MNL-1007 REV A
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The process of apodization affects both the resolution and the noise level of FT-IR spectra:
•
Resolution:
the information near the interferogram’s centerburst
determines the shape of the single-beam spectrum, and this
low-resolution spectrum is largely unchanged when the interferogram is multiplied by an apodization function. Conversely, the
sinusoids of narrow spectral features take much longer to decay and are therefore far more attenuated when the spectral
data are apodized, leading to degradation of resolution.
•
Noise:
lowspatial-frequency noise and very broad bands are also largely unaffected by apodization. Highspatial-frequency
noise is attenuated on apodization. On the average, the root mean square (rms) noise on the spectrum is decreased on
apodization and the SNR increases.
The specific apodization function that should be used for a certain application is chosen by experimentation [1].
Zero Padding
Spectra being measured on FT-IR spectrometers consist of intensity values at equally spaced intervals. The discrete nature of the
data makes it highly unlikely that the maximum absorption of any band will correspond exactly with one of these data points. In
addition, the slope of the spectrum can appear to change markedly from one point to the next. To eliminate these problems, some
form of interpolation or alternative data processing is necessary. One of the most useful of these is zero padding (also known as
Zero Filling).
Considering complex Fourier transformation of an interferogram that consists of N data points, the real and imaginary spectra each
contain N/2 data points. Increasing the number of points by N (adding N/2 zeros to each end of the double-sided interferogram)
increases the number of data points per resolution element to two. This process may be continued until the data point spacing in the
calculated spectrum is so small that no information is lost visually [2].
Available options of zero padding are: 0,1,3,7 corresponding to
•
0: No zero padding is applied
•
1: N zeros are added to the interferogram
•
3: 3N zeros are added to the interferogram
•
7: 7N zeros are added to the interferogram
