Line-scan camera.

A camera that images one row of pixels at a time off material moving past it, assembling the picture line by line, where an area-scan camera grabs a whole frame at once. It only produces a usable image when it is synced to a known, steady line speed. Mismatch the camera to the motion and the image smears or tears, which is why putting one behind a dryer tunnel is an engineering project rather than a bolt-on.