An apparatus for determining the perimeter of an object being scanned comprises a movable arm adapted to follow the surface of an object being scanned as the arm is orbited around the object; the arm having one end being adapted to maintain contact with the surface as the arm is orbited around the object; and an encoder operably connected to the arm for determining the movement of the lever arm at each angular position around the object. An opaque cup is positioned at one end of the lever arm immediately over a point on said surface at which a laser beam impinges on the surface of the object being scanned such that reflection from the surface is retained within the cup.