An instrument in which a mirror is automatically moved so that it reflects sunlight in a constant direction. It is used with a pyrheliometer to make continuous measurements of solar radiation.

• 定日镜一种仪器，装有一面会自动调控的镜子，以便不断朝某一恒定方向反射日光。与太阳热量计同时使用，以保持对太阳辐射的不间断测量