This work presents the measurement of the dielectric anisotropy of selected nematic liquid crystals in the 0.01-6 GHz range, and comparison of the measurements with the birefringence values at optical frequencies. The effect of alignment on the reproducibility of dielectric measurements was also measured. It was found that a good surface alignment is essential to achieve reproducibility. Finally, it was experimentally confirmed that there is a strong correlation between the dielectric anisotropy in the microwave region and the birefringence at visible frequencies.