The optical absorptivity spectra for samples of LaH x and NdH x were also measured by Peterman et al. [36] at 4.2K at near-normal incidence. From the absorption data, they deduced the real and imaginary part of the dielectric constant, and hence the optical conductivity by using Kramers-Kronig analysis. They found a relatively broad feature for LaH 2.87 . However, their samples were polycrystals and the use of Kramers-Kronig analysis might enlarge the **data** uncertainties. On the other hand, a more recent and accurate measurement reported by Griessen et al. [39] sheds new light on the optical conductivity curve. The optical transmission spectra of the insulating phase YH 3−δ were measured as functions of the photon frequencyhω and of hydrogen vacancy concentration δ. The effect of the vacancy appears to reduce the overall transmission spectra quite evenly between hω = 0.5 eV and 2 eV. In our theory, the δ-dependent conductivity σ(ω) athω < 2.8 eV mainly determined by the optical transition from the vacancy state to the conduction bounds. The transition energy from the valence bands to the vacancy state is larger because of the larger energy difference between the two states and because of the Coulomb repulsion of the doubly occupied electron states on the same vacancy site. Since the vacancy state is highly localized, the transition matrix largely depends on the density of states of the conduction bands, which is expected to lack pronounced feature.