data and which includes the contribution of interband electrons [@Lambrecht00].

The corrections to the Casimir effect are conveniently represented in terms of factors measuring the reduction of the force and energy with respect to the ideal limit of perfect mirrors $$\begin{aligned}
F&=&\eta_{\rm F}F_{\rm Cas}  \ \ , \ \   \eta_{\rm F} < 1   \  \ \textrm{and} \nonumber  \\  
E&=&\eta_{\rm E}E_{\rm Cas}  \ \ , \ \   \eta_{\rm E} < 1.\end{aligned}$$ The results of the calculations are plotted on Fig.(\[fig:1\]) for Au-covered mirrors. They are shown as $\eta_{\rm F}$ varying versus the ratio of the cavity length $L$ to the plasma wavelength $\lambda_{\rm P}$.

For metals used in recent experiments, the plasma wavelength lies around $0.1\mu$m ($136$nm for Au and Cu). At large distances $L\gg\lambda_{\rm P}$, the ideal Casimir formula is recovered ($\eta_{\rm F}\rightarrow 1$), as expected. At short distances, a significant reduction of the force is obtained, with a change in the power law for the variation of the force with distance. This change can be understood as the result of the Coulomb interaction of surface plasmons at the two vacuum interfaces [@GenetAFLdB04; @Henkel04]. This interpretation may be actually generalized to arbitrary distances at the price of a full electromagnetic 