Abstract: A thermometer is provided for remotely measuring the temperature of a heat source. The device utilizes an infrared sensor for detecting the temperature of a remote object and an output of the sensor is processed and used to drive a meter or other display device. An improved resonant circuit is provided for modulating the radiant energy impinging on the sensor to produce a pulsed output from the sensor. The modulating circuit includes a piezoelectric reed which deflects under an applied frequency and is located in the optical path of the system. The reed serves as an active element in an improved phase-locked loop used to drive and stabilize the optical chopper.