The aim of this study is to extend, for a larger range of mineral volume (V(1)) and different aqueous immersion media, the model for calculating the refractive index of the non-mineral phase of the human dental enamel proposed by Sousa et al. Published experimental birefringence data of carious and developing human enamel in air and aqueous immersion media with different refractive indexes were interpreted. For 48%</=V(1)</= 95%, quantitative estimates on the non-mineral contents consistent with the values described in the literature were derived. New polynomial relationships of the non-mineral contents as a function of the mineral content (48%</=V(1)</= 95%) are introduced, which allows computing the theoretical mean enamel birefringence in water and air for most of the points of developing, carious and mature enamel with a difference of one order of magnitude in relation to the classical model for the non-mineral content's refractive index and closer to the experimental data. For enamel birefringence in immersion media other than water and air, a less expressive improvement was obtained. Perspectives for future studies are discussed.