The field dependent birefringence and the tilt angle are measured for a series of four electroclinic liquid crystals. The zero-field birefringence decreased significantly and monotonically with the length of the siloxane side chain. For all four samples, the birefringence and the electroclinic tilt angle increased substantially with the application of a field. At 10 V/µm, the electroclinic tilt angle in DSiKN65 and TSiKN105 exceeded 22.5 degrees and the phase retardation due to the change in the birefringence exceeded a quarter wave. In all four liquid crystals, the electroclinic tilt angle and the field-dependent birefringence were shown to be strongly correlated, suggesting a common origin. Moreover, this correlation persists even at high fields where both processes undergo saturation. A simple orientational model, based on a distribution in the molecular optical axis, was developed to explain the large change in birefringence in the substituted compounds as well as the dependence of δn on electric field.