To optimize the operation of an installation for the manufacture of clinker, which installation comprises a rotary tubular kiln having an open discharge end for the clinker produced in the kiln, a cooler for the clinker connected to the discharge end, a hood covering the open discharge end and the cooler, hot clinker falling from the open discharge end into the cooler in a downward direction and cooling air passing in the cooler through the hot clinker and heated by the hot clinker, and a burner mounted in the hood and extending into the open discharge end of the rotary tubular kiln, the calorific energy radiated by the falling hot clinker is measured across the atmosphere of the hood to establish a control value, the measured control value is compared with a predetermined reference value for the radiated calorific energy, and at least one operating parameter of the installation is controlled in response to the difference between the control and reference values.