We introduce a novel method for the analysis of the dependence of the optical parameters of Hydrogenated Silicon-rich Carbide (SRC:H) on the alloy composition by using a ternary-3D diagram. The SRC:H samples were fabricated using the very high frequency (VHF) plasma enhanced chemical vapour deposition technique. The composition is given as Si-% + C-% + H-% = 100, with the carbon fraction C-f = C%AC% + Si%) varying between 0.18 and 0.72. The actual composition was determined by Rutherford Back Scattering analysis. The optical constants are determined by means of Reflectance and Transmittance UV-visible spectroscopy, and are modelled by means of the Jellison-Tauc-Lorentz-with Gaussian Band Tail model. The unique properties of the ternary-3D diagram give unexpected insight on the origin of the dependence of optical properties on composition. It is shown that the wavelength position of the Lorentz oscillator is governed by Cf with virtually no influence of H-%. Conversely, the refractive index is strongly affected by H-%, besides C-f. We show that using the ternary-3D diagram approach it is possible to separate the contributions of the different components, and derive the analytical dependence between optical parameters and composition. In the second part of the paper such findings are used for a reversed analysis, that is, we employ the obtained analytical formulations in order to retrieve the SRC:H composition within better than +/- 4% absolute error for all components.