The dark level of the LW array is the sum of 2 effects, a leakage of charges generated during the commutation of the reset transistor, and thermal charge generation in the photoconductors.
The first effect dominates for integration time up to 10 seconds. For longer integration time, charge generation dominates. However, in the ground calibration data, a small contribution from the cryostat background cannot be excluded. This behavior should be verified in flight.
The dark frame pattern (Figure 
) exhibits a strong line
effect with a separation between odd and even lines. The mean dark
signal changes by a few ADU at each switch on/off of the array, but the
pattern remains very stable (
0.3 ADU). A measurement of a dark
frame, with 5 sec. integration time, will be provided for each orbit,
with an accuracy of 0.3 ADU rms, as part of the common user
calibration. Observers needing a very precise absolute photometry of
faint objects, or whishing to detect very faint objects, may require a
dark measurement at the end of their observations (see also
section ).