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Room-temperature optical constants of crystalline Bi2Sr2CaCu2O 8 + δ 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 ⩽ 2 κ / n ⩽ 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.