1. An apparatus for rapid and precise scanning of a three-dimensional (3-D) occlusal profile of a dental cast, comprising: an optical projection unit, including a set of optical lenses and semiconductor laser devices for generating a light source by means of two vertically symmetrical semiconductor laser diodes arranged at suitable orientation of inclination, wherein the light source generates a structured light pattern after passing through the optical lenses, wherein the structured light pattern is subsequently projected onto an object to be measured through the optical lenses; an image capturing unit, including three sets of optical lenses and image sensing devices situated at various positions for acquiring images of a deformed structured light pattern representing the occlusal profile to be measured, wherein the image sensing devices output information representing the images to a main control unit; a precision rotating platform unit, including a precision rotating platform and a dental cast vise, wherein the precision rotating platform unit is able to precisely rotate the object to be measured to various measuring orientations for complete scanning of the 3-D occlusal profile of a single tooth; and a main control unit, connected to the optical projection unit and the image capturing unit, for modulating the structured light pattern output by the optical projection unit and processing the image information acquired by the image capturing unit.