Spectral Windowing

Windowing premultiplies input data supplied to the FFT with a value that smoothly decreases to zero at each end of data.

The purpose is to reduce "leakage" aberrations in the FFT output that are introduced by sudden changes in the data at the start and end of data. Much research has been applied to the selection of the proper "windowing function", so Igor´s FFT supplies many of them:

Blackman361, Blackman367, Blackman492, Blackman474
Cos1, Cos2, Cos3, Cos4
KaiserBessel20, KaiserBessel25, KaiserBessel30
Poisson2, Poisson3, Poisson4

Here is a demonstration of how windowing can improve the Fourier Transform result:

Graphs showing that windowing reduces aberrations.

Igor provides a standalone Hanning command to window data without involving the FFT. The

statement loads the Parzen, Welch, Kaiser, Hamming, BlackmanHarris3, and KaiserBessel functions which also window data without involving the FFT.