The mixed reactants pass an observation cell that allows the reaction to be followed spectrophotometrically, typically by ultraviolet spectroscopy, fluorescence spectroscopy, circular dichroism or light scattering, and it is now common to combine several of these.
Observation cuvette with a short light path (0.75 to 1.5mm) are usually preferred for fluorescence measurements to reduce self-absorption effects. Observation cuvette with longer light path (0.5 cm to 1 cm) are preferred for absorbance measurements. Modern stopped-flow can accommodate different models of cells and it is possible to change the cuvette between two experiments. 
For stopped-flow X-ray measurements, a quartz capillary with thin wall is used to minimize quartz absorption. Simultaneous x-ray and absorbance measurements are possible in the same capillary.