Actually, FTIR never measures absorption, but transmittance or reflectance. Somewhat naively, it is assumed that what is not transmitted must be absorbed, on this the definition of transmittance absorbance is based (A = -log T). It makes much more sense to use absorptance instead, which is defined as 1-R-T and is, in contrast to absorbance, proportional to the electric field intensity inside the sample. Furthermore, it is compatible with Maxwell's equations while absorbance is not. See e.g. Article Employing Theories Far beyond Their Limits-The Case of the (...

FTIR is an absorption spectroscopy - or reflection, depending on the geometry!

UV-vis absorption spectroscopy probes transitions between electronic states in a material. In some materials, transitions between these states can be of lower energy, and so are probed in the Near Infra Red.

The mid IR (usually covered by FTIR) probes vibrational states in a material.

Mr. Ali,

FTIR can be absorption, transmission or reflectance spectroscopy.

Actually, FTIR never measures absorption, but transmittance or reflectance. Somewhat naively, it is assumed that what is not transmitted must be absorbed, on this the definition of transmittance absorbance is based (A = -log T). It makes much more sense to use absorptance instead, which is defined as 1-R-T and is, in contrast to absorbance, proportional to the electric field intensity inside the sample. Furthermore, it is compatible with Maxwell's equations while absorbance is not. See e.g. Article Employing Theories Far beyond Their Limits-The Case of the (...

As the absorption of electromagnetic radiation, e.g. IR radiation, follows the Beer-Lambert law, i.e. the radiation is decreasing exponentially with the penetration depth in the actual material, it is often helpful to plot the spectra on a logarithmic absorbance scale versus wavelength or wave number. Hence, a representative spectrum is chosen from each of the samples and plotted on a logarithmic absorbance scale for quantitative studies. Mathematically and physically it follows that a doubling of the logarithmic absorbance, also called optical density, is interpreted as a doubling of material thickness or a doubling of concentration of absorption active agents. In general, a spectrometer measures either the transmittance or the reflectance of a sample, i.e. measuring the radiation which is collected by the detector from either the transmitted or the reflected beam. Furthermore: T + A + R = 1 (100 %) where T = transmittance, A = absorbance and R = reflectance between 0 to 1 (or between 0 to 100 %). That is, the absorbance is not measured directly, but is calculated from the measured transmittance and reflectance (if both can be measured). When a spectrometer gives you the absorbance A’ on the logarithmic form, i.e. the optical density OD which is a common output from many spectrophotometers, it is in reality a calculation from the measured transmittance ignoring the reflectance.

The mathematical relationships are shown in the following (note that the common (Briggs) logarithm is denoted lg = log10 and the natural logarithm is denoted ln = loge, the latter one not directly used below):

The optical density OD is defined as the absorbance A' written on a logarithmic form by

OD = A’ = log10(1/T) = alog10(e)x = a’x

which is deduced from the Beer-Lambert law given by

I = I0e-ax

where the transmittance T is given by

T = I/I0

where the transmitted radiation intensity I decreases exponentially with the penetration length or depth x, I0 is the incident radiation intensity, and a and a' denote absorption coefficients depending what form is used.

For further information see e.g. chapter 7.6.3 in the following article (may be requested through Research Gate): B. P. Jelle, ”Solar Radiation Glazing Factors for Window Panes, Glass Structures and Electrochromic Windows in Buildings - Measurement and Calculation”, Solar Energy Materials and Solar Cells, 116, 291-323, 2013. In addition, the following articles may also be of interest: (a) B. P. Jelle, A. Gustavsen, T.-N. Nilsen and T. Jacobsen, ”Solar Material Protection Factor (SMPF) and Solar Skin Protection Factor (SSPF) for Window Panes and other Glass Structures in Buildings”, Solar Energy Materials and Solar Cells, 91, 342-354, 2007, (b) B. P. Jelle, T.-N. Nilsen, P. J. Hovde and A. Gustavsen, ”Accelerated Climate Aging of Building Materials and their Characterization by Fourier Transform Infrared Radiation Analysis”, Journal of Building Physics, 36, 99-112, 2012, and (c) B. P. Jelle and T.-N. Nilsen, ”Comparison of Accelerated Climate Ageing Methods of Polymer Building Materials by Attenuated Total Reflectance Fourier Transform Infrared Radiation Spectroscopy”, Construction and Building Materials, 25, 2122-2132, 2011.

Dear colleague! We should distinguish between the mechanism of excitation, range of wavelength, points of detection geometry and the way to get a spectrum. IR is the world of molecule vibrations including local modes of impurity atoms ... of collective vibrations as phonons and plasmons etc. Excitations of higher energy you have to study in the UV and VIS. Anyway, if you use a "white" beam for probing these excitations are detectable as absorption lines or bands in the spectra. There are a lot of different detection geometries, i. e. the position of source, sample and detector can vary depending on the problem you have to solve and on aspects of polarization and selection rules you might take into account. Finally, FT is an powerful alternative to record a spectrum instead of using the classic way of gratings. Wkr, M. Herms.

In the wavelength and the goal.

The wavelength of infrared light is longer than uv/vis.

Infrared absorption by molecules corresponds to differences in vibration energy. Infrared spectroscopy can therefore be used to identify molecular vibrations and uniquely recognize compounds. Compounds consisting of more than a handful of atoms have very many narrow infrared absorption bands.

Uv/vis absorption by molecules correspond to differences in the (covalent) bonding of atoms. Many compounds do not have any visible absorption. Compounds with metal ions or with (conjugated) double bonds often show uv or vis absorption in a single broad band that can help identify the compound but rarely deliver enough information to identify it unambiguously by itself.