Abstract: A method and apparatus for monitoring the fiber surface development of pulps due to the refining or beating of mechanical or chemical pulps; an optical cell, fabricated with light-transparent windows, permits an incident collimated beam of light to pass through pulp stock flowing through the cell. The beam is transmitted through a polarizing filter prior to entering the cell, and through a second, crossed, polarizing filter, on exiting from the cell. The exit beam converges onto a photodetector. A gradual variation in the mass of fibers of the pulp stock is established over a predetermined range and this produces a change in the intensity of the transmitted light. As the light intensity changes, it passes through a maximum value; this is accompanied by a similar change in the photodetector electrical output signal.