The central temperature of the infrared image and the temperature variation range of the entire infrared image vary depending on the observation object. Therefore, the core temperature and temperature variation range required to be displayed should be adjustable. Since the static operating point of the amplifier is determined, the selection of the central temperature can be achieved by changing the DC level of the video signal (as the input signal of the amplifier), and the selection of the temperature range can be achieved by changing the gain of the amplifier.