A radiation sensor includes an integrated circuit radiation sensor chip (1A) including first (7) and second (8) thermopile junctions connected in series to form a thermopile (7,8) within a dielectric stack (3). The first thermopile junction (7) is insulated from a substrate (2) of the chip. A resistive heater (6) in the dielectric stack for heating the first thermopile junction is coupled to a calibration circuit (67) for calibrating responsivity of the thermopile (7,8). The calibration circuit causes a current flow in the heater and multiplies the current by a resulting voltage across the heater to determine power dissipation. A resulting thermoelectric voltage (Vout) of the thermopile (7,8) is divided by the power to provide the responsivity of the sensor.