Abstract: An apparatus and method for controlling yarn coatings such as sizing are provided which includes nuclear radiation detectors which measure the attenuated radiation through processed yarn. Before and after radiation measurements are signaled to a computer which is programmed to generate a signal which controls squeeze rollers that adjust, according to specific size requirements. A second embodiment of the invention is provided for processing tire cord webs and utilizes digital line scan cameras in conjunction with the radiation detector sensors. Line scan cameras provide an opacity measure to determine yarn count and maintain an accurate density measurement even as lateral shifts in the web during processing occur.