A system for measuring periodic displacement of an object in periodic motion at a first frequency includes a light source module, a camera module and a processor. The light source module is configured to periodically illuminate the object at a second frequency that is different from the first frequency and that is related to a sum of the first frequency and a first constant. The camera module is configured to take pictures of the object. The processor is in communication with the camera module in order to receive the pictures, and is configured to determine periodic displacement of the object based on the pictures.