Apparatuses and methods are disclosed for performing a light-absorption measurement on a test sample and a compliance measurement on a reference sample. A method includes moving a reference sample receptacle carrier of an apparatus to position a first reference sample receptacle received in the carrier at a second position in a light path, directing light from an illumination system to a detection system along the light path to perform a light-absorption measurement on a first reference sample at the second position, moving the carrier to position the first receptacle out of the light path, placing a test sample receptacle in a test sample receptacle holder of the apparatus in the light path at a first position different from the second position, and directing light along the light path to perform a light-absorption measurement on a test sample in the test sample receptacle holder at the first position.