A method of continuously casting molten steel to produce high surface quality slab by preventing depressions and cracking. Radiation from a plurality of optical radiation sources travels along a first optical path. A waveform DB stores prerecorded waveform data together with pitch mark data imparted thereto. The strap extending between the loops can be adjusted between the loops so that when the first and second loops are positioned about the elongate object, the strap is sized to prevent either loop from being removed from the object.