In FT-IR measurement, we generally use the term "Collect Background" or "Collect Samples".
Background means a spectrum of before-cell beams with no IR absorption.
It can be either in interferogam or single beam spectrum, but usually we use single beam spectrum.
[Before and After Sample Cell]
As we have seen, absorption occurs if it pass thorugh an IR active sample. (See the following diagram)
Transmission Factor is defined as Tλ = I'λ / Iλ
and Absorbance unit = -logT(log is common logarithms)
where I' λ is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and Iλ is the intensity of the light before it enters the sample or incident light intensity (or power).
In the field of FT-IR spectroscopy, Absorbance is mainly used because it complies with Beer-Lambert law of Absorbance unit = ε* L * C
where ε = Absorption coefficient
L = Optical Cell Path Length
C = Concentration of a sample
In "Basic FTIR Principle (2)", we described, "plotting amplitudes of decomposed waves will form a spectrum over wide-range wavelength called Single Beam spectrum". Background single beam is a one with no absorption because of being purged by IR-inactive gas, and sample single beam has IR absorption in some wavenumber region.
The above sample is the one taken under amnbient air and shows shows absorption becase of CO2 and H2O.
In the same way, transmittance spectrum and absorbance spectrum can be get by plotting results at each λ in series.
Basic FTIR theory(1)
Basic FTIR theory(2)
Basic FTIR theory(3)
Spectra of Single Beam(Background and Sample), Transmission factor, and Absorbance