the invention relates to an apparatus for monitoring extruded products moving in an inline extrusion process so as to affect quality control of the process by continuously measuring dimensional parameters and determining the existence of contaminants in the extrusion . the apparatus makes use of terahertz radiation , which is adapted to provide a curtain of parallel rays of the radiation , which is scanned across the product as the product passes there - through in a linear manner . the composition of the omitted radiation received after the scanning process is subject to an imaging analysis to determine the dimensional parameters and contaminant free integrity of the extrusion process .