Abstract: A dual channel photo-optical clot detection system for the automatic determination of prothrombin times and activated partial thromboplastin times and other related coagulation tests. A circular tray is utilized, and is vacuum formed to provide cuvettes having uniform wall thicknesses in two circular paths and arranged with radially aligned cuvettes. A single light source is placed between the radial spaced cuvettes and thus provides a common source for two separate detectors measuring the clot time of samples in each cuvette. The automatic dual channel operation provides a fixed time for activated partial thromboplastin time test so that fixed incubation time for the tests can be obtained. Thus, the sample to be tested is maintained at a low temperature until only two stations before measurement, and, accordingly, fixed incubation periods can be determined and exact conditions can be maintained for accurate testing.