An apparatus is disclosed that develops output signals representing the degree of finished state of a processed medium during a process operation, such as the lignin content of wood pulp during delignification or, alternatively, the brightness during the bleaching process. Broad spectrum light energy from a light energy source is injected into the processed medium and the resultant reflected light energy is collected from a first light collector located near the point of injection and a second light collector located far from the point of injection. The light energy collected by each light collector is conveyed to a respective light analyzer associated with each light collector. Each light analyzer divides the detected light into distinct spectral wavelengths and generates output signals representing the intensity of light energy received in each wavelength. A feedback arrangement conducts the light energy emitted by the source to a location proximate the point of injection and then to a light analyzer. The light analyzer associated with the feedback arrangement divides the light emitted by the light source into distinct spectral wavelengths and generates output signals representing the intensity of the light energy emitted by the source in each wavelength. The output signals can be used by a process monitoring system to examine the degree of finished state of the processed medium.