A method for determining the relative phase retardations and orientations of the optical axes of a multi-layered birefringent wood or cellulose fibre specimen, which is an intact cellulosic fibre with two opposite walls with the same thicknesses in S 1, S 2 and S 3 layers, the two outside layers S 1 and S 3 having microfibrils oriented transversely with respect to the fibre longitudinal axis, and the middle dominant layer S 2 having microfibrils wound in a helix at fibril angle θ forming optical axes orientated at ± θ at the two opposite walls, respectively, comprising the steps of: producing circularly polarized light in a dark field or a bright field, said light having a plurality of well-defined wavelengths, wherein the plurality is at least the same number as the number of parameters to be determined in the multi-layered specimen under evaluation; impinging the circularly polarized light on the specimen to be evaluated; recording and measuring the light intensities of the different wavelengths of the plurality of well-defined wavelengths emergent from the specimen; and (i) in the case of the dark field, determining the at least one parameter from the light intensities of the emergent wavelengths by fitting the data with Equation if both S 1 and S 3 layers are not so thin as to be insignificant: or by fitting the data with Equation if the S 1 and S 3 layers are so thin as to be insignificant: wherein θ is the fibril angle of a fibre in the S 2 layer, I fibre,dark (ΔS 1, ΔS 2, ΔS 3, θ) denotes the light intensities of the emergent wavelengths, ΔS 1, ΔS 2, ΔS 3 each denoting relative phase retardations of S 1, S 2 and S 3 layers, respectively, and I o denotes intensity of the light incident on the specimen; or (ii) in the case of bright field, determining at least one parameter from an equation corresponding to Equation or derivable from the equation I bright = I 0 - I dark, wherein I bright and I dark denote the transmitted light intensity under the bright field circularly polarised light system and the dark-field circularly polarised light system, respectively, by fitting the data with the bright field equation corresponding to the dark field Equation or, wherein the method is performed on-line in real time. producing circularly polarized light in a dark field or a bright field, said light having a plurality of well-defined wavelengths, wherein the plurality is at least the same number as the number of parameters to be determined in the multi-layered specimen under evaluation; impinging the circularly polarized light on the specimen to be evaluated; recording and measuring the light intensities of the different wavelengths of the plurality of well-defined wavelengths emergent from the specimen; and (i) in the case of the dark field, determining the at least one parameter from the light intensities of the emergent wavelengths by fitting the data with Equation if both S 1 and S 3 layers are not so thin as to be insignificant: or by fitting the data with Equation if the S 1 and S 3 layers are so thin as to be insignificant: wherein θ is the fibril angle of a fibre in the S 2 layer, I fibre,dark (ΔS 1, ΔS 2, ΔS 3, θ) denotes the light intensities of the emergent wavelengths, ΔS 1, ΔS 2, ΔS 3 each denoting relative phase retardations of S 1, S 2 and S 3 layers, respectively, and I o denotes intensity of the light incident on the specimen; or (ii) in the case of bright field, determining at least one parameter from an equation corresponding to Equation or derivable from the equation I bright = I 0 - I dark, wherein I bright and I dark denote the transmitted light intensity under the bright field circularly polarised light system and the dark-field circularly polarised light system, respectively, by fitting the data with the bright field equation corresponding to the dark field Equation or, wherein the method is performed on-line in real time. (i) in the case of the dark field, determining the at least one parameter from the light intensities of the emergent wavelengths by fitting the data with Equation if both S 1 and S 3 layers are not so thin as to be insignificant: or by fitting the data with Equation if the S 1 and S 3 layers are so thin as to be insignificant: wherein θ is the fibril angle of a fibre in the S 2 layer, I fibre,dark (ΔS 1, ΔS 2, ΔS 3, θ) denotes the light intensities of the emergent wavelengths, ΔS 1, ΔS 2, ΔS 3 each denoting relative phase retardations of S 1, S 2 and S 3 layers, respectively, and I o denotes intensity of the light incident on the specimen; or or by fitting the data with Equation if the S 1 and S 3 layers are so thin as to be insignificant: wherein θ is the fibril angle of a fibre in the S 2 layer, I fibre,dark (ΔS 1, ΔS 2, ΔS 3, θ) denotes the light intensities of the emergent wavelengths, ΔS 1, ΔS 2, ΔS 3 each denoting relative phase retardations of S 1, S 2 and S 3 layers, respectively, and I o denotes intensity of the light incident on the specimen; or (ii) in the case of bright field, determining at least one parameter from an equation corresponding to Equation or derivable from the equation I bright = I 0 - I dark, wherein I bright and I dark denote the transmitted light intensity under the bright field circularly polarised light system and the dark-field circularly polarised light system, respectively, by fitting the data with the bright field equation corresponding to the dark field Equation or, wherein the method is performed on-line in real time.