  Laboratory (laser and Z-pinch) opacity measurements of well-characterized
plasmas provide data to assist inertial confinement fusion, astrophysics and
atomic-physics research. In order to test the atomic-physics codes devoted to
the calculation of radiative properties of hot plasmas, such experiments must
fulfill a number of requirements. In this work, we discuss some sources of
uncertainty in absorption-spectroscopy experiments, concerning areal mass,
background emission, intensity of the backlighter and self-emission of the
plasma. We also study the impact of spatial non-uniformities of the sample.
