An apparatus for determining a vertical position of at least one interface between a first component and at least one second component, the components (104, 106) being comprised as different layers in a sample container, the apparatus comprising: - a first sensing unit comprising a first laser diode configured to emit light having a first wavelength and a first light detector configured to generate a first sensing signal in response to an intensity of light having the first wavelength being applied to the first light detector, - a second sensing unit vertically spaced by a given vertical distance from the first sensing unit and comprising a second laser diode configured to emit light having a second wavelength and a second light detector configured to generate a second sensing signal in response to an intensity of light having the second wavelength being applied to the second light detector, - a driving unit configured to move the sample container relative to the first sensing unit and the second sensing unit, - 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 sensing signal and the second sensing signal such that first sensing signal 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, characterized by - a calibration device comprising a plurality of bars and a plurality of openings arranged between the bars, wherein the driving unit is configured to move the calibration device relative to the first sensing unit and the second sensing unit, wherein the vertical position determining unit is configured to determine a vertical orientation of the driving unit relative to the first sensing unit and the second sensing unit based on the first and second signals (124, 136) generated during a movement of the calibration device relative to the first sensing unit and the second sensing unit. - a first sensing unit comprising a first laser diode configured to emit light having a first wavelength and a first light detector configured to generate a first sensing signal in response to an intensity of light having the first wavelength being applied to the first light detector, - a second sensing unit vertically spaced by a given vertical distance from the first sensing unit and comprising a second laser diode configured to emit light having a second wavelength and a second light detector configured to generate a second sensing signal in response to an intensity of light having the second wavelength being applied to the second light detector, - a driving unit configured to move the sample container relative to the first sensing unit and the second sensing unit, - 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 sensing signal and the second sensing signal such that first sensing signal 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, characterized by - a calibration device comprising a plurality of bars and a plurality of openings arranged between the bars, wherein the driving unit is configured to move the calibration device relative to the first sensing unit and the second sensing unit, wherein the vertical position determining unit is configured to determine a vertical orientation of the driving unit relative to the first sensing unit and the second sensing unit based on the first and second signals (124, 136) generated during a movement of the calibration device relative to the first sensing unit and the second sensing unit.