We present SEDs for single-age, single-metallicity stellar populations (SSPs) covering the full optical spectral range at resolution (FWHM = 2.3Å). These SEDs can be regarded as our base models, as we combine scaled-solar isochrones with an empirical stellar spectral library (MILES), which follows the chemical evolution pattern of the solar neighbourhood. The models rely as much as possible on empirical ingredients as also employ extensive photometric libraries. Thanks to the unprecedented parameter coverage of the MILES library we synthesize SSP SEDs from intermediate- to very-old age regimes, and the metallicity from super-solar to [M/H] = -2.3, all for a suite of IMF shapes and slopes. We propose a new Line Index System (LIS), based on flux-calibrated spectra, to avoid the intrinsic uncertainties associated with the Lick/IDS system and provide more appropriate, uniform, spectral resolution.