PURPOSE:To scatter the vapor to the outside from the surface of a test piece, upon which the light having passed through an inner air one of dual annular air curtains impinges, by forming the dual annular air curtains so that air may be injected from the helical groove of the outer annular curtain. CONSTITUTION:A lasser is used to measure the moving speed of a steel plate in a non-contact manner. Dual annular air curtains 7 and 25 are formed so that the laser beam 4 from a sensor and the resultant reflected light 5 may pass through the inner air curtain 7. A purge air 7 is blown onto a test piece 6 form the nozzle 1 thus constructed thereby to remove water 8 and vapor 9. The spurting speed of the inner air curtain 7 is made larger than that of the outer air curtain 25 so that the purge air 7 may be prevented from being mixed with the vapor. The outer air curtain 25 is made to be injected air 29 from the annular clearance 27 and to have a vortex component so that the purge air 27 may be prevented from being mixed with the vapor.
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