The colour temp. control system for an object uses two sources of radiation. The appts. includes an input element (1) near the object, a unit (2) to set the required value, a setting unit (3) with a scale (4) and a monochromator (5). The latter is followed by a radiation modular (6) and receiver (7), which produces signals amplified by an amplifier (8) with output terminals (11) connected through to the energy source (15/16) and a furnace heater (17) for the heated object (18). Between the input element, typically a lens, and the monochromator is an optically combined absorber (13) and prism (12) with connections to the setting unit (3) and scale (4).