An apparatus for determining a vertical position of an interface between a first component and a second component comprising different layers in a sample container comprises a first unit comprising a first emitting light, a first optics, and a first detector; a second unit vertically spaced from the first unit comprising a second emitting light, a second optics, and a second detector; a driving unit to move the first unit and the second unit relative to the sample container; a position sensing unit to output a position sensing signal indicative of a vertical position of the sample container; and a vertical position determining unit to calculate the vertical position of the interface.