An apparatus for performing a light-absorption measurement on a test sample and a compliance light absorption measurement on at least one reference sample, the apparatus comprising: an illumination system configured to direct light along a light path; a test sample receptacle holder configured to receive the test sample receptacle at a first position in the light path for a test sample measurement; a reference sample receptacle carrier including at least one holding area configured to receive a reference sample receptacle for a reference sample measurement; a detection system configured to detect absorption of light by the test sample during the test sample measurement and configured to detect absorption of light by a reference sample during the reference sample measurement; and an actuator connected to the reference sample receptacle carrier by a linear conveyor, the actuator configured to move the reference sample receptacle carrier to position the reference sample receptacle received in a respective one of the at least one holding area at a second position in the light path at a first point in time, and at a third position out of the light path at a second point in time, wherein the first position and the second position are situated in the light path at different locations along the light path; characterized in that the apparatus further comprises a support member coupled to the illumination system, the test sample receptacle holder and the detection system; and a carrier assembly that includes the reference sample receptacle carrier, the actuator, the linear conveyor, and a cover, wherein the carrier assembly is configured to be removably attached to the support member, and wherein the cover is configured to at least partially shield the reference sample receptacle carrier, the actuator and the linear conveyor from an environment outside of the apparatus. an illumination system configured to direct light along a light path; a test sample receptacle holder configured to receive the test sample receptacle at a first position in the light path for a test sample measurement; a reference sample receptacle carrier including at least one holding area configured to receive a reference sample receptacle for a reference sample measurement; a detection system configured to detect absorption of light by the test sample during the test sample measurement and configured to detect absorption of light by a reference sample during the reference sample measurement; and an actuator connected to the reference sample receptacle carrier by a linear conveyor, the actuator configured to move the reference sample receptacle carrier to position the reference sample receptacle received in a respective one of the at least one holding area at a second position in the light path at a first point in time, and at a third position out of the light path at a second point in time, wherein the first position and the second position are situated in the light path at different locations along the light path; characterized in that the apparatus further comprises a support member coupled to the illumination system, the test sample receptacle holder and the detection system; and a carrier assembly that includes the reference sample receptacle carrier, the actuator, the linear conveyor, and a cover, wherein the carrier assembly is configured to be removably attached to the support member, and wherein the cover is configured to at least partially shield the reference sample receptacle carrier, the actuator and the linear conveyor from an environment outside of the apparatus.