Assays (100) may be performed with a luminometer (400) having a chassis (405) that may include a reaction vessel chamber (610). The luminometer (400) may also include a light passage (640) that intersects the reaction vessel chamber (610). The luminometer (400) may also include a cap (415) that, when in a closed configuration, prevents light emitted by external sources from entering the reaction vessel chamber (610) and from entering the light passage (640). The cap (415) may provide access to the reaction vessel chamber (610) when in an open configuration. The luminometer (400) may also include a calibration light source (460) optically coupled to one end of the light passage (640) and a light detector (630) optically coupled to another end of the light passage (640). The light detector (630) may include a sensing element for receiving light from the light passage (640).