  Room-temperature optical constants of crystalline
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ were determined using data extracted from
Brillouin light scattering spectra. Optical extinction
coefficient-to-refractive index ratios at a wavelength of 532 nm were obtained
from bulk phonon peak linewidth and frequency shift measurements and range from
$0.19 \leq 2\kappa/n \leq 0.29$ for directions close to the crystallographic
$c$-axis. These ratios, and optical extinction coefficients, absorption
coefficients, and imaginary parts of the dielectric function determined from
these ratios and known refractive index, are in general agreement with values
found in optical reflectance studies, but are 5-7 times larger than those
extracted from optical interference measurements.
