In this thesis formation of ligament and fibers, formation of larger clusters of fibers and their further accumulation and formation of so called primary layer is investigated. Due to difficult measuring techniques in real industrial environment of the process, two laboratory experimental studies were conducted on model spinner and model collecting chamber. Measuring method was performed by means of visualization with high speed camera and computer analysis of the data. Study on model spinner was focused on formation of fibers generated by perforated rotor, their intertwining and formation of typical larger structures. Results were presented in form of multiple regression models. Study on model collecting chamber was conducted to develop measuring method for determining primary layer mass distribution. The principle of this method is absorption of light through fibrous material. Relation between operation parameters and primary layer mass distribution and its texture was found. This relation was quantitatively formulated in form of multiple regression models. Developed method for measuring mass distribution of accumulated fibers was then used in real industrial plant, where it enabled quantitative evaluation of primary layer characteristics and their relation to operating parameters. Better understanding of the process led to the design of new technical solution for the collecting chamber which is patent protected.