Methods for the continuous determination of the properties of a flow of wood fibers for use in the production of fiberboard are disclosed including determining reference value characteristics for calibration samples of the fibers, determining predetermined relationships between those reference value characteristics and measured spectral values for the fibers utilizing multivariant statistical regression methods, measuring reflectance spectral values for the fibers illuminated by light, and determining the fiber characteristics relating to the properties from the measured spectral values utilizing the predetermined relationship.