1. A method for determining an optimal exposure of a structured light based three dimensional (3D) camera system including a projecting means for illuminating a predetermined pattern on a target object, an image capturing means for capturing an image of the target object with the pattern projected, and a processing means for reconstructing 3D data based on the captured image, wherein the method comprises the steps of: obtaining an image of a target object with a predetermined pattern projected thereon, which is illuminated from a projecting means, and an image of the target object without a predetermined pattern projected thereon; and determining automatically an optimal exposure level of the structured light based 3D camera system using said two kinds of images.