  The distribution in frequency of optical spectral weight remaining under the
real part of the optical conductivity in the superconducting state of a d-wave
superconductor depends on impurity concentration, on the strength of the
impurity potential as well as on temperature and there is some residual
absorption even at T = 0. In BCS theory the important weight is confined to the
microwave region if the scattering is sufficiently weak. In an Eliashberg
formulation substantial additional weight is to be found in the incoherent,
boson assisted background which falls in the infrared and is not significantly
depleted by the formation of the condensate, although it is shifted as a result
of the opening of a superconducting gap.
