A synthetic projection system determines analyte concentration, such as blood glucose concentration, from a spectral-energy change associated with an uncharacterized instance of a medium in which the analyte is likely present. The projection system is factory calibrated for different instances of the medium, without needing instance-specific training or calibration. The projection system includes a set of projector curves, each relating spectral-energy change values obtained by analyzing reference medium samples to analyte concentrations in those samples. Each projector curve also corresponds to a respective range of energy-change gradients, determined using a group of surrogate media characterized according to analyte concentrations measured using a reference system. A spectral-energy-change gradient for the uncharacterized medium may be computed to select one of the projectors curves. Analyte concentration in the uncharacterized medium can be reliably computed at a specified high level of accuracy using the spectral-energy change associated therewith and the selected curve.