Method and apparatus for optically testing (e.g., using a laser beam) an operating integrated circuit (device under test--DUT) that actively control the operating temperature of the DUT. This is chiefly useful with flip-chip packaged ICs. The temperature of the DUT varies with its operating power consumption, and this fluctuation in temperature adversely affects the results obtained during optical probing or other optical testing. Furthermore, the DUT may be damaged if its temperature exceeds design limits. The temperature of the DUT is controlled by thermally contacting the exposed backside surface of the DUT die to a diamond film heat conductor, an associated heat sink structure, and at least one thermoelectric device. The thermoelectric device is controlled by a temperature sensor proximal to the DUT. By controlling the amount and direction of the electrical current supplied to the thermoelectric device in response to the sensed temperature, the temperature of the DUT is maintained.