  During the commissioning of the first of the two detectors of the Double
Chooz experiment, an unexpected and dominant background caused by the emission
of light inside the optical volume has been observed. A specific study of the
ensemble of phenomena called "Light Noise" has been carried out in-situ, and in
an external laboratory, in order to characterize the signals and to identify
the possible processes underlying the effect. Some mechanisms of instrumental
noise originating from the PMTs were identified and it has been found that the
leading one arises from the light emission localized on the photomultiplier
base and produced by the combined effect of heat and high voltage across the
transparent epoxy resin covering the electric components. The correlation of
the rate and the amplitude of the signal with the temperature has been
observed. For the first detector in operation the induced background has been
mitigated using online and offline analysis selections based on timing and
light pattern of the signals, while a modification of the photomultiplier
assembly has been implemented for the second detector in order to blacken the
PMT bases.
