treatment of the plasmonic as well as ordinary photonic modes [@Intravaia05; @Intravaia07]. The plasma model is sufficient for a first description of the variation of the force with distance but it is not sufficient for a precise comparison.

First, the relaxation of the conduction electrons has to be accounted for. Then, interband transitions are reached for metals like Au, Cu or Al for photon energies of a few eV and their effect on the optical response has to be taken into account for evaluating the Casimir force at short (sub-micron) distances. This can be done by using tabulated optical data which are integrated using causality relations [@Lambrecht00]. The result of the corresponding evaluation is shown on Fig.(\[fig:1\]). It is worth stressing that calculations are sensitive to the existing differences in optical data between different tabulated sets [@Pirozhenko06]. This means that an imperfect knowledge of the optical properties of the mirrors used in the experiment is a source of uncertainty in the experiment-theory comparison. Ideally, if the aim is to have a reliable theoretical evaluation of the Casimir force to be compared with experiments, it is necessary to measure the reflection amplitudes *in situ*.

Silicon slab mirrors {#sec:5}
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As stressed 