With polychromatic light, the output signal from a scanning interferometer is the sum of all cosine waves for all optical frequencies. This is the interferogram, related to the spectrum through the Fourier transform. A simplified interferogram from a light wave of three discrete frequencies is shown in the figure. The cosine waves are below and their sum, the interferogram, is above. The interferogram, a function of time, is difficult to interpret directly. The spectrum, a function of frequency, is more easily interpreted. The spectrum is calculated from the interferogram by computing its cosine Fourier transform.

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