A folding top for a convertible is proposed, comprising a folding-top cloth and a folding-top linkage which is provided on both sides with a main link which is mounted on a main bearing and to which a frame part, which extends substantially in the longitudinal direction of the vehicle in the closed position of the top, and an intermediate link are coupled. An incident angle of a light beam entering into an exit face of light is gradually reduces by repeating total reflection on these two faces so as to make is smaller than a critical angle, and to emit the light beams outward of the light emitting device. The data processing means generates a payload size signal indicative of the size of the payload.