Abstract: Gap bulk density (GBD) of a support mat surrounding a catalyst substrate in a catalytic converter is calculated using an average gap width optically determined by a camera system. Mat weight determined at an assembly station is bar coded and placed in a bar code label attached to the converter under test. A programmable controller calculates an average gap width from a plurality of camera readings. GBD is then calculated using mat weight and dimensions, and the GBD is compared to an acceptable range to determine pass/failure of the converter.