The invention relates to a method, comprising illuminating an area of a wood fibre web by directing ultraviolet light to an area of the web by an ultraviolet light source, capturing an image of at least part of the illuminated area by an imaging device comprising a colour filter, transmitting image data of the captured image to an image data processing device, and analysing if the captured image comprises a fluorescent liquid stain emitting fluorescent light passed through the colour filter. The invention further relates to a machine vision system and a computer program product for detecting fluorescent liquid stains from a wood fibre web.