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Shortly after their invention (as a distance indicator), surface brightness fluctuation (SBF) magnitudes have been recognized as indicators of stellar populations. Since the radiation from external galaxies is dominated by their stellar populations between 0.05 and 5 $\mu$m, we would like to know what we can learn from SBF magnitudes over this entire wavelength range. SBF magnitudes can be measured even if individual stars cannot be resolved. In the case that the brightest stars are seen, SBF magnitudes give a first-order approximation of the steepness of the luminosity function for fainter stars. In the ultraviolet ($\lambda < 2800$\AA ) SBF magnitudes are sensitive to the brightest UV-emitting population of stars, which are post-asymptotic giant branch (PAGB) stars in populations with ages greater than a few tenths of a Gyr. In the 2800--4000\AA\ region, SBF magnitudes can be used to detect the presence of hot horizontal branch stars, especially in populations of near-solar metallicity, if PAGB stars are calibrated out. In the 4000\AA --1.5$\mu$m region, SBF magnitudes measure the average temperature of the red giant branch (RGB), with slightly more sensitivity than ordinary broad-band colors. In the 1.5--5 $\mu$m region, in addition to RGB temperature, the SBF magnitudes are also sensitive to the presence of young (0.1--5 Gyr) populations which have bolometrically luminous asymptotic giant branch (AGB) stars. Unfortunately, the lifetimes of the most luminous AGB stars are presently uncertain, which makes age estimation from SBF magnitudes difficult.
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