Although optical components in Fourier transform infrared (FTIR) spectrometers are preferably wedged, in practice, infrared spectra typically suffer from the effects of optical resonances (“channeling”) affecting the retrieval of weakly absorbing gases. This study investigates the level of channeling of each FTIR spectrometer within the Network for the Detection of Atmospheric Composition Change (NDACC). Dedicated spectra were recorded by more than 20 NDACC FTIR spectrometers using a laboratory mid-infrared source and two detectors. In the indium antimonide (InSb) detector domain (1900–5000 cm⁻¹), we found that the amplitude of the most pronounced channeling frequency amounts to 0.1 ‰ to 2.0 ‰ of the spectral background level, with a mean of (0.68±0.48) ‰ and a median of 0.60 ‰. In the mercury cadmium telluride (HgCdTe) detector domain (700–1300 cm⁻¹), we find even stronger effects, with the largest amplitude ranging from 0.3 ‰ to 21 ‰ with a mean of (2.45±4.50) ‰ and a median of 1.2 ‰. For both detectors, the leading channeling frequencies are 0.9 and 0.11 or 0.23 cm⁻¹ in most spectrometers. The observed spectral frequencies of 0.11 and 0.23 cm⁻¹ correspond to the optical thickness of the beam splitter substrate. The 0.9 cm⁻¹ channeling is caused by the air gap in between the beam splitter and compensator plate. Since the air gap is a significant source of channeling and the corresponding amplitude differs strongly between spectrometers, we propose new beam splitters with the wedge of the air gap increased to at least 0.8^∘. We tested the insertion of spacers in a beam splitter's air gap to demonstrate that increasing the wedge of the air gap decreases the 0.9 cm⁻¹ channeling amplitude significantly. A wedge of the air gap of 0.8^∘ reduces the channeling amplitude by about 50 %, while a wedge of about 2^∘ removes the 0.9 cm⁻¹ channeling completely. This study shows the potential for reducing channeling in the FTIR spectrometers operated by the NDACC, thereby increasing the quality of recorded spectra across the network.

中文翻译：

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大气成分变化检测网络（NDACC）的傅立叶变换红外光谱仪的表征和降低光学共振的潜力

尽管优选将傅立叶变换红外（FTIR）光谱仪中的光学组件楔入，但实际上，红外光谱通常会受到光学共振（“通道”）的影响，影响弱吸收气体的回收。这项研究调查网络中每个FTIR光谱仪的通道水平，以检测大气成分变化（NDACC）。专用光谱由20多个NDACC FTIR光谱仪使用实验室中红外源和两个检测器记录。在锑化铟（InSb）检测器域（1900–5000  cm ^-1）中，我们发现最明显的通道频率的幅度为光谱背景水平的0.1  ‰至2.0  ‰，平均值为（0.68±0.48）  ‰和0.60 ‰的中位数 。在碲化汞镉（HgCdTe）检测器域（700–1300  cm ^-1）中，我们发现了更强的影响，其最大振幅范围为0.3  ‰至21  ‰，平均值为（2.45±4.50）  ‰，中位数为1.2  ‰。对于两个检测器，在大多数光谱仪中，领先的引导频率是0.9和0.11或0.23  cm ^-1。的0.11和0.23观察到的光谱频率 厘米^-1对应于分束器衬底的光学厚度。0.9 厘米^-1通道是由分束器和补偿板之间的气隙引起的。由于空气间隙是窜的显著源和强烈的光谱仪之间的相应振幅的不同，提出了新的分束器与所述空气间隙增大到至少为0.8的楔^∘。我们测试了在分束器的气隙中插入垫片的情况，以证明增加气隙的楔度会显着降低0.9  cm ^-1的通道振幅。气隙的楔形为^0.8∘会减小通道幅度约50  ％，而楔形的^2φ会去除0.9  cm ^-1完全引导。这项研究表明了减少NDACC操作的FTIR光谱仪中通道的潜力，从而提高了整个网络记录光谱的质量。