The Birefringence of Wood Pulp Fibres and The Thickness of the S1 and S3 Layers

The birefringence of pulped softwood fibres has been measured and found for several species to decrease with decreasing cell-wall thickness. This is explained theoretically by the compensation of the retardation of light in the S2 layer by the transversely wound S1 and S3 layers. These layers are of similar thickness for all fibres and thus have a greater effect on thin-walled fibres. Measurement of birefringence over a range of fibre wall thicknesses permits a determination of the thickness of the S1 and S3 layers, and the results are in good agreement with data from other methods

The Measurement of Fibril Angle of Wood Fibers Using Polarized Light

The fibril angle of the S2 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 S1 and S3 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 S1 and S3 layers, for the light path used in the mercury reflectance method of Page. For certain thicknesses of the S2 layer, the presence of the S1 and S3 layers can introduce serious errors in the fibril angle measurement. However, there is a range of S1, S2, and S3 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