This invention provides a novel, rapid method and device for determining the relative phase retardation of different layers 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 circularly 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 circularly 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 circularly 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).