area of the beam at any focus-that is, the throughput, is greater for an interfer- ometer than for a monochromator. For interferograms to be transformed into spectra, they must be digitized. In essentially all modern Fourier transform Fourier Transform infrared (FTIR) spectrometers, this goal is achieved by sampling at equal inter- Infrared Spectrometry vals of optical path difference using the sinusoidal interferogram from a helium- neon laser beam which is passed through Peter R. Griffiths a different region of the beamsplitter of the interferometer. The infrared interfer- ogram is generally digitized once per wavelength of the laser interferogram The origin of Fourier transform tech- lengths even at the centerburst, so that a (typically at a zero-crossing). This meth- niques for the measurement of high-qual- typical "real" interferogram in this re- od of sampling gives rise to the third ity infrared spectra can be traced back to gion is not symmetrical, but usually has major advantage of FTIR spectrometry, the turn of the century, when Michelson the form shown in Fig. lA. which is its extremely high wave number first described the interferometer that The interferogram is related to the precision. now bears his name (1). In this instru- spectrum through its Fourier transform A fourth benefit, which results in part ment an input collimated b
