  The Casimir force between two metallic plates is affected by their roughness
state. This effect is usually calculated through the so-called `proximity force
approximation' which is only valid for small enough wavevectors in the spectrum
of the roughness profile. We introduce here a more general description with a
wavevector-dependent roughness sensitivity of the Casimir effect. Since the
proximity force approximation underestimates the effect, a measurement of the
roughness spectrum is needed to achieve the desired level of accuracy in the
theory-experiment comparison.
