The noise of the array in the LW channel presents several components: a
high frequency term which is well approximated by a constant readout
noise of 
180 e
/px rms added to the photon shot noise, and
a low frequency term which becomes important after several tens of
readouts (depending on the flux intensity and the integration time). At
high level, 
e/px, an amplifier noise becomes dominant
and limits the Signal to Noise ratio to 
in a single exposure.
The optimum performance is obtained with a signal between 2
and 1
e/px.