The interferometer light source controls fringe formation and data acquisition. Long coherence (laser) simultaneously creates fringes over all cavity lengths, from less than a micrometer to many hundreds of meters. Whereas a broadband, white light source creates fringes in a narrow micrometer wide region. The source wavelength, spectrum and variability control data acquisition: If fixed wavelength mechanical phase shifting is required, if variable wavelength or spectrally controlled, fixed cavity length acquisition is possible.