Accurate dispersive refractive indices for GaSb and AlAsSb are highly desirable for the design of GaSb-based resonant cavity devices operating in the 2-8 μm range. This dissertation reports on the development of two novel methods for the application of the well-known Swanepoel method in the case where normal incidence reflectance spectra are unavailable. By utilising linear regression of oblique angle reflectance spectra to determine normal incidence values, the required input data for the standard Swanepoel method can be calculated without direct measurement of normal incidence reflectance. Dispersive refractive indices calculated from this first method are shown to be in good agreement with literature data and are applied to the modelling of a DBR stopband, where they show significant improvement in agreement between modelled and measured reflectance when compared to experimental literature. The second method determines non-dispersive infrared refractive indices by a second-order Taylor expansion of Swanepoel’s equations and is shown to be capable of predicting a reasonable trend in the refractive index of dilute (InAs_0.91Sb_0.09)_xGaSb_1−x alloy with increasing indium content.