Higher to noise ratios can be achieved in FTIR spectra compared to spectra from dispersive IR instruments. How is such a useful feat achieved in FTIR
You can read this site.
http://chemistry.oregonstate.edu/courses/ch361-464/ch362/irinstrs.htm
Part way down the first page you see the following:
All of the source energy gets to the sample, improving the inherent signal-to-noise ratio.
The digitization and computer interface allows multiple scans to be collected, also dramatically improving the signal-to-noise ratio.