coherent/incoherent gain of fft window

Hi

I'm right now dealing wiht calculation of signal to noise ration from 
an fft transform. Specificly I would like to know more about the
coherent and incoherent gain of a fft window (such as Hanning etc.).

I have read that if you are calculating the SNR by dividing the signal
bin with the sum of the power of the other bins, then you also have
to include a correction factor due to the difference between the
coherent and incoherent gain of the window applied.

Does anyone have a reference on this topic or could explain it a bit
more in detail?

Regards
/Johan