A new version of the Chang method for the determination of the optical birefringence of nematic liquid crystals

We report a new experimental technique for the measurement of the optical birefringence An of nematic liquid crystals. This new version of Chang's method is verified for (p-etoxy-benzylidene)-p-n-butylaniline (EBBA). The cell filled with EBBA is placed between crossed polarizers which make an angle of 450 with the optical axis. The wavelength dependence of the light intensity transmitted by the analyzer has been recorded at various temperatures. In the Chang method, by using the wavelengths A4 and A,+1 of two consecutive minima (), < A,i+1), and by considering that An(A4) =. An(A,+1) the optical birefringence is calculated according to the relation An= A A,+ald(A,+1-Ad), where d is the thickness of the cell. The accuracy of Chang's method is affected on the one hand by considering only of wavelengths corresponding to minima of the transmitted light and by the other hand by completely neglecting the dispersion of optical birefringence. The method proposed here solves both weaknesses by considering all the experimental data, contained into a large spectral range, and by using the dispersion of the optical birefringence given by the three-band model. By applying a nonlinear fitting procedure on the recorded experimental data we have obtained the parameters involved in the expressions of the optical birefringence and we have computed the optical birefringence of EBBA at these temperatures. Our results are in a very good agreement with the corresponding ones obtained by using the Talbot-Rayleigh method. Unlike the Talbot-Rayleigh method, which mainly determines the ordinary and extraordinary refractive indices, that can be further used at the computation of optical birefringence, our method allows for the direct determination of the birefringence, without other intermediate steps.