A measuring apparatus comprises a detector device for detecting a variable to be measured, and a controller operative to control the detector device and generate an output signal indicative of the magnitude of the variable being measured. The detector device comprises a housing on which are mounted two Peltier-Seebeck detectors, the detectors being arranged on the housing such that only the first Peltier-Seebeck detector is exposed, in use, to the variable to be measured. The controller is operative to generate the output signal based on the output of the first Peltier-Seebeck detector and the output of the second Peltier-Seebeck detector so as to account for the effect of the ambient heat on each Peltier-Seebeck detector.