  The temperature dependent redistribution of the spectral weight of the
$CuO_2$ plane derived conduction band of the $YBa_2Cu_3O_{6.9}$ high
temperature superconductor (T_c = 92.7 K) was studied with wide-band (from 0.01
to 5.6 eV) spectroscopic ellipsometry. A superconductivity - induced transfer
of the spectral weight involving a high energy scale in excess of 1 eV was
observed. Correspondingly, the charge carrier spectral weight was shown to
decrease in the superconducting state. The ellipsometric data also provide
detailed information about the evolution of the optical self-energy in the
normal and superconducting states.
