This invention provides a novel, rapid method and device for determining the
relative phase retardation of a multi-layered specimen, which is related to the thicknesses of its
layers and walls, and the orientations of its optical axes. An intact wood
pulp fibre is a typical multi-layered birefringent specimen. This new method is based on the change of polarization of polarized light that passes through a specimen composed of birefringent
layers with different
optical axis orientations, such as directions of cellulosic microfibrils oriented differently in various
layers of wood fibres. In particular, a novel solution is found to relate the emerging
light intensity from an intact
wood fibre to the incident
light intensity, the
wavelength of the light, and the
relative phase retardations of various layers and orientations of their cellulosic microfibrils, such as
fibril angle, in a circular polarized light
system. This new method evaluates the
transmitted light intensities of multiple predetermined wavelengths simultaneously to determine the optical and physical properties of a multi-layered specimen being measured. A device for determining the
relative phase retardation (retardance) of fibre walls and the
fibril angle of intact wood fibres in accordance with the presented invention comprises a
light source with well defined multi-wavelengths, an achromatic circular polarized light
system, appropriate imaging
optics, a multi-channel camera such as a digital color camera that has two or more
wavelength (color) detection channels, and an
image processing and
data analysis system. The measurements take
advantage of the
birefringence of cellulosic microfibrils, and thus require neither
sample preparation nor
high resolution optics. Specimen alignment is not required as specimens such as wood fibres are evaluated under circular polarized light. Compared with other methods, this invention is more rapid, accurate, and robust. This method can be automated, and implemented in a fibre flow-through system, thus allowing a
rapid assessment of wood
pulp fibre properties (on-line in real time).