Interferogram transmittance

Nowadays, most instruments use a FT-infrared (FT-IR) system, a mathematical operation used to translate a complex curve into its component curves. In an FT-IR instrument, the complex curve is an interferogram, or the sum of the constructive and destructive interferences generated by overlapping light waves, and the component curves are the IR spectrum. The standard IR spectrum is calculated from the Fourier-transformed interferogram, giving a spectrum in percent transmittance (%T) versus light frequency (cm ). 

Here T v) is the power or intensity transmittance of the sample and qi(v) is the phase shift introduced by it. The wave reflected at the movable mirror can still be written in the form of Eq. (2.3b). The interferogram with the sample in one arm of the Michelson interferometer is then for monochromatic radiation... 

Fig. 33. Transmission (a) and reflection measurements (b) by means of asymmetric Fourier transform spectroscopy. The samples are polyethylene (PET) and mylar (transmission), and KBr (reflection). The difierent curves show background and sample interferograms as well as power transmittance (reflectance) and phase angle spectra. Data taken from Ref. 59)...

Let us now turn to the intensity errors and discuss the most frequent ones in detail Figs. 39a)—d) demonstrate some of them in the interferogram and the effect in the spectrum. A minor intensity error is that sometimes the computed spectrum happens to be wrong by a constant factor due to some change in the amplifier gain between background and sample measurement (Fig. 39 a). Tliis does not affect the structures in the spectrum but is important when refractive index or absorption coefficient are evaluated from the reflectance or transmittance, respectively. 

AamI Absorbance spectrum i Transmittance spectrum 1 I Single chaimel spectrum of the reference l> I I Single chaimel spectrum of the sample jIu-J Raman spectrum -Ft.l Interferogram of the sample I Audit Trail I Information... 

When the default settings are loaded, the spectrum window is located in the right pane of the OPUS user interface. After a file has been loaded, all spectral features of this file such as spectra and interferograms can be displayed. As an example, the transmittance spectrum, the single channel sample spectrum, the sample inter-ferogram, and the single channel reference spectrum of the MIR file INDIGO are displayed in Fig. 3.10. 

THE USE OF ASYMMETRIC INTERFEROGRAMS IN TRANSMITTANCE MEASUREMENTS. FROM COLLOQUE SUR LES METHODES NOUVELLES DE SPECTROSCOPIEINSTRUMENTALE, 2ND,... 

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