A convenient and accurate technique for measurement the birefringence of liquid crystals at discrete wavelength is described. The method is based on the phase difference which occurs when monochromatic polarized light propagates through a medium with an anisotropic refractive index in the presence of ac electric fields. Absolute birefringence is measured with one experimental run by using PEM (photoelastic modulator) while two independent experiments are necessary in the conventional optical method. Birefringence measurements at 0.6328 $\mu$m in a nematic mixture are reported with different cell thickness. Absolute birefringence of the cell with some thickness agree with one of different thickness. We conclude that the method developed here is both accurate and reliable. And threshold voltage is measured from this experimental data. From the existing literature data of the dielectric anisotropy and measured threshold voltage, splay elastic constant is calculated. In addition, we develop the method which measure the transient time in a twisted nematic cell in the presence of ac electric field. The DATA 6000 (Data Precision Co.) is used in the experiment for the rapid data acquisition. Rise and decay time of the light transmission is observed with different cell thickness, voltage and frequency.