The Measurement of Fibril Angle of Wood Fibers Using Polarized Light

The fibril angle of the S 2 layer of wood pulp fibres can be determined microscopically from the extinction position of intact cell walls between crossed polars. It is always assumed in this method that the S 1 and S 3 layers are so thin that they do not affect the state of polarization of light. The complete theory of the extinction position has been derived, taking into account the contribution of the S 1 and S 3 layers, for the light path used in the mercury reflectance method of Page. For certain thicknesses of the S 2 layer, the presence of the S 1 and S 3 layers can introduce serious errors in the fibril angle measurement. However, there is a range of S 1 , S 2 , and S 3 thicknesses for which the error is small and the fibres of most softwoods lie within this range. For thick-walled species the method must be used with caution.