A system for coupling photometers to an incubation ring for use in in vitro diagnostics comprises one or more light sources, and an incubation ring assembly, and two photometers. An incubation ring assembly comprises an internal trough and an external trough. Each trough comprises (a) an internal wall comprising an internal aperture and (b) an external wall comprising an external aperture. A first photometer comprises: a first optics housing directing light from the light sources through the external aperture of the internal trough, and a first detector positioned to receive the light through the internal aperture of the internal trough. A second photometer comprises a second optics housing directing the light from the light sources through the internal aperture of the external trough, and a second detector positioned to receive the light through the external aperture of the external trough.