Figure 12.

The device used to test the accuracy of the system. The glass plate ($400\mathrm{mm}\times 500\mathrm{mm}$) is painted with white matt paint, in which 69 marked points are struck on with known 3D coordinates in $o_w{x}_w{y}_w{z}_w$ measured accurately by TRITOP system. A ball arm with two standard spheres is put on this glass plate to test the accuracy of the system, the standard radii of sphere 1 and sphere 2 and their distance are 20.030 mm, 20.057 mm, 198.513 mm, respectively.