The visibility amplitude bias (there is also a closure phase bias, but this one is nominally corrected by the raw data averaging pipeline) is related to non-Poisson detector statistics due to after-pulsing, and to fixed pattern noise due to non-linearities in the modulation stroke. We determine the bias using so-called incoherent scans, which record data even though the delay lines are deliberately set away from the white light fringe. ... Averaging triple products.. ... Background data loaded. ...