Once more thanks for all the reactions.
I checked the reactions again this morning, and it is clear that the right answer/suggestion came from David San Segundo Bello (imec). Already in one of the very first reactions, he mentioned a possible drift of the LED light source due to an AC variation on top of the DC voltage. Afterwards Guy Meynants (CMOSIS) repeated the answer of David, but also added to it the method to check it out, namely by means of noise measurements. That actually completed the story. So I think it is fair to give both guys a bottle of wine.
Albert, 09-09-2013.