This paper presents an innovative and powerful imaging based measuring technology for online measurement of paper formation. The optical sensor installed on a traverse system is able to perform robust and reliable imaging and image processing on fast paper machines. The velocity of the paper track can be up to 30m/s still ensuring a comprehensive measurement of paper formation in industrial environment. The measuring principle of the introduced measurement method is based on extensive analysis of porosity of the paper. Paper images are taken online with transmitted light and analysed with 2D Fourier transform extracting spectral paper formation parameters. The paper presents the system concept and explains special hardware and software solutions. The performance of the sensor is verified with a functional prototype.