The bulk dielectric constant (i.e. relative permittivity) is assumed to be given by the Lyddane–Sachs–Teller relation:

$$ \epsilon_{\text{bulk}}(\omega) = \epsilon_{\infty} + (\epsilon_0-\epsilon_{\infty}) \frac{\omega_{\text{TO}}}{\omega^2_{\text{TO}}-\omega^2}$$