An apparatus for determining a vertical position of at least one interface between a first component and at least one second component, the components comprised as different layers in a sample container. The apparatus comprises a first sensing unit and a first light detector configured to generate a first sensing signal, a second sensing unit comprising a second light detector configured to generate a second sensing signal, a driving unit configured to move the sample container, a position sensing unit configured to output a position sensing signal indicative of a vertical position of the sample container, a vertical position determining unit configured to match the first and the second sensing signal such that first and the second sensing signal correspond to identical vertical positions, and to determine the vertical position of the at least one interface in response to the matched sensing signals and the position sensing signal.