Quality and condition monitoring based on spectrum separating measurement

A method of controlling the quality and/or condition of a fibre web in a process for manufacturing and/or finishing the fibre web, which comprises:

• monitoring the fibre web with at least one optical spectrum separating measurement device;
• determining at least one quality variable of the fibre web;
• performing temporal and spatial measurements of transmitted, reflected, or emitted electromagnetic radiation from the fibre web, synchronously with a movement of the fibre web, using the at least one optical spectrum separating measurement device, and storing the temporal and spatial measurements in the form of spectral data;
• generating a continuous quality variable chart using the spectral data, wherein the continuous quality variable chart is substantially continuous in the direction of movement of the fibre web and represents the at least one quality variable as a function of the position of the at least one quality variable on the fibre web;
• dividing the continuous quality variable chart into successive matched partial charts having a cycle length, wherein the cycle length corresponds to a length of influence of a rotating/moving means in the process for manufacturing and/or finishing the fibre web;
• detecting deviations and/or discontinuities of the at least one quality variable from the successive matched partial charts;
• and
• detecting malfunctioning of the rotating/moving means using the detected deviations and/or discontinuities of the at least one quality variable.