Abstract

In all infrared spectrometers the output from the source (which varies greatly in intensity with wavelength) is focused through the entire optical system and eventually to the detector. Energy and hence detector output vs. wavelength is, as a consequence, a steep curve and shows evidence of water vapor and carbon dioxide absorptions. In dispersion instruments it is normal to analyze the radiation in such a way that the transmission through the sample is compared with that which has avoided the sample completely, i.e., <i>I</i>_trans is compared with the background source emission <i>I</i>₀. In older instruments, this was achieved electromechanically with the use of the so-called "null balance" system; more recently, it has been achieved with the use of the digital electronic technique referred to normally as "ratio recording." With either method the absorption spectrum of the sample appears as a series of bands on an otherwise flat background. If source emission attenuated by carbon dioxide or water absorption is high at specific wavelengths, and it frequently is, no useful energy reaches the detector, and hence the spectrum displayed at these wavelengths is meaningless. Further, optical imbalance and/or instability in the level of water vapor and carbon dioxide can lead to uneven backgrounds.

Water vapor absorption of carbon dioxide laser radiation

M. S. Shumate, R. T. Menzies, J. S. Margolis, and L.-G. Rosengren
Appl. Opt. 15(10) 2480-2488 (1976)

Measurement of atmospheric carbon dioxide and water vapor in built-up urban areas in the Gandhinagar-Ahmedabad region in India using a portable tunable diode laser spectroscopy system

Anirban Roy, Neetesh Kumar Sharma, Arup Lal Chakraborty, and Abhishek Upadhyay
Appl. Opt. 56(31) H57-H66 (2017)

Design of an open path near-infrared diode laser sensor: application to oxygen, water, and carbon dioxide vapor detection

H. Riris, C. B. Carlisle, L. W. Carr, D. E. Cooper, R. U. Martinelli, and R. J. Menna
Appl. Opt. 33(30) 7059-7066 (1994)

Refractive Indices of Water Vapor and Carbon Dioxide at Low Pressure

Kenneth B. Newbound
J. Opt. Soc. Am. 39(10) 835-840 (1949)

Transmittance of Carbon Dioxide and Water Vapor Over Stratospheric Slant Paths

Gilbert N. Plass
Appl. Opt. 3(4) 479-482 (1964)