The lack of a scanner calibration target representative of the image medium (substrate and colorants) often limits the accuracy of scanner color calibration. In this paper, a color calibration method is presented for photographic input media that does not require a calibration target. Using characteristic spectral measurements from the image(s) to be scanned, a model for the spectra on the medium is obtained through a principal component analysis. The spectral sensitivity of the scanner provides a model for its operation. By using a set-theoretic estimation method that combines the models for the scanner and the medium, the spectral reflectance of the input corresponding to a given set of scanner RGB values can be determined. This provides a spectral calibration for the scanner, which can readily be transformed into a color calibration under any suitable viewing illuminant. Results from simulations and actual calibrations demonstrate the value of the new method.