You recieve the character report cluster from the OCR read3 VI, use it to perform manual treshold in the image using the standard treshold functions.
OCR will perform very poorly on the kind of images you supplied, you will probably need some self made function to determine the character bounding rectangle.
By the way, isnt the example you supplied an image of some marked metalurgical product e.g. ingot. I remember those things having such dotted characters.