A new theoretical calibration of surface-brightness fluctuations (SBF) for single age, single metallicity stellar populations is presented for the optical and near-IR broad-band filters, as well as for the HST WFPC2 filters. The IAC-star code is used together with two Padua and the Teramo stellar evolution libraries. A set of single-burst stellar populations with a wide range of ages (3Gy-15Gy) and metallicities (Z=0.0001-0.03) have been computed. The present theoretical calibration shows that the analysis of near-IR SBF provides a very powerful tool in the study and characterization of unresolved stellar populations. This, together with the high spatial resolution, limiting magnitude and image quality of CIRCE@GTC, makes this unique to study and characterize the age and metallicity of unresolved early type stellar populations.