# For the high-frequency permittivity of a material, do I need to look beyond its value in the optical energies region?

I was trying to find $$\varepsilon_\infty$$ for InSb, and the value I found on Kittel p. 416 was 15.6. But in the literature, even at 6 eV, $$\varepsilon (\omega )$$ is still in the negative (at -3.835) but that's beyond the optical region and has not reached the positive value of 15.6 shown in Kittel.

Yet, $$\varepsilon_\infty$$ is supposed to be synonymous with $$\varepsilon_\text{opt}$$ i.e. that the high-frequency limit is reached at optical frequencies (in visible region).

So really my question is: for finding $$\varepsilon_\infty$$ it's not enough to go look in optical region right, so the use of $$\varepsilon_\text{opt}$$ is basically misleading and wrong, right?