Abstract: A method for correcting optical path length measurement errors of a measurement scanner at a laser processing optical unit includes incoupling a measurement beam of the measurement scanner for distance measurement purposes coaxially into a processing laser beam, moving the measurement beam laterally in an x-y plane over a workpiece in a vicinity of the processing laser beam, measuring distance values by the measurement scanner at different measurement points of the workpiece, and correcting the distance values in a z-direction by change values. The change values are obtained from calculated or previously known optical path lengths of the measurement beam at different selection points in the x-y plane.

Abstract: The present invention provides erythrocyte-retention substrates of the general formula:R.sub.1 --Sp--R.sub.2in which Sp is a bridge serving as a spacer, R.sub.1 is a strongly polar group and R.sub.2 is also a strongly polar group or, when Sp is an electron-conductive group, R.sub.2 can also be a polarizable group.The present invention also provides a process for the separation of erythrocytes from whole blood, wherein whole blood is brought into contact with one of the above substrates and subsequently filtered through a filter layer and the separated plasma obtained.