:What should be made clearer in my opinion is the fact that the Fourier theorem  implies that the whole spectrum is actually phase-locked and that thus the bandwidth/coherence_time relationship can not be applied to natural light. 

 :Anyway, is there a reference for the statement that white light has a coherence time of only 10 periods or so? Is this merely a theoretical figure based on the spectral width of white light or has it been measured? And if the latter, how does one know that the coherence time is actually not determined by the length of the individual light pulses (as given by the individual atomic emissions) rather than the combined bandwidth. I found actually a reference where the coherence length of white light has been measured to be about 200 micrometers, which is subtantially more than the generally quoted value of a few micrometers (see http://www.iop.org/EJ/abstract/0143-0807/24/4/363 ). 

 :Thomas