The aim of this study is to determine the optical anisotropy of three different samples of thermo-tropic polyester fibers of structure Poly(chloro-1,4-phenylene terephthalate-co-4,4'-oxybisbenzoate) and its copolymers (PCPT-co-CPO). The molar fraction of disruptor units ([CPO]) ranging from 0.40 to 0.60. The variable wavelength interferometry, VAWI technique, was used to determine the birefringence and spectral dispersion properties of such of these fibers. The Cauchy's dispersion formula and its related constants were determined using the spectral dispersion curves of the birefringence. A mathematical formula was derived for direct measurement of the birefringence profile of highly birefringent polymer fibers using VAWI technique. The effect of varying copolymer molar fractions of the three samples of thermo-tropic polyester fibers was investigated throughout the spectral dispersion curves, the Cauchy's formula constants and the birefringence profiles. Microinterferograms are given for illustration. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012