The optical reflection spectrum of incommensurately modulated calaverite is presented. Using a Kramers-Kronig analysis the dielectric function is derived from the reflection spectrum. The results for the imaginary part of the dielectric function are qualitatively discussed in terms of absorption by phonon, free electron and interband scattering. The phonon contribution clearly shows the influence of the incommensurability by the activity of phonons with k not=0. The free electron scattering can be understood in terms of a simple Drude model. The interband scattering is found at a lower frequency than predicted from electronic band structure calculations.