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).