Surface scanning system for selective deposition modeling

A closed loop selective deposition modeling apparatus having a surface scanning system for actively monitoring the surface height of a layer of a three-dimensional object as it is being built by selectively dispensing a build material. The surface scanning system directs a beam of energy on the surface of the object that establishes an illumination zone which emits scattered light, and has a detector which senses the scattered light and produces a response indicative of the surface condition of the object. The response is processed to establish a plurality of height data signals that are further processed to produce feedback data. The feedback data is then utilized to selectively dispense the build material to desired locations on the surface of the object to therein dimensionally normalize the layer of the object being formed.