Abstract: Thermal conditions inside an oven are measured by a transducer device (1) having plates providing heat absorbent and heat reflective surfaces (2,4) and a thermocouple for measuring the temperature difference between them as a measure of radiant heat. The plates are mounted on a thin thermally insulating layer secured to a heat sink in the form of a copper mass (7,9), and a second thermocouple measures the temperature difference between the heat absorbent surface and the heat sink as a measure of heat flux through the surface. A Peltier cooler (55) or a flowing liquid can be used alternatively as a heat sink. Heat flux alone can be measured by omission of the heat reflective surface, and where the heat sink is a flowing fluid the heat gained can be measured.