Frequency distribution not determined by the temporal variation of the pulse??? In his book Modern Optical Spectroscopy, William Parson says Light from an incoherent source such as a xenon flash lamp contains a distribution of frequencies that are unrelated to the length of the pulse because atoms or ions with many different energies contribute to the emission. http://books.google.com/books?id=O3... frequencies in Short pulses of light&f=false This assertion sounds strange if we recall the well-known fact that the Fourier transform of the pulse is the broader the shorter is the pulse. Maybe there is a hint in the word "incoherent" which he used. This word is used very often in spectroscopy, but I never found a clear and simple definition. My understanding is that incoherent light is "light whose many spectral components have random phases", i.e. light whose E(t) is chaotic. Is this right? If so, even a chaotic light in a pulse still can be Fourier transformed and the relation between the length of the pulse and the distribution of frequencies holds.