to reach the resolution needed for very low gas concentrations in the infrared. The interferometer takes a different route and records wavenumbers rather than wavelengths — the reciprocal of wavelength, expressed in cm⁻¹. Incoming light meets a beamsplitter at 45°: half is reflected, half transmitted. Both beams return from mirrors and recombine at the beamsplitter, and a sensor watches the result. With all parts still, nothing useful happens. Move one mirror slowly and the sensor sees alternating constructive and destructive interference: a sinusoid whose period reflects the wavenumber of the light. That signal is an interferogram. Several colours give a sum of sinusoids with slightly different periods; a continuous spectrum gives a complex interferogram built from an enormous number of them. A Fourier transform, pure mathematics inside a computer, converts that interferogram into a spectrum. From there the DOAS algorithm takes over. The detour is worth it because the interferometer delivers the infrared resolution a spectrometer cannot, while in the ultraviolet it is the spectrometer that performs.
The two bands cover different molecules. Many classical pollutants — SO2, NOx among them — are measured in the ultraviolet, while a group of other molecules absorbs characteristically in the infrared. For multi-component flue gas, running UV-DOAS and FTIR-DOAS side by side is therefore a mature configuration: each analyser covers the components it handles best, and both feed the same data acquisition and reporting chain. The infrared region also contains strong absorption by water vapour and CO2, which places higher demands on reference handling and on keeping the spectrum clean. In hot, wet, dusty plants, periodic calibration and performance checks plus clean, stable optics are the daily routine that protects long-term data quality. Decide at design stage which component is assigned to which band; retrofitting that split later usually means adding hardware instead of reconfiguring it.
Inside the Gas Analyser: the Interferometer — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2021-03-16. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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