A method and arrangement are disclosed for measuring a flow rate of optically non-homogeneous material in a process pipe. The non-homogeneous material can be illuminated through a window. Images are taken with a camera, through a window, of illuminated non-homogeneous material. Correlation between temporally successive images determines travel performed by the non-homogeneous material in the process pipe between capture of temporally successive images. Velocity of the non-homogeneous material is determined by the time difference between the successive images and the travel.