PURPOSE:To discover the generation of the surface flaw of heat buckle, etc., in an early stage by making scanning to a transverse direction in plural positions at the same time with laser beam to the surface of a strip. CONSTITUTION:Plural laser oscillators 1a and optical systems are provided in the inner part of a projector 1. The surface of strip 5 is irradiated with the laser spot from the projector 1 and the scanning is made to the transverse direction in the plural positions in parallel to the running direction of the strip 5 so as to contain a counter-bored part in a hearth roll. This reflected beam is received with a photoelectric conversion element and the obtd. signal is treated. By this method, error judement due to the intrusion of the noise, etc., is reduced, and symptom at early stage of the heat buckle can be checked from a little deformed quantity.
展开▼
