A sensor device formed on a semiconductor substrate. The device comprises a thermal radiation sensor including a sensing cell and a referencing cell which are co-operable for providing a first output signal indicative of the temperature fluctuation resulting from incident radiation. A gradient sensor including a pair of cells spatially located on the semiconductor substrate is provided which are co-operable to provide a second output signal indicative of the temperature gradient across the semiconductor substrate for facilitating calibrating the first output signal. At least one of the cells of the gradient sensor is not common to the cells of the thermal radiation sensor.